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En route adjustments


Rev. 21 — page content was last changed 9 February 2013
Flight Planning and Navigation

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Situation awareness is the backbone of en route navigation. Apart from maintaining awareness of the weather, the traffic and the state of the aircraft; the pilot must continually monitor progress against the flight plan — where am I now? where am I going? — and make the heading adjustments necessary to maintain the planned route. 'What shall I do when I get there?' — if progress is slower than planned, then diversion to an alternate airfield may be necessary — and there are standard procedures to be implemented when uncertain of position.




      Content


8.1 Setting compass heading

Before we reach the set-course point of our flight we have to determine the compass deviation known to occur at that magnetic heading — if the deviation has been measured and if there is a compass correction card displayed in the aircraft. Such a card might look like this, although it is more likely to be expressed in 30° rather than the 45° intervals shown:
Compass correction card
Heading magnetic045°090°135°180°225°270°315°360°
Compass correction+2°+5°+2°0-2°-5°-2°0


To find the heading to set on the compass just add the deviation value to, or subtract it from the magnetic heading. For example, our heading on the flight from Oxford to Warraway Mountain is 079° magnetic. Interpolating between the values shown for 045° and 090°, the correction for 079° is +4°. Thus the compass heading to steer is 083°, the magnetic heading will then be 079° and the true heading will be 090°.



8.2 Monitoring and recording flight progress

Position fixing methods
Monitoring involves checking from clock to map to ground — anticipating what should be in view a few minutes ahead — plus intermittent position fixing to establish the track made good, and estimating the track error and the actual ground speed. The essential navigation instruments are just the compass and the clock — to provide direction and ground speed.

Deviations from the track required occur because:
  • the pilot is not maintaining the planned heading or has set the wrong heading; e.g. the heading for a different leg.
  • unrecognised compass error (deviation) causes the heading flown not to be the planned magnetic heading. There should be no extraneous ferrous materials (e.g. the rings on a binder) near the compass, or larger items anywhere in the cockpit.
  • the wind velocity is substantially different from that used for the flight plan or it was applied incorrectly during flight planning
  • the required track direction was incorrectly measured — or converted to magnetic — during flight plan preparation. In eastern Australia, where magnetic variation reaches 13°, reversed application of variation when calculating the magnetic heading can result in a very large heading error.

There are basically two methods of fixing the aircraft's position. The first and the most common in light aircraft navigation is by identifying a landmark close to the aircraft. The second is by identifying (or establishing) two or more lines of position [LOP] such that their point of intersection provides the position fix. A line of position is a line drawn, or an existing line feature, on the chart indicating that the aircraft's position is somewhere along it. Note that there should be a reasonable angular difference, maybe more than 30°, between two LOPs in order to derive a useful position fix.

A LOP might be:
  • a clearly identifiable physical identity such as a long road, railway or river that the aircraft is about to fly across — but without any supporting indication of exactly where the crossing is being made

  • a visual bearing from an identifiable landmark. Or an instrument bearing from a radio beacon, which we will discuss in the 'Supplementary navigation techniques' module

  • a transit. When two identifiable landmarks are seen to be in line with each other the aircraft must be somewhere on the extended sight line. It is a good practice to identify suitable landmark pairs in the flight planning stage, and mark on the chart the transit line between them, extending it across the track required — thus pre-marking the LOP.


Running fixIt is not always possible to derive two LOPs, more or less simultaneously, to get a position fix at their intersection. However, if a second LOP can be derived within, say, 10 or 15 minutes of the first then a running fix can be ascertained. This is done by estimating the distance flown along the track during the lapsed time and then shifting the initial LOP that distance. The intersection of the shifted line and the second LOP is the position fix. (There is another form of running fix involving two bearings from the same object but this is really a technique applicable to radio navigation aids and we will look at it in the 'Supplementary navigation techniques' module.)
Recording progress
An accurate running log of LOPs or position fixes, segment start and finish times, magnetic headings, and fuel consumption must be maintained — it is very difficult to remember times and headings flown — and the log need only be a blank note pad. For example the first note made is the engine start-up time and the second is the set heading time. The minutes elapsed between those times is then noted on the fuel log 'Airfield departure' line in the 'Actual' column.

All events logged should be preceded by the time and all the magnetic headings flown must be logged, even when that heading is the planned heading. As each route segment is commenced, log the ETA for the next waypoint or major landmark, which will be the current time plus the flight plan ETI (or an adjusted ETI) for that segment. As each route segment is completed, the elapsed time is transferred to the fuel log.



8.3 Track error adjustments

The track plot below shows the first leg of our planned flight between Oxford and Tottenham. The green lines are the 10° drift lines and the pink marks along the track are the 10 nm distance marks. In this example the first three marks are distance from Oxford, the last three are distance from Warraway Mountain, and the longer mark is the track midpoint.

Instead of distance marks some navigators favour time marks at, say, 10-minute intervals. However time marks really don't correlate that well with charts — time notations should be shown in the log.

nav plot segment


As it is remarkably easy to set off in the wrong direction — reversed application of variation for instance — it is advisable to note a landmark as a means of verifying that, at the set-course point, you really are setting off along the required track. The sun's position provides a gross indication of heading, which will at least confirm that you are not flying the reciprocal course. Starting off in the wrong direction, without realisation, really makes progress monitoring difficult when you are flying over relatively featureless terrain. In addition, a position fix must be acquired within 15 to 20 minutes of the set heading time.

At any time after departure, when the aircraft's position has been pinpointed and found to be off track, heading adjustments will be necessary. Initially this is to regain the required track and then to maintain it; or, alternatively, for a new heading to track directly to the next turning point. There are several methods for calculating and applying heading adjustments — a pencil and paper will be handy.
Double track error method
This is the recommended method if the position fix shows the aircraft to be less than halfway along the leg — hence the reason for marking that midpoint on the chart. The procedure is as follows:

      1.   Using the diverging 10° drift lines estimate the track error — the difference in degrees between the track required and the track made good. The track error is also referred to as the opening angle or sometimes as the drift angle. (The term 'drift angle' in this context is deprecated, as it normally refers to the angular difference between the heading flown and the track made good.)

For example let's say, on our Oxford to Warraway Mountain segment, we find ourselves crossing the railway line at Trida and estimate the track error as 6° north of required track; i.e. the track made good is 077° magnetic. We log the time and note 24 minutes have lapsed since departing the set-course point.

      2.   Double the track error and add or subtract that value from the planned heading to arrive at the heading to regain track or the intercept. If the drift was to the right of track the new heading must be to the left of the original heading and, conversely, if drift was to the left the new heading must be to the right of the original. The time we must remain on this new heading, until intercepting the required track, is roughly equivalent to the time flown on the original heading.

Although we have used the terms 'left' and 'right' you will find it more helpful, when considering position, to think in terms of 'north', 'south', south-west' etc.

nav plot segment


For example track error is 6° north (left) and original heading 079° magnetic, thus the heading to regain track is 079 plus 12 = 091° magnetic (096° compass) and we fly that for 24 minutes — the same time as that flown on the original heading.

      3.  After it is visually evident that the required track has been reached, or the required time has passed, subtract the track error and turn onto the new heading to maintain the required track and log the event.

For example the track error was 6° and the heading to regain track is 091° magnetic, thus the heading to maintain track is 091 minus 6 = 085° magnetic (090° compass) and we fly that until either a new position fix is obtained or we reach the waypoint.

Theoretically this method doesn't work if the position fix is past the halfway point, because the point at which the required track is finally regained would be past the waypoint.
Track error/closing angle method
The recommended method if the position fix shows the aircraft to be more than halfway along the leg, or if you choose to fly directly to the waypoint at any time, is as follows:

      1.   Using the diverging 10° drift lines estimate the track error or opening angle — the difference in degrees between the track required and the track made good. Then using the converging drift lines estimate the direct track to the waypoint, and the angle between that track (the new required track) and the original required track. This is usually called the closing angle.

nav plot segment


For example, let's say on our Oxford to Warraway Mountain segment we fix our position as one mile south of the Dundooboo Ridge with an estimated track error of 7° north of required track and the closing angle to the waypoint is about 9°.

      2.   Add the track error and closing angle, and apply the value as a correction to the original heading. If the drift was to the left of the required track the new heading will be to the right and vice versa.

For example, 7 plus 9 is 16°, drift was to the left of the original heading 079° magnetic, thus the heading — to track directly to the waypoint — is 095° magnetic (100° compass).
Flight direct to landmark
A third method might be employed if after getting a position fix a landmark known to be on, or close to, the required track is positively identified.

      •   Having pinpointed your position use the diverging 10° drift lines to estimate the track error, then fly directly to the identified on-track landmark. To maintain the required track it will be necessary to turn onto a new heading when overhead the landmark. The new heading will of course be the original heading plus/minus the track error.
Utilising the 1-in-60 rule
The 1-in-60 rule of thumb can be used to determine track error, given distance travelled and distance off track. It replaces the use of drift lines but the latter is much the easier method to use in flight because the angle is easier to estimate than the on-chart distance off track, and the mental arithmetic is easier. However, just to keep you informed, here is the 1-in-60 method for track error calculation.

      1.   Having pinpointed the aircraft's position, estimate the distance off track and the distance travelled along the leg. The track error = the distance off track [DO] divided by the distance travelled [DT] × 60; i.e. the track error = DO/DT × 60.

Or conversely the distance off track [DO] = track error/60 × DT

For example, let's revert to our Oxford to Warraway Mountain segment where, after 24 minutes flight, we pinpoint our position at Trida. Trida is about 3 nm north of required track and 22 nm distant from the departure point. Thus 3/22 × 60 = 8° track error. The track error we estimated using the drift lines was 6°, but that's basic navigation for you.

      2.   To regain the required track double the track error and when the required track is reached, or the time has elapsed, subtract half the error and take up the new heading. In this aspect it's the same technique as the double track error method.

      3.   Or to track direct to the next waypoint calculate the closing angle — which will equal the distance off track [DO] divided by the distance to go [DTG] × 60; i.e. closing angle = DO/DTG × 60.

For example, Trida is 52 nm distant from Warraway Mountain. Thus 3/52 × 60 = 3° closing angle. The new heading is the original heading plus track error plus closing angle = 079 + 8 +3 = 090° magnetic — as in the track error/closing angle method.

Each time the aircraft's position is pinpointed and the heading is adjusted, a re-calculation of the ground speed and ETI for the segment should be made on the running log. The 1-in-60 rule has use in this aspect of navigation, see 'Re-assessing wind velocity'.
Diversions — 30° and 60° dog-legs
There is another navigational adjustment technique occasionally mentioned as a standard method of diversion around hazards or no-fly areas such as towns. As no such diversions would be necessary if the flight plan is properly prepared, such techniques should be of practical use only when something of interest is spotted off track and you divert for a little sight-seeing.

The method is as follows:

      •   When you judge the point of interest is about 30° off your current heading, alter course 30° towards the target. Note the time when the target is reached and then turn 60° in the reverse direction and fly that heading for the same time as the first part of the dog-leg, then revert to the original heading. The time lost during the two legs (which has to be added to the ETI for the segment) is one third of the time flown on the first (or either) leg and, of course, you have to add to the ETI any time spent circling over the target.

      •   The 60°dog-leg is much the same except that you alter course when the target is 60° off the original heading, the alteration to return is 120° and the time lost flying the dog-leg is the time spent on the first (or either) leg.

60° sightseeing dogleg
      For example, if we were about halfway along our track from Oxford to Warraway Mountain, we thought it a good idea to have a look around the Warranary Hill. Thus we turn 60° left from our original heading of 079° magnetic to 019° magnetic, fly that for, say, six minutes then turn right 120° to 139° magnetic and fly that heading for six minutes. Then turn left 60° back on to our original heading of 079° and add six minutes to our ETI and/or ETA.



8.4 Recalculating ETI/ETA and fuel consumption

Checking ground speed
Being off-track, because of a variation in wind, is much more likely to be noticed quickly than if on track but adversely affected by a stronger than expected headwind, or tailwind. Whenever a position fix is obtained, whether it is on-track or off-track, it is advisable to calculate the ground speed achieved and to re-estimate the ETI for the leg, ETA at the next waypoint and at the destination. Ground speed [GS] in knots is distance travelled [DT] / elapsed time in minutes [ET] × 60;
i.e. GS = DT/ET × 60. For example, reverting to our Oxford to Warraway Mountain segment where, after 24 minutes flight, we pinpoint our position at Trida, 22 nm distant from the departure point and about 52 nm from the next turning point.

Ground speed = DT/ET × 60 = 22/24 × 60 = 55 knots.
Recalculating ETI
Time in minutes to the next checkpoint, turning point or destination = distance to go [DTG] / GS × 60; i.e. time = DTG/GS × 60.

e.g. Time to the next turning point = DTG/GS × 60 = 52/55 × 60 = 57 minutes.

There is a slightly simpler means to estimate the time to the next turning point. A ground speed of 60 knots is one nm per minute so at 60 knots the distance of 52 nm will be covered in 52 minutes. However, our ground speed of 55 knots is about 10% less so the time will be 10% greater than 52 = 57 minutes.

Our ground speed at 55 knots is nearly 20% less than the flight plan figure of 67 knots and our ETI for the first leg is now 57 + 24 = 81 minutes; some 15 minutes — or more than 20% — greater than the flight plan figure of 66 minutes. This decreased performance is most likely caused by an easterly wind speed much greater than the forecast. The summed flight plan en route time was 144 minutes and 20% of this is about 30 minutes — which makes a significant inroad into our 40-minute above-reserve fuel margin. We will have to closely monitor progress, as we are getting near to the point of considering diversion to an alternate airfield.
Re-assessing wind velocity
The headwind component of the wind speed must be our true airspeed (75 knots) minus our ground speed (55 knots); i.e. 20 knots. However, the crosswind component is difficult to estimate because we don't know what caused the drift away from the track required. It could be unadjusted compass deviation, poor heading holding or changed wind velocity — or a combination of all three. However, let's assume we found ourselves over Trida only because the wind varied substantially from that assumed in the flight plan. If so, what is the crosswind component?

To measure the crosswind component we need to measure the drift angle — the angular difference between the heading flown and the track made good [TMG]. For example, at Trida TMG was estimated as 6° left of the track required (083° magnetic) thus 077° magnetic and the heading flown was 079° magnetic. The drift angle is then 2° left, rather than the expected 4° right.

Using the 1-in-60 rule, the crosswind component in knots = the drift angle/60 × TAS in knots.
For our Trida position fix, the crosswind component is thus 2/60 × 75 = 2.5 knots. We can now estimate the wind direction by first ascertaining the ratio of crosswind to headwind, which in this case is 2.5/20, and then, using the 1-in-60 rule, calculate the wind angle relative to the aircraft's heading.

The equation is: wind angle = crosswind/headwind × 60 = 2.5/20 × 60 = 7°.

As the drift recorded was to the left, the wind must be coming from 7° to the right of heading, the heading being flown was 079° magnetic thus the wind direction is 086° magnetic or 097° true and its speed must be very close to 20 knots.
Summary of track angle definitions
Track error — the angular difference between the track required and the track made good. You may come across the term cross track error [XTE] which refers to the distance off track.

Opening angle — another term for angular track error.

Drift angle — the angular difference between the heading flown and the track made good.

Closing angle — the angular difference between the direct track to the waypoint and the original required track, measured at the waypoint.

Wind angle — the angular difference between the heading flown and the estimated wind direction; i.e. the wind relative to the aircraft rather than the ground.

Checking fuel consumption
Fuel flow indications are monitored for abnormalities as part of the continuing in-flight instrument scan. A calculation of consumption rate should be made at half-hourly or hourly intervals to check for any significant variation from the hourly consumption rate used in the flight plan. Fuel consumption should always be measured in terms of time not distance.



8.5 Diverting to an alternate airfield

During flight the pilot should always be aware of the general direction of the planned alternate airfields so that, should a diversion be deemed necessary, the aircraft can then — after verifying current position — be headed in the general direction of the selected alternate without unnecessary delay. The mental calculations required to refine the heading, estimate distance, ETA and fuel requirement are then done without wasting time and fuel.

For example, let's say that we reach our turning point at Warraway Mountain 85 minutes after the set heading point; i.e. the actual time interval [ATI] is 19 minutes, or nearly 30%, greater than our ETI of 66 minutes. Under these circumstances we could expect that the total en route time would also increase by 25% to 30%, even though the next two legs are swinging out of wind by 30° or more. This adds around 40 minutes to total time and reduces the planned fuel margin above reserve to zero so a diversion to Condobolin is warranted. The bearing is about due east so we would note the time and alter heading slightly to 079° magnetic.

Looking at the chart the distance from Warraway Mountain to Condobolin is easily estimated (knowing that the distance between the meridians on the grid of this WAC is 25 nm) at about 55 nm and the bearing is not quite due east, say 088° true. Our ground speed will not alter from that established between Oxford and Warraway Mountain where we covered the distance of 74 nm in 85 minutes — 74/85 × 60 = 52 knots. The ETI from Warraway Mountain to Condobolin will be 55/52 × 60 = 63 minutes and our fuel log will look something like this:

Fuel calculation and fuel log
Cruise fuel flow:16 litres/hr
Usable fuel loaded:64 litres
Endurance:240 mins
 
 EstimateActual
Airfield departure:10 mins12
Climb penalty:6 mins6
 
En route:  
Oxford – Warraway:66 mins85
Warraway – junction Cond'n:48 63 mins 
Junction – Tottenham:30 mins 
Estimated time en route:144 148 mins 
 
Airfield arrival:10 mins 
Fixed reserve:30 mins 
Total fuel required:200 206 mins 
Fuel margin
(endurance –total required)
40 34 mins 




8.6 Line-of-sight distance and landmarks

Knowledge and use of landmarks is an essential part of light aircraft pilotage, thus on cross-country flights it is useful to know at what distance any landmark, particularly those distinguished by height and shape, might be discernible. The rule-of-thumb is, given unlimited visibility and an eagle eye, the maximum optical line-of-sight (LOS) distance, in nautical miles, is equal to the square root of the observer's height in feet. More precisely, it is 1.06 times the square root of the height. By the way, VHF transmissions are also LOS.

Theoretical LOS distance to horizon
Observer height (feet) Maximum LOS distance (nm)
103.2
10010
100032
10 000100


The theoretical distance in nautical miles at which a landmark may be seen is near enough to the sum of the square root of the height of the top of the landmark (in feet) and the square root of the observer's height. Theoretically then a pilot flying at 10 000 feet might first see the highest point of an island, with an elevation of 1000 feet, from 132 nm away (100 + 32).

However, in cross-country flight, the only landmarks readily discernible at long distance are hills or mountains, particularly sentinel types. For example, in south-eastern Australia, Mt Ulandra, Mt Major and The Rock. Even then, for firm identification, you may need to have the top few hundred feet in view and be less than the maximum distance from the landmark. Haze, residual dust and smoke greatly reduce visibility, particularly below inversions and in the friction layer.

When doing the calculation for LOS distance the basic elevation of the general intervening terrain must be deducted from the elevation of the landmark — and from the observer's altitude. In the table below, The Rock has an elevation of 1800 feet and the intervening terrain elevation is 800 feet, so the top 500 feet of the sentinel begins 500 feet above the general terrain. The third column of the table shows the LOS distance from observer height above the terrain, the fourth column shows the LOS distance from a point 500 feet below the summit and the last column — the sum of columns three and four — shows the maximum distance at which all the top 500 feet might be seen, above the horizon, by an observer at a recommended light aircraft hemispherical cruising level.

LOS distance to sentinel landmarks
Observer altitudeObserver height above terrainObserver LOS distanceThe Rock top 500 ft LOS distanceDistance at which The Rock visible
feetfeetnmnmnm
1500700272350
25001700422365
35002700522375
45003700612384


An eagle-eyed observer flying at 4500 feet, on a remarkably clear day, could see The Rock, well above the horizon, from as far as Young, Griffith, Deniliquin or Benalla. If you were heading for Yabba North, at 3500 feet, you could see Mt Major (nine miles south of Yabba) from Culcairn, Jerilderie or Bendigo.

Estimating the square root: mental calculation is easier if you ignore the two least significant digits of the height, then estimate the square root of the remaining one or two digits and multiply by 10. For example; height 3000 feet, ignore 00, the square root of 30 is between 5 and 6 — say 5.5 and multiply by 10 = 55 nm LOS distance. Another example; height 700 feet, ignore 00, the square root of 7 is between 2 and 3 — say 2.5, multiply by 10 = 25 nm LOS distance.





8.7 Procedure when lost

There are occasions during a cross-country flight when the pilot is uncertain about the aircraft's position, particularly when there are considerable distances between verifiable landmarks and a near-track landmark has not come into view. If proper flight planning and checking procedures are followed, and actual versus planned flight progress is continually monitored and recorded, then probably the only way to become really lost — in fine weather and reasonable visibility — is if an en route heading adjustment is incorrectly calculated or implemented, or if a turning point is overflown without noticing.

There are a few rules that must be followed if thought to be lost or caught in a difficult situation;

      1. Fly the aeroplane! You must not concentrate all attention on the navigation problem — keep the normal scan going otherwise you can readily lose control of the aircraft.

      2. If the ETA at the next waypoint has not yet, or only recently, lapsed then hold the heading — resist the temptation to start wandering about searching for landmarks.

      3. However, if the ETA at the waypoint has long passed then choose a landmark below the aircraft, log the time and then orbit the landmark while you carry out a quick recheck of the running log and previous mental DR, and start the procedure detailed in the next paragraph. But don't forget rule 1: "Fly the aeroplane!". There is no point in wasting fuel while doing this so reduce power and airspeed to the best endurance setting for a safe flight speed. If no obvious error is found that will provide the basis for a position estimate then proceed with rule 4.

      4. Check the time elapsed since the last position fix and estimate the distance covered in that time. On the chart draw a line of position [LOP] across the track (the original or an intercept) at the estimated distance from the last fix. The line should extend about 1 nm either side of track, for each 5 minutes flown since the fix; i.e. if it is 30 minutes since your last positive fix then the line will extend roughly 6 nm either side. Then draw a rough circle with the LOP as the diameter (see diagram below) — your most probable position [MPP] is somewhere within that circle of uncertainty. Find the most prominent features on the map within the circle and then try to locate them on the ground. The 1 nm per 5 minutes is based on ground speeds around 50 or 60 knots; if ground speeds are around 100 knots then make it 2 nm per 5 minutes. 'Most probable' means maybe an 80% chance.

      5. If below 3000 feet agl then climb a little, cloud base permitting. The theoretical line-of-sight distance at 4000 feet agl is 65 nm all round. This provides sufficient coverage to pick up all the major landmarks — near and middle distance — which aren't concealed by terrain or atmospheric conditions. If climbing takes you above an inversion layer you may find surface visibility is better just below the inversion. Remember that on a bright day, scattered cloud shadows may make some landmarks difficult to pick up even if relatively close. Reduce power to best endurance.

      6."Read from ground to map!" Normally in flight, the navigator should be continually identifying features on the map and waiting for the next one to come up on track, within an estimated time. When uncertain of position, the procedure is reversed — look for two or more large features on the ground and then identify features on the chart that are in the same juxtaposition. Prominent line features are best although, quite often, a spot feature is easily identified — for example the names of grazing or farming properties are shown on the charts and their owners, particularly those with an airstrip, often paint the name on a roof, in large letters. If you see a prominent line feature, then fly along it until you can derive a fix from an intersect or a verifiable landmark.

      7. If necessary "assess the wind!" Whilst over the orbiting landmark turn onto a quadrantal heading, e.g. north, and fly that heading for one minute then turn 90°, e.g. west, and fly that for one minute. Systematically scan the surrounds for an identifiable landmark, starting with the area closest to the aircraft then moving out to the middle distance. Repeat for two more anticlockwise turns and after 4 minutes have elapsed you should arrive back near the starting point. If you have held to the headings and the timing, then the ground distance and direction of the arrival point from the orbiting landmark should provide a reasonable estimate of the wind velocity; e.g. if the arrival point is about 1 nm north-west then the wind speed must be 15 knots from the south-east. Of course if you are a poor judge of ground distance (which applies to many/most of us) then the indicated wind speed is not calculable but at least you know the direction and have a gross indication of the speed.

Expanding square search
      8."Start an expanding square search!" Starting over the orbiting landmark turn onto a quadrantal heading, e.g. north, and fly that heading for 2 minutes then turn 90°, e.g. west, and fly that for 2 minutes. Log the times and headings. Systematically scan the surrounds starting with the area closest to the aircraft then moving out to the middle distance. Repeat for two more legs but fly these for 3 minutes each. The next 2 legs are flown for 4 minutes each and so the expanding pattern is repeated, extending each pair by one minute, until a position is pinpointed or you are well outside the circle of uncertainty and a precautionary landing might be a wise action. Do not fly around in increasing circles, always fly planned (and logged) headings and durations.

      9. "Don't stay up too late!" Be prepared to make a precautionary landing well before the fuel content reaches the 30-minute reserve figure and well before oncoming twilight reduces visibility at ground level. You need to ensure that a precautionary landing isn't downgraded to a forced landing because of fuel exhaustion. Try to select a suitable site near a house. Remember after you have landed you still have to secure the aircraft, protect it from stock (cattle licking the skin do a lot of damage) and perhaps get some help — very difficult in the bush and near impossible in the dark!

A 'precautionary' landing is an emergency landing under power at a prepared landing ground or some other suitable, but unprepared, site. If you have read your insurance policy carefully you may find that damages claims are limited if you make a precautionary landing at a 'non-prepared landing zone'.

There are many circumstances where a precautionary landing is a wise move. Among them are:
  • occupant illness or a frightened passenger
  • deteriorating weather
  • oncoming darkness
  • fuel reaching reserve level
  • lost and you decide to obtain help on the ground
  • engine running rough (although this might be considered a forced landing.)
The technique for precautionary landings at other than a prepared landing ground is essentially the same as that for short field landings except that additional low-level passes should be made to check the hazards, taking particular care in locating and avoiding wires. Map out the landing/run-out path and also determine the escape route in the event of an aborted landing.

And lastly:

      10."Communicate!" Share the problem. See the next module in the "Coping with Emergencies Guide' — Safety and emergency communication procedures.



[The next section in the airmanship and safety sequence is the following section 8.8 describing Dangers of flight into cloud or when lacking visual references]





8.8 Dangers of flight into cloud or when lacking visual references

Vestibular system illusions
When walking, a person's prime sense of orientation is provided by visual references. When vision is severely degraded, the vestibular system in the inner ears — which senses motion and gravity (thus roll, pitch and yaw) — generally allows us to keep our balance when walking without using visual references. However, the vestibular system is not designed for high speed or angular motion, and cannot be used as an in-flight back-up system; i.e. you cannot close your eyes and continue to fly straight and level. Motion of the fluid within the ears' semicircular canals is affected by inertia and will feed quite erroneous prompts to the brain, resulting in various types and levels of vertigo.

For example, without the external visual references of clear sky, terrain or a horizon, forward deceleration tends to give a pitching-down sensation whilst forward acceleration gives a pitching-up sensation. Once settled into a constant-rate turn, the sensation is of not turning at all; but when the turn is halted, the sensation is then of turning in the opposite direction. In addition, the vestibular system will not detect slow rates of bank, so that if the aircraft is banking at the rate of one or two degrees per second the vestibular system will not send any prompts to the brain — it will consider the aircraft is still flying straight and level, while any associated speed changes may provide contrary sensations. For example, if the aircraft is slowly banking and accelerating in a descending turn, the sensation may well be one of pitching-up. Such sensations disorient the pilot.

See the effect spiral instability may have on an aircraft and pilot in cloud.
Spatial disorientation
Aircraft accidents caused by spatial disorientation are usually fatal and occur when VFR flight is continued in adverse visibility conditions — cloud, fog, smoke, haze, showers, oncoming darkness and combinations thereof. Pilots who have not been trained to fly solely by visual reference to the indicators in an 'instrument flight' panel/display will soon find themselves experiencing spatial disorientation should they inadvertently or deliberately enter instrument meteorological conditions [IMC] where the external visual references — by which they normally orient themselves in visual meteorological conditions — are lost. The same applies to any atmospheric condition where the visual references — horizon (principally), terrain and clear sky — are lost or just significantly reduced; see white-out/flat light for example.

Thus, a non-instrument rated pilot would be unable to maintain controlled flight in cloud, or maybe even in conditions where the horizon disappears, and even an instrument-rated pilot cannot fly in cloud without the minimum IFR instrumentation. Nor can an instrument-rated pilot in an IFR aircraft fly where the aircraft can't out-climb rising terrain, whether it is concealed or not. In addition many horrific accidents have occurred when an IFR pilot has descended below the area 'lowest safe altitude' in IMC and impacted the terrain; such events are classified as controlled flight into terrain.

Note: even a pilot who is well experienced in flying in IMC may occasionally experience a phenomenon called 'the leans'. This might occur when the IFR instrumented aircraft has been inadvertently allowed to slowly bank a few degrees and the pilot then makes a quick correction to level the wings. The vestibular system doesn't register the slow initial bank but does register the wing levelling as an opposite direction bank (away from a wings level attitude) and the pilot's brain produces a leaning sensation while also perceiving from the instrument readings that the aircraft is flying straight and level. The reaction — which can persist for quite a while — may be for the pilot to lean sideways in her/his seat so that everything feels right!



[The next section in the airmanship and safety sequence is the following section 8.9 describing Pressing on in deteriorating conditions]





8.9 Pressing on in deteriorating conditions

Most fatal excursions into IMC by light aircraft seem to occur when the pilot freely elects to find a path through or over high terrain beneath an overlaying cloud cover, in order to maintain a perceived time schedule, but without ensuring that there is a clear way out or back.

For example a Jabiru pilot, who held a current RA-Aus CFI approval and had accumulated some 10 000 flight hours experience mostly in commercial general aviation, elected to cross the Great Dividing Range east to west in the northern NSW region after being frustrated from doing so the previous day by low cloud. The aircraft crashed on track, first impacting trees and then the rising terrain of a rainforest covered ridge, approximately 200 feet below the top of the ridge. The aircraft made its initial impact with wings level, travelling on a westerly heading consistent with its track. The aircraft initially contacted a tree some 80 feet agl shearing off the starboard wing then continuing another 100 metres where the port wing was removed by another tree. The fuselage continued for another 100 metres where the engine still running at high rpm was buried a half-metre in the forest floor.

"The accident was consistent with the operation of an aircraft in marginal visibility, close below the cloudbase where the pilot inadvertently enters IMC conditions possibly due to a slight lowering of the overcast. In this situation one option is to lower the nose attitude slightly, establishing a small rate of descent with the expectation of regaining VMC within two or three seconds. This scenario would account for the slightly nose-low attitude at the point of contact with the first tree and for the marginally greater than cruise airspeed indication at the time of impact."

For another example of 'pressing on' read this fatal accident report.

Apart from accepting that you will not be able to cope with adverse weather conditions encountered at low levels, and thus positively resisting that urge to press on or get home (which urge seems to become quite strong once you have passed the half-way point), the following rules can save your life.

      "Stay in the clear!"   Watch what is developing around you — including behind you. Don't fly towards worsening weather — if you have to change course, fly towards better conditions/terrain. In conditions where the METARs indicate little spread between temperature and dewpoint — or the air just feels cooler and damp, perhaps a bit drizzly — watch out for mist, fog, fractus or scud suddenly forming. This is particularly in valleys, across ridge-line saddles or on wooded slopes, and more so in the late afternoon which, when combined with a compunction to get there before dark, can lead to disaster. Also the gaps in a layer of broken cloud — in front, behind, above or below — may start to close in at any time and perhaps very rapidly. When any doubt exists, make a 180° turn or divert towards better conditions. The accident reports cite too many instances of light aircraft 'controlled flight into terrain' or 'continued VFR flight in adverse weather' because of increasing cloud cover, a lowering cloud base or reducing visibility; probably because the pilot thought 'I'll just go a little bit further and see what the conditions look like there'.

      "Be wary of lowering cloud and rising ground!" If you can't see a gap between the horizon and the overlaying cloud base, be absolutely sure you can proceed and be very careful that: (a) you are not gradually climbing and losing airspeed; and (b) that you don't get into a position where poor visibility precludes making a 180° turn without entering cloud.

      "Be wary of valleys!" If you can't see the tops of surrounding hills because of cloud, don't fly into a valley unless: (a) you can clearly see the exit and the horizon is quite clear and well defined at that end; and (b) the valley is clearly wide enough to do a 'U' turn at any time. In addition, possible turbulent downflows over the windward slopes warrant some precautions when turning.

      "Be wary of concealed CB and squall lines!" When flying below low and mid-level unbroken or broken cloud layers ,cumulonimbus and squall line development may be concealed from view and you may suddenly encounter extreme turbulence, wind shear and very heavy rain with consequent loss of VMC in the worst possible conditions.
Don't get caught on top!
If caught above what appears to be an extensive cloud layer it is generally wise to turn 180° and climb for a better line-of-sight distance while returning to clearer sky. But remember the wind velocity changes with height, so what may be a favourable wind at low level may be unfavourable at height.

      "Be wary of sucker holes!" If caught out above a cloud layer be extremely wary of descending through a hole in the layer. Such holes tend to suck you in but — if the hole starts to fill or proves not to be wide enough to conduct a safe slow-speed descending turn — disorientation may spit you out the bottom. Also you have to be sure that, having descended through the hole, the height of the cloud base, terrain and visibility will allow safe onward passage in VMC under the cloud cover, or at least the option of a safe precautionary landing. Once descending in a sucker hole it may well prove impossible to climb out of it — without entering cloud — if you change your mind.

There are several articles, contained in the online version of CASA's magazine Flight Safety Australia, which are recommended reading. See the section titled Micrometeorological event effects and VFR incursions into IMC in our index to those magazine articles, be sure to read "178 seconds to live".

Also read 'Wind shear and turbulence' in the 'Decreasing your exposure to risk' modules.


[ The next section in the airmanship and safety sequence is the complete module describing the
Safety and emergency communication procedures ]



The next module in the 'Coping with emergencies guide' deals with Comfort and survival in a remote environment.
Further reading
The online version of CASA's magazine Flight Safety Australia contains some articles relating to en route navigation that are recommended reading. I have made a categorised index of the articles of interest to recreational pilots contained in Flight Safety Australia since 1998 and I suggest you read the articles noted under "En route and navigation", "Micrometeorological event effects and VFR incursions into IMC" and "Fuel management".

If you don't find an article in the magazine index it will be included within the ATSB supplement for that issue. All articles are in pdf format requiring the Adobe Acrobat reader.

Groundschool – Flight planning & Navigation Guide

| Guide contents | 1. Australian airspace regulations | 2. Charts & compass | 3. Route planning |

| 4. Effect of wind | 5. Flight plan completion | 6. Safety audit | 7. Airmanship & flight discipline |

| [8. En route adjustments] | 9. Supplementary navigation techniques |

| 10. Global Positioning System | 11. Using the ADF | 12. Electronic planning & navigation |

| 13. ADS-B surveillance technology |


Supplementary documents

| Operations at non-controlled airfields | Safety during take-off & landing |

Next - other navigation techniques The next section of the Flight Planning & Navigation Guide discusses supplementary navigation techniques




Copyright © 2001–2009 John Brandon     [contact information]