Research on turbulence and wind shear

[John Brandon]

G'day all,

 

I am currently writing the safety articles for the July and August issues of the RA-Aus journal and need a little information regarding the statements made in various light aircraft POH/Flight Manuals about operations in turbulence.

 

For example the POH for the Piper PA32, a six seat, 300 hp retrac in FAR23 'normal' category states:

 

"In keeping with good operating practice used in all aircraft, it is recommended that when turbulent air is encountered or expected, the airspeed should be reduced to maneuvering speed [i.e. Va] to reduce the structural loads caused by gusts and to allow for inadvertent speed build-ups which may occur as a result of the turbulence or of distractions caused by the conditions."

 

Would you mind checking your POH and advising if essentially the same is stated or if more options are added or if it is substantially different?

 

cheers

 

John Brandon

 

 

[ahlocks]

G'day John,

 

 

 

 

 

 

 

 

 

There's nothing in the Sportstar POH about turbulence.:ah_oh:

 

 

 

 

 

 

 

 

 

The Sportstar LSA training supplement supplied with the aircraft does include this though.:thumb_up:

 

 

 

---------

 

 

 

4.17.5 Flight in Turbulence

 

 

 

 

 

Flight in turbulence is a compromise between staying below VA to prevent a sudden gust from causing an overstress condition and having enough speed to be able to keep adequate control of the aircraft when a decreasing performance gust occurs.

 

A speed of about 65 KIAS will accomplish this – it would take a huge gust to put you above VA or below VS, and a gust that large would indicate you shouldn’t be airborne anyway.

 

The correct technique is to set the throttle for this speed, and try to maintain level flight. Don’t chase the airspeed and if you lose or gain altitude, simply climb or descend slowly to regain your original altitude.

 

 

 

 

 

 

 

 

[Guest J430]

The problem is how do you define TURBULENCE, light-med light-moderate- mod heavy- heavy-severe ????

 

I have asked an old and wise pilot of some 30,000 hours and he defined it as when you have trouble reaading the instruments its time to back off.:confused:

 

So the typical thermal and mechanical turbulence you get most days I can only assume to be acceptable and keep plugging away.

 

When it is rather severe.... back off, a recent crash in Victoria is most likely due to this problem.

 

I would be interested to hear what other experienced folk think. It is certainly not described in any manual I have read.

 

Cheers

 

J:wave:

 

 

[Smokey]

John

 

I've sent you a PM with some POH info on Superpup & Foxbat

 

Mark

 

 

[facthunter]

Define.

 

Light- Little bumps they are there, but no great discomfort.

 

Medium- you have to put a bit of effort to pilot the aircraft but it's not stressful. You are aware of the seatbelt on your body restraining you. Pity, would have been a good day for flying otherwise.

 

Severe- you have to put in a concentrated effort just to keep the aeroplane under control and you become apprehensive about your welfare. Look at the wings bending! You hope it stops and you wish you were somewhere else. ( This is not the textbook definition.) Nev..

 

 

[John Brandon]

The problem is how do you define TURBULENCE, light-med light-moderate- mod heavy- heavy-severe ????I have asked an old and wise pilot of some 30,000 hours and he defined it as when you have trouble reaading the instruments its time to back off.:confused:

 

So the typical thermal and mechanical turbulence you get most days I can only assume to be acceptable and keep plugging away.

 

When it is rather severe.... back off, a recent crash in Victoria is most likely due to this problem.

 

I would be interested to hear what other experienced folk think. It is certainly not described in any manual I have read.

 

Cheers

 

J:wave:

G'day J430,

You have not responded to my query and are departing from the thread, however, in response here are the first few paragraphs of the article Im working on:

 

6.1 Defining turbulence and wind shear

 

It is usual to classify all the changes in atmospheric motion that significantly disturb aircraft flight as turbulence but in some wind shear events there may be no air turbulence involved. It is difficult to define the degree of turbulence or the load effects of shear in a way that is meaningful to a recreational aviation pilot. Measuring by the airflow velocity change or the gust velocity measured in feet per second doesn't really enable the pilot to judge how turbulent the conditions are in her/his circumstances, particularly so if the instrument panel is not equipped with an accelerometer. However the following is partly based on an old ICAO turbulence scale which, though classifying by the induced positive or negative accelerations [only as measured near the aircraft cg] does provide a descriptive definition of sorts appropriate for 3-axis aeroplanes but perhaps not flexible wing weight-shift aircraft and certainly not meaningful for our powered parachutists.

 

Very low – below 0.05g; light pitch, yaw and roll oscillations experienced.

 

Low – 0.05 to 0.2g; might include light 'chop' i.e. slight, rapid, rhythmic bumps and oscillations without significant changes in altitude or attitude. Like driving a boat through a choppy sea. Also known as 'cobblestoning' — like driving at moderate speed on a corrugated gravel road.

 

Moderate – 0.2 to 0.5g; turbulence is becoming significant and the ride produces strong intermittent uncomfortable jolts with attitude upsets and airspeed variations but aircraft remains in control. Head hitting cockpit roof structure if clearance small or harness allows.

 

Severe – 0.5 to 1.5g; aircraft handling in all axes made difficult but not dangerous [if occupants and objects properly secured] except near the surface. Large abrupt changes in altitude/attitude with significant variations in indicated airspeed. Cockpit instruments difficult to read mainly because of problems with re-focus.

 

Very severe – above 1.5g; aircraft violently tossed about combined with extreme handling difficulty, structural damage possible.

 

The wake vortices from larger aeroplanes or the rotor downwash from helicopters add another form of turbulence extremely hazardous to all recreational aircraft particularly because of strong rotational effects leading to sudden height loss. Such vortices must be anticipated and avoided.

 

Wind shear – in aviation terms wind shear is a sudden but sustained “variation in wind along the flight path of a pattern, intensity and duration, that displaces the aircraft abruptly from its intended path and sufficiently that substantial and timely control action is needed” and is probably the greatest hazard to flight at low levels in visual meteorological conditions but its effect is short-lived. The flight path displacement is initiated by a substantial change in lift generation associated with the aircraft's inertia [see note 1 following]. The shearing action between air layers with substantially differing velocities — or vertical gusts and their surrounds — may also induce strong turbulent eddies or breaking waves at the shearing level or interface."

 

cheers

 

John

 

 

[djpacro]

The 8KCAB Airplane Flight Manual states:

 

"GREEN ARC extends from power-off stall speed (Vs1) to maximum structural cruising speed (Vno).

 

YELLOW ARC extends from maximum structural cruising speed to never-exceed apeed (Vne). Operate in this range with caution and only in smooth air."

 

The POH (I.e. not approved by the FAA) goes further:

 

“In severe turbulence do not exceed 121 mph. Maintain a constant nose attitude rather than flying by reference to the altimeter and airspeed indicator.â€

 

Maneuvering speed (Va) at gross weight in normal category is 121 mph.

 

The above approach and similar text is what I have seen in other FAR 23 certified airplanes.

 

Just had a quick look at the Airtourer T-6 AFM – similar notes wrt max structural cruise speed but I didn't see anything about severe turbulence and Va.

 

The old British airworthiness regulations had a specific turbulence penetration speed, Vb. They used a different set of design gust loads. Seems that “everyone†uses stuff similar to FAR 23 now.

 

May be worth noting that load factor arising from a gust is a function of the gust velocity, speed of the aircraft and inversely proportional to the aircraft wing loading.

 

 

[John Brandon]

The above approach and similar text is what I have seen in other FAR 23 certified airplanes.

Just had a quick look at the Airtourer T-6 AFM – similar notes wrt max structural cruise speed but I didn't see anything about severe turbulence and Va.

 

The old British airworthiness regulations had a specific turbulence penetration speed, Vb. They used a different set of design gust loads. Seems that “everyone” uses stuff similar to FAR 23 now.

Thanks for that. It is interesting that the T6/24 Airtourer AFM doesn't mention handling in turbulence but at least it does provide Va for normal and aerobatic category.

 

Vb, the design speed for maximum gust intensity, still exists in FAR23.335 though it's unlikely to be specified by the designer of an aircraft below 6000 lb. Probably more applicable to the higher end commuter category. Seems to me that most FAR23 and FAR23 Appendix A light singles stick with just Va as a speed to fly in "turbulence' but I'd like to see something from the European designed JAR23 and CS-VLA aircraft.

 

cheers

 

John

 

John

 

 

[John Brandon]

JohnI've sent you a PM with some POH info on Superpup & Foxbat

 

Mark

Thanks Mark but they haven't turned up yet - 10.00 pm

John

 

 

[djpacro]

I'd been working with earlier amdts of Far 23. Yes, Vb is there for those with a recent certification basis - I haven't done any sums but I'd guess that the gust envelope would be within the maneuver envelope for many small aeroplanes. That'd put Vb=Vc per that reg.

 

 

[John Brandon]

I'd been working with earlier amdts of Far 23. Yes, Vb is there for those with a recent certification basis - I haven't done any sums but I'd guess that the gust envelope would be within the maneuver envelope for many small aeroplanes. That'd put Vb=Vc per that reg.

Sorry for the delayed reply, it was our fortnightly shopping day yesterday — big trip into Albury.

Vb and Vc are not the same but are derived from the 'corners' of the aircraft gust envelope. B is Vb, C is Vc and D is Vd. Vb is related to the 66 fps gust velocity required to be considered by designers of commuter category aircraft.

 

Va doesn't appear in the gust envelope only the manoeuvring envelope [or the combined envelope] at corner A. in the combined envelope Vb would be between Va and Vc and, I guess, closer to Va than Vc.

 

I've added a V-n diagram which might be helpful but the quality is not the best. The Vb gust line is the 66 fps line for commuter category, the Vc and Vd gust lines are the 50 fps and 25 fps lines respectively for all categories.

 

cheers

 

John

 

 

 

[djpacro]

The designer gets to choose Vb to some extent. It doesn't have to be more than Vc. It must not be less than the intersection of the Vb gust line and the stall line.

 

That sample chart shows Vb to be about halfway between Va and Vc obviously chosen where it is as the gust load factor at Vb gets to be the same as it is for Vc. Choosing a higher Vb puts a higher load on the structure. Choosing a lower Vb is maybe not rational but something a designer may want to do just to simplify life.

 

That sample flight envelope coincidentally shows the intersection at Va so the designer may choose to take Vb = Va. (That intersection would not normally be at Va).

 

Depending on the aircraft characteristics (and especially if the limit manoeuvre load factor is 4.4 for utility rather than 3.8 for normal category) the gust load factors may turn out to fall entirely within the manoeuvre load factors. In such a situation the designer may choose to take Vb = Vc as there is no structural penalty.

 

PS I started to do some quick sums using approx data for a Cessna 150 but time is running short.