Probable Cause, Dan Gryder

[APenNameAndThatA]

On 10/02/2021 at 7:51 AM, old man emu said:

It is hard to reply correctly to that statement because we need to specify when the density changes. Normally we fly on Area QNH. The acronym QNH is one of the Q(uestion) code names developed, circa 1909, for use in morse code. To concisely ask for atmospheric pressure at mean sea level (MSL), the operator would transmit the letters QNH. This was understood to mean "I have a question. What is the atmospheric pressure at Nil Height", i.e. at mean sea level.

 

These are the boundaries of Area QNH for Australia. http://www.pilotpracticeexams.com/courses/wp-content/uploads/2016/09/Image-2019-02-26-at-10.22.52-AM.png

 

We know that pressure systems move across Australia from west to east, so while an Area QNH is current for three hours, commencing 0100 UTC and each three hours thereafter, an altimeter set to Area QNH will be representative to within ±5 hPa of any actual QNH of any location within the defined area. If a pressure system, usually a Low, is crossing the continent, the actual air pressure is constantly changing, but we don't usually keep adjusting the subscale as we fly along within a QNH zone.

 

Interestingly, if we are flying towards a low pressure area, and our altimeter is set to Area QNH, the altimeter will gradually overread our actual height because the air pressure is dropping and the altimeter interprets that as the aircraft climbing. So if we keep flying at a constant indicate altitude based on a QNH derived altimeter reading, we could fly into the ground.

 

So, getting back to the original quote, if we fly within a QNH Zone using the Area QNH, we don't alter the sub-scale until Air Services changes it at the end of the three hour validity period. However, if we cross a QNH Zone boundary, we need to check with Air Services for the current Area QNH in that zone, and possibly adjust the sub scale.

 

If an altimeter has reminder markings to mark Vso - Stall speed or minimum flight speed in landing configuration they have a safety factor of 1.3. In practice that should take care of local air density changes on altimeter indications at airfields in Australia, which for the whole continent average out at about 1000' AMSL.

Safety factors have nothing to do with QNH. What often happens is that you cut and paste stuff from the internet, and then add add an erroneous conclusion. That would not be a problem, except people up vote your comments as if they believe you. 

Edited February 12, 2021 by APenNameAndThatA

[ClintonB]

I thought Dan’s whole point was to employ the KISS principle. The yellow tape goes on your minimum manoeuvre speed which happens to be 1.3Vso for a visual reminder of DONT MANOURVE BELOW THIS SPEED, to prevent us over thinking this thing and bending our airplane/selves.

Flightchops channel has an episode on YouTube with Dan explaining this.

I did manoeuvre once below a safe speed, and by some miracle did not kill myself and daughter, if I had marked a speed and knew don’t let that needle go past the line, it very likely would not have happened. I did seek remedial training before next flight where instructor and myself came up with a number that you don’t turn or fly below. It has worked ever since. I will be taping my ASI when I go to airfield next.

 

[Thruster88]

On 09/02/2021 at 7:54 PM, skippydiesel said:

OOOOOooohhhh! Me thinks those fast aircraft, where Maximum Structural Cruise (Vno) is close to Never Exceed (Vne) just might benefit from being fairly accurate at the fast end of the scale. Just a slight (unnoticed) decent from high speed cruise, might put you &your aircraft over Vne , within moments, with potentially alarming results.

Structural failure due to exceeding Vne or Vno or Va is very rare compared to stalling and spinning accidents in high performance and low performance piston aircraft. Pilots of such high performance aircraft would not normally allow the airspeed to go "unnoticed". It seems a blatant disregard or just plain hooning is usually the probable cause 

  V speeds are always IAS indicated airspeed unless the POH states otherwise.

[aro]

On 11/02/2021 at 9:26 AM, old man emu said:

I suppose I should have said "could" instead of "should" to allow for personal preference in the way the information provided by the ASI was displayed.

Let's take Vs1 for a C172 as an example, which is 48 KIAS. If you take off at Moorabbin on a 35C day and climb to 7000 feet where it is 20C, Vs1 in TAS* is initially 50 and increases to 55 as you climb. What value "could" you mark on the TAS scale?

 

*In fact it is not really TAS because at low speeds there is significant error in the indicated values. The actual TAS values are 55 and 61. We would need a new concept, "Indicated True Airspeed".

[old man emu]

The Air Speed Indicator is not a precision instrument. It is an "indicator"

 

CASA Aviation Order 108.56 says, 

3          Test procedure and accuracy

   3.1     Airspeed indicators

            Airspeed indicators must be tested against an appropriate test instrument. The scale errors at the major graduations of the scale must not exceed ± 4 knots up to the maximum speed of the aircraft, when tested first with the pressure increasing and then with the pressure decreasing. Operation must be smooth and continuous.

 

Civil Aviation Order 100.5 (General requirements in respect of maintenance of Australian aircraft) 2011, Appendix 1 Additional maintenance requirements — pitot-static systems, pressure altimeters, airspeed indicators and fuel quantity gauges at Paragraph 5 says the same thing.

 

So, if an ASI can be in error of the correct value by plus or minus 4 knots, and the manufacturer's stated stall speed is X knots, based on design factors, how can you place your trust and your neck in what a needle is pointing to?

 

There is also another fly in the ointment - pneumatic lag and attenuation.

 

During unsteady flight, pneumatic lag and attenuation may affect pressure measurements. Pressure variations propagate as waves through the pneumatic tubing to the ASI The wave propagation is damped by frictional attenuation along the walls of the tubing and fluid viscosity. This damping produces a magnitude attenuation and a phase lag. After the wave reaches the ASI, it is reflected back up the tube. Depending upon frequency distribution of the incoming wave energy and tubing length, the reflected wave may cancel or reinforce incoming pressure wave. If the waves cancel each other out, further spectral attenuation occurs. If the waves reinforce each other, the power of the incoming wave is amplified and resonance occurs.

 

[facthunter]

The Lag, sometimes  called hysteresis, in an ASI is pretty minimal if able to be picked up at all.. At the rate of increase or decrease in our planes speed (acceleration) you can for all practical purposes IGNORE it.  I've NEVER ever heard mention of it on  an any aircraft endorsement training course  I've ever done. Plenty of ASI's have an instrument error and some have a pitot related POSITION error. When in discussion one says "Indicated" airspeed takes account of this, you are talking principles. Not an individual instruments "possible" characteristics in some hypothetical situation... .  Nev. .

[skippydiesel]

17 hours ago, Thruster88 said:

Structural failure due to exceeding Vne or Vno or Va is very rare compared to stalling and spinning accidents in high performance and low performance piston aircraft. Pilots of such high performance aircraft would not normally allow the airspeed to go "unnoticed". It seems a blatant disregard or just plain hooning is usually the probable cause 

  V speeds are always IAS indicated airspeed unless the POH states otherwise.

Thruster,

 

I accept your point that low speed / close to the terrain stalls, are by far the most significant contributors to speed related accidents. This does not mean that your ASI being accurate at the high speed end of the envelope  is not equally desirable particularly in faster aircraft.

 

 I challenge you to check out some of the RAA complaint  (can meet the standards) aircraft offerings, that are capable of quit extraordinary (claimed) High Speed Cruise (Vno) and see how many are, at this speed, only a few knots below Never Exceed (Vne).

 

These slippy little demons, accelerate very fast and being small light aircraft, can have departures from straight and level, just because the pilot/passenger leans forward.

 

Sure many of these aircraft are fitted with auto pilot that will reduce the chance of an inadvertence decent (increase speed above Vne) but you still need an accurate VSI to set the aircraft up within its design limits.

 

A wingless decent from 10,000 ft is going to give you a lot of time to contemplate your assured demise.

[old man emu]

3 hours ago, facthunter said:

The Lag, sometimes  called hysteresis, in an ASI is pretty minimal if able to be picked up at all.. At the rate of increase or decrease in our planes speed (acceleration) you can for all practical purposes IGNORE it.  When in discussion one says "Indicated" airspeed takes account of this, you are talking principles. Not an individual instruments "possible" characteristics in some hypothetical situation... .  Nev. .

I picked up that bit about Lag while reading about ASIs in military aircraft in this https://apps.dtic.mil/dtic/tr/fulltext/u2/a104830.pdf

You've probably never seen Lag because you have probably never watched an instrument tech do your four-yearly ASI check. Who does?

 

You are correct when you say, When in discussion, you are talking principles. Unfortunately people seem to have the need to drag in an individual instruments "possible" characteristics which bogs things down.  

 

Hypotheticals are OK for discussing principles, but we do, in fact, live in a real world where reality rules. One just has to make it clear early on in the discussion whether the discussion is about principles or reality. Once that is agreed upon, the discussion can proceed accordingly.

[facthunter]

Vibration aids the reduction of hysteresis in instruments. Most IVSI's have a little vibrator in them for this reason. If you place the MIC on them you can check it. V1 is a critical go no go Point which is for convenience is called  as  a speed which corresponds to the point on the runway where you can stop or reach 35 feet in the remaining distance, on a take off If an engine fails in a multi engined plane. There is no more critical call made in Aviation than this one based on what the ASI reads. What the instrument repair shop uses is "Their" business when they repair something..  Pilot's have the job of flying the planes within the rules,they operate to with airworthy planes and in accordance with the Planes POH. ASI's are compared with each other where two systems exist. A comparator light will illuminate where there's disagreement.. Your ALTIMETER is checked at each turn around using the aerodromes height above sea level and a QNH reference. If it's outside a given figure It's U/S. Nev

[Thruster88]

2 hours ago, old man emu said:

I picked up that bit about Lag while reading about ASIs in military aircraft in this https://apps.dtic.mil/dtic/tr/fulltext/u2/a104830.pdf

You've probably never seen Lag because you have probably never watched an instrument tech do your four-yearly ASI check. Who does?

I have seen airspeed indicators being checked at work and can say that there is no lag, even when the pitot tube is out at the wing tip.

 

This is a thread about a SIMPLE way to save lives by marking the airspeed indicator with (1) approach speed 1.3 x stall speed which is also the best angle of climb speed for most. (2) minimum manoeuvring speed 1.4 x stall speed which is also the best glide speed for most aircraft. See the vid in post 1.

[old man emu]

1 hour ago, Thruster88 said:

This is a thread about a SIMPLE way to save lives by marking the airspeed indicator

C'mon Thruster. You have been around here long enough to know that the SIMPLE pretty soon drifts to the COMPLICATED, then to the HYPOTHETICAL and finally ad absurdum.

 

However, your point about marking the ASI to provide a visual reference to the boundaries of safe flight is quite valid and should be encouraged.

 

 

[aro]

26 minutes ago, old man emu said:

You have been around here long enough to know that the SIMPLE pretty soon drifts to the COMPLICATED, then to the HYPOTHETICAL and finally ad absurdum.

Marking the airspeed indicator in IAS seems pretty simple. There is one person who brought TAS into the thread via attempts to use the lift equation.

 

I have a couple of comments on the original video:

• Adding markings to the indicator that could be mistaken at a glance for the needle seems unwise. If you're going to do it, perhaps use short markings at the edge, similar to eg. VNE.
• I am not convinced that the relationships between the different airspeeds (Vref, Vy etc) that he talks about always holds. I would have to do more research on that one.
• In particular, Vx, Vy will have different flap settings than Vref so any resemblance is likely to be coincidental. 

[old man emu]

When you look at the numbers, using a factor of 1.3 makes a pretty big change. A factor of 1.3 is, in reality 30%. 

 

I only have POH's for a C-172 and Piper Cub to get some speeds from, but according to them the flapped stall speed of the C-172 is 40 kts, so a 30% fudge factor takes that to 52 kts and the minimum manoeuvering speed fudge factor of 1.4 takes it to 56 kts.  (For the Cub, stall is 37.5  1.3 = 49 and 1.4 = 52)

 

Those are pretty big fudge factors. Not that there's anything wrong with extra safety padding, but wouldn't they restrict operations at low speed - say during search operations?

[aro]

9 hours ago, old man emu said:

I only have POH's for a C-172 and Piper Cub to get some speeds from, but according to them the flapped stall speed of the C-172 is 40 kts, so a 30% fudge factor takes that to 52 kts and the minimum manoeuvering speed fudge factor of 1.4 takes it to 56 kts

This is a good illustration of the problems calculating your own factors. The speeds need to be calculated based on Calibrated Airspeed (CAS) which is IAS corrected for errors due to the location of the pitot/static ports on the airframe. These errors are significant at low speed in a C172.

 

Stall speed with flaps in a C172 is 48 KCAS. Without flap is 53 KCAS. So 1.3 x Vs0 is 63 KCAS. If you want to calculate a minimum maneuvering speed of 1.4 Vs1 that is 74 KCAS. Before use these need to be converted to IAS, but the errors are smaller at these speeds.

 

74 KCAS is about 75 KIAS. 63 KCAS is 60 KIAS. 60KIAS is the lower bound of the book approach speed (60-70 KIAS) so it works out.

 

9 hours ago, old man emu said:

wouldn't they restrict operations at low speed - say during search operations?

 

These are pretty slow speeds to be operating a C172, particularly at MTOW. 70-75 KIAS is probably reasonable for slow operations with 10-20 degrees of flap. If you need slower, you probably want a helicopter.

[Bruce Tuncks]

My ASI is already marked. But I need an audio warning in case I forget to look at it...  Oops, I have just re-invented the stall warning.

When I installed that stall-warning, I didn't like how far it was around on the underside of the wing, so I moved it up a few mm to go off earlier.

What's the use of a warning if it happens too late, thought I.

The result is that I can make the warning go off at altitude easily enough, but I still never hear it go off on landing. I think the Jabiru just doesn't reach anything like stalling A of A on landing... if you draw a line from the sub-fin to the main wheels, you will see what I mean.

[old man emu]

The thing is, what is the IAS when the stall warning device begins to squawk? it's a warning device, so it must be designed to activate before the airspeed gets to the stall speed.

34 minutes ago, Bruce Tuncks said:

I think the Jabiru just doesn't reach anything like stalling A of A on landing

Maybe that's why they are noted for their float between flare and touch down. Perhaps between flare and tough down they simply have ballistic motion.

 

The red dot is the fall from flare height to ground simply due to Gravity. The blue dot shows the combination of horizontal velocity and fall due to gravity.  Notice that at each time interval, the dots are on the same line because the acceleration due to gravity is the same. The displacement of the blue dot is due to its horizontal velocity at launch, which for the red dot is 0 m/s.

[Bruce Tuncks]

thanks OME. My son-in-law is a shooter and although he knows his stuff, he will like to see it displayed like that.

I personally don't think a flaring and floating Jabiru though is ballistic. If so, you would feel the lack of g force. I reckon the float comes from the conservative approach speed of 60 knots compared with the touch-down speed of 40 knots or a bit less. But I have to admit that a slightly high flare can end with a decisive drop.   

On the topic of speed control on landing... A glider exercise was to cover the ASI and see how well the pilot flew and in particular maintained a safe speed near the ground. Most pilots flew the circuit  about 5 knots faster with no ASI.

After flying gliders, I never thought that the Jabiru had a long float time. In a glider, you would float right along and past the airstrip without those nice airbrakes.

 

[facthunter]

Engine idle speed affects a jabiru. It will affect ALL planes but seems more noticeable on Jabs I think it's just that they are pretty CLEAN in ground effect.

 Folks, all the talk of extra marking is BS. Don't complicate what doesn't need to be. Last time I heard any reference to it ,adding permanent marking to the ASI is not legal. Moveable BUGS on the instrument rim are more common, but are usually used when flap retraction schedules are complex  etc .. We are NOT going to fly constantly calculating margins and getting it right without being distracted from more important things. Your plane is primarily flown to a reference attitude for it's particular situation.  You KNOW you need to add power (and speed) in a turn and you practice it till it becomes second nature.  entering a turn you add some and as you come out of a turn and get wings level you take some off. Naturally you only do this if the turn is of some duration. Small heading adjustments tracking on final  don't need it. . Lowering the nose can equal a power increase if you are too high already. This is ALL basic stuff.. Nev

[old man emu]

Have you ever tried to maintain a constant speed in a car while driving in free-flowing traffic? There is too much going on "outside the cockpit" to be constantly monitoring an indicator that you know is affected by a multitude of variables, the most common of which is the diameter of the outer circumference of the tyres. Personally, after having compared engine RPM to a GPS derived speed reading, I rely on my tacho to monitor speed. This is because there are fewer divisions on the tacho scale, so the information I need is immediately identifiable. Also, I can look up the RPM value for best torque output and let the engine work away at that, because for 99% of the time, I don't need a herd of horsepower to get the results I want. 

 

Now switch over to flying circuits in a plane.  Early in a career, a pilot is as busy as a one-armed paper hanger, especially on base and finals. As experience grows, the pilot gets as busy as a one-armed paper hanger's supervisor. In any case, no pilot has the time to stare at an ASI to decipher a reading when close to the ground.

 

49 minutes ago, facthunter said:

Last time I heard any reference to it ,adding permanent marking to the ASI is not legal.

Are you able to identify the reference for that?

 

It would be need for someone to know as " ignorantia legis neminem excusat".  A criticism, of course, is that in the present age, with the proliferation of legislative instruments that touch upon almost every aspect of modern life, it is extraordinarily difficult for an ordinary citizen (who has not had the benefit of legal training) to remain apprised of his or her rights and responsibilities under the law.

 

" ignorantia legis neminem excusat" = ignorance of the law excuses no one

[facthunter]

Not from memory, but it would have been from some worthy source in the Australian context. Probably relates to certified aircraft but I can see reasons for it  for standardisation  . Mind you the vane on a spring on the Gypsy moth interplane strut is a big departure from the average ASI presentation. Nev

[pmccarthy]

THis is a good example of excess float due to excessive speed due to a dodgy ASI. Start at 18.00 if you don't want to enjoy the whole thing.

 

[FlyBoy1960]

On 09/02/2021 at 9:06 PM, kgwilson said:

The arcs and numbers on the ASI in the picture are all fixed so that must be for a specific aircraft. My ASI had nothing so I got circles cut out of stick on vinyl the same circumference as the glass, cut bits off and stuck them on the glass. I've got plenty left over if I need to adjust it or do it again.

 

 

it is illegal to stick the vinyl onto the glass !   there have been multiple accidents reported where the glass has simply rotated within the instrument. These are not pressurised instruments or anything like that and only rely on a small rubber fitting to stop the glass from rotating. I remember reading an article not so long ago on a Savage CUB aircraft in the USA where the glass had rotated, not much maybe 1/10 of a turn but that was enough to cause a massive screwup and a stall spin that claimed two lives.  This accident had video and commentary and you can see the glass rotating throughout the flight. It would go one way and then the other. Please don't do it because it is a stupid idea. They are easy enough to pull apart the basic ultralight ones and put the stickers on the inside of the instrument on the actual face and there are also companies that do this for you for about $80

[Thruster88]

The instrument case of an ASI is sealed to get correct static pressure. If the glass is able to rotate then the instrument is Unservicable and or a cheap piece of crap.

[kgwilson]

My ASI is not very accurate at low airspeed. I can fly it pretty much stalled at height, still have control & the ASI can be indicating 20-30 knots. It is the turn from base to final that kills too many pilots when they stall too low in the turn. I just never make that turn at less than 60 knots & often (if I have a bit of height to spare will use a bit of speed to lose the height. With full flap I can bleed that off pretty quick & once on late final never even look at the ASI. Once in ground effect even a tiny bit of throttle and I'll float the entire runway. My arcs are on the glass & I have no marks for anything. How this could turn is beyond me & I am not about to try pulling the instrument apart.

 

Just to see what I did as an experiment some years ago I had covered it so only my passenger (another pilot) could see it with instructions to scream at me if I got slower than my minimums. I averaged 10-15 knots faster every time. I have an aversion to flying slow near the ground except in ground effect. 

 

 

[dan gryder]

Thanks, I saw the OP and thought I'd add some clarification.

First, there is nothing "Illegal" about adding a reference to the cover of your ASI.

 

On larger aircraft there are slide able "bugs" that get set for each takeoff and landing dependent upon weight and conditions.  The bugs offer a minimum yielded net effective stall "buffer" of 30% even in a 30 degree bank holding altitude.

 

The concept for the GA pilot is to mark the airspeed indicator in colors as is already done for us by FAA.  The colors they chose long before me

were Green, Red, Yellows, White.   The FAA mandated that wings level stall speeds be color coded on each install to show where the plane will stall at clean and dirty (bottom of the green and bottom of the white...)  The problem is that those "reminders" don't give us the 30% buffer that they want us to have at all times.

 

The concept is called DMMS and is intended to slow down the number one killer of GA pilots;  LOSS OF SPEED AWARENESS. 

Of the 188 fatal GA accidents in the year 2020, 42 were day, vfr, loss of speed awareness in the traffic pattern!

DMMS is Defined Minimum Maneuver Speed and you can find it for your plane by multiplying your own Vs1  (Bottom of the green, clean wings level stall speed) by 1.404     If you don't have one then take your plane up and see where you get first indication at and add 40.0% to that.

 

More training on stall /spin recovery is a great idea but all these stalls occur below 1000 and even the best of the best cant recover from that low.

Prevention is all we got.

 

So you can placard two speeds:

 

Vref is 1.3 Vso which is also Vx                                                              Mark it!

 

DMMS is 1.404 Vs1 which is also best glide and it is also Vy.              Mark it!

 

Do the math for your plane and see how the numbers fall out for your operation.  DMMS is called the dumb ass speed because I am a dumb ass

and I cant remember where my keys are, let alone the min speed to fly in all the planes I fly all the time...but I want that 30% buffer

for a rainy day.   Don't do math in public, just mark it and let me know if it helps remind you of min speed to fly.   The 42 losses in 2020 were all within traffic pattern and below 1000.   I am a fan of AOA but due to cost and STC paperwork problems, the airlines have gone with purely ASI references and it works for them.

We don't ever hear of an airliner stalling in the pattern.

 

Military uses AOA on some planes but for us low down hash slingers, a piece of neon sticky note and a calculator will bring a world of

buffer to keep you flying another day.

 

Write me at Dan@DanGryder if I can help.  I'm not selling anything.  I don't have any product or sales or revenue.  Just trying to help

solve this senseless fatal problem in GA.   Thank You!   Dan 

 

   

Edited February 17, 2021 by dan gryder
typos