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Vulnerability of light GA to loss of airspeed indication

With most GA aircraft, losing airspeed does precisely nothing. The pitot system isn’t connected to anything except the ASI instrument. Losing static will affect the altimeter and will bugger up altitude hold in an autopilot, but not pitch hold, or the roll attitude or the ability to fly a heading or a track.

A few bigger ones use airspeed for autopilot functions e.g. the TBM700’s KFC325 uses it to modify the control loop parameters. I am sure later autopilots in fast aircraft do the same.

This danger came up here most recently, with an SR22 chute pull following a reported loss of ASI. What may be relevant is that the Cirrus has envelope protection and will pitch down if airspeed bleeds off too far. Does anyone know what the constraints are on this? One would expect some sort of inhibit e.g. if the engine is developing power, otherwise a pitot heater failure would result in a rapid transition way past Vne, and disintegration. It could generate a lot of sudden pilot workload if the pilot is not familiar with the system.

Some avionics, notably the older versions of the Aspen EFD, also have a big loss of functionality if airspeed info is lost. This is because while mechanical gyros have for decades been certified standalone, AHRS versions cannot be certified as “primary” for attitude unless there is a separate verification, which can be either airdata or GPS. The Aspen EFD solution is discussed here (and that thread contains an update on a fix they did several years later). The Garmin solution is discussed here and isn’t vulnerable to a loss of airspeed (but is vulnerable to a loss of GPS).

Another discussion, relevant to bigger aircraft, is here. This highlights the need for the pilot to be familiar with these details of the aircraft systems.

Then, there is the separate discussion of how would you land the plane with the ASI not showing anything, or showing something misleading?

Shoreham EGKA, United Kingdom

Peter wrote:

Then, there is the separate discussion of how would you land the plane with the ASI not showing anything, or showing something misleading?

We do this at each annual proficiency test at our club – we’re told to remember the RPM for approach (fixed pitch prop) and continue to “fly the numbers” (just RPM instead of IAS). Flare is looking outside / listening for the stall horn anyway. A constant speed prop pushed all the way forward is effectively a fixed-pitch prop, right?

EPKP - Kraków, Poland

I fly DA42 on pitch and engine load – approach at 40% load with flaps in APP position and pitch -5 give 105 KIAS, load 30% with flaps in LDG give 78 KIAS.

LDZA LDVA, Croatia

Peter wrote:

Then, there is the separate discussion of how would you land the plane with the ASI not showing anything, or showing something misleading?

On old aircraft POH standard cloud-break is: low power or idle, trim for 1.3*VS0 (you should know this trim setting by heart), hand-off yoke controls and fly small heading corrections with the rudder (on whatever instrument left: turn coordinator or even compass), to fly S&L say at 1000ft agl use power, on all SEPs 2/3 of max rated power (easy with a fixed prop) should set you straight & level on a safe cruising speed

Trim for 1.3*VS0 is usually few 3-5 degrees bellow horizon and VSI around 500-700fpm for any SEP (stalls at 60kts and glide at 1/10), I tried this on all aircraft I flew so far for fun and had to use it for real on a glider with clogged ASI and flat turn coordinator battery

Safest ways to land deficient aircraft are most likely hand-off low power glides or hand-off on chute

Last Edited by Ibra at 31 Jan 11:59
Paris/Essex, France/UK, United Kingdom

Ibra wrote:

you should know this trim setting by heart

How can you when the CG position can differ substantially due to loading?

ESKC (Uppsala/Sundbro), Sweden

Very true, only try playing with this one pax usually full fuel, no power and clean config

Not sure how much the trim position is sensitive to GC/load profile on zero power near that 1.3*VS0?
But yes trim changes vs GC/load is very pronounced at high power with speeds close to VS0

On the Mooney I usually fly, trim all the way forward & gear down & full flaps & low power does set me for 80-85kts hands off landing under various loads

Paris/Essex, France/UK, United Kingdom

In the other hand, I know that aircraft does not fly by itself on takeoff trim position under any load configuration, it needs a firm “rotate at 55kts” to liftoff, trimming it all the way back does that job for me but need to push forward and trim down to climb, I had a discussion with someone who told me that will look very bad if trim is found in any other position than the one for take-off

Last Edited by Ibra at 31 Jan 13:52
Paris/Essex, France/UK, United Kingdom

I’d argue that 80% of landing w/o ASI is choosing the right airfield!

For me it would be an Airport im VMC with ILS and and 2000m runway. If that is not available, a 1500m runway at a field where the terrain is obstacle free enmough so that the visual approach feature of the GTN works will be fine as well.

With that I have a 3° glideslope. The only thing I have to remember (and which is printed on my checklist) is that in landing conf with 2nd notch flaps I need 17 inch MAP for a stable approach speed. W/O ASI I would chose 20 inch that would lead to about 110kt. When overhead the runway, I land like in a seaplane with glassy water which should not take more than 1200m for me.

That‘s at least my plan at the desk…


Way too much dependence upon positions of controls in this discussion, it’s going to catch you out one day. Trim is there to allow you to relieve the flight control load, not to fly the plane because you aren’t. Similarly flaps are available to allow you to adjust lift and drag to change the configuration of the plane, so you can continue to fly it. Propeller/engine RPM is not a good substitute for assuring that you are maintaining flying speed by the feel of the airplane. If you get used to using a certain power setting at a particular phase of the approach, you are cheating yourself out of the skills development. You depend upon a certain power setting to fly an approach, and the engine quits – you still have to fly the approach.

Instead, feel the plane, feel the settling in mush as you approach the stall. That feeling will always be there, independent of trim setting, flaps setting, or power setting. When you are confident that you can fly just faster than that settling seat/mushing speed, you can fly a no ASI approach safely at any trim/flap/power setting. All you have to do for any landing is arrive over the numbers on the runway centerline, at a suitable height, and flying speed to have some reserve energy to flare. If you’ve judged your numbers height well, you need less speed reserve for a power off flare. If you get the height really precise, too slow a speed won’t be as much a problem, as you can begin your flare anyway.

I’ve had a number of ASI failures on a number of types, both land and sea flying, The only difference in my landing technique has been to allow a little faster approach speed by feel, and a lesser expectation of a short landing. Until you’re flying the heavy metal, avoid flying the numbers and settings, and fly the feel. If you can’t feel it, pay more attention, all modern GA planes have a good feel and you approach the stall – it’s designed in for you.

Home runway, in central Ontario, Canada, Canada

In the Robin the stall warner would chirp on landing, if didn’t I’d consider myself as having landed too fast. The handling right up to that point was slightly mushy but still in perfect control.
If I had an ASI failure I’d pop off to somewhere safe to try out my stall warner, then return home flying by feel and deliberately look to ‘squeak’ the stall warner a couple of times on the approach.
Doing it deliberately and pre-ready to lower the nose with a dab of power is extremely safe, except maybe for a low hours student.
Once established like this, I don’t think I’d consider the ASI again and just continue normally. If it was into less than 600m I may work a little harder.
In the Cessna I’m not as confident. I know the stall warner works and have flown slowly with it going, and stalled with it going, it seems to be set up ok, but if I look at any of my rare ‘no real room for improvement’ landings the stall warner hasn’t gone off and when it has, it hasn’t been so pretty. I haven’t yet got round to testing the stall warner for the approach, independently from a check-ride.
I’d still be unstressed to land at 900m without an ASI just not at a short field.

United Kingdom
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