Menu Sign In Contact FAQ
Welcome to our forums

Vacuum failure and loss of control in retractables - what is the relationship ?

So here I am researching vacuum driven vs. all electric AI, and I find this article (link below) which refers to a FAA study concluding that vacuum failure in a retractable gear aircraft is more likely to cause loss of control compared to a fixed gear aircraft. Quote from this article:

Joint ASF/FAA Study On Vacuum Pump Failures

“Air Safety Foundation recommends that pilots of complex aircraft lower their landing gear if they suspect a vacuum failure, to help maintain stability and to lower airspeed”

I can see why flying slow might help in a vacuum loss scenario.
How does having fixed gear make an aircraft more stable than retractables ?
Correctly trimmed, dont all aircraft fly in a stable manner ? I just dont get it.

Could it be the study’s outcome was due to pilot sampling or anything specific about the aircrafts used ?
I have not flown the Archer or the Bonanza to have an opinion on either aircrafts “stability” in flight.

United Kingdom

Having read that, I think all it’s proving is that a major failure requiring attentive hand-flying has a greater tendency to lead to an adverse outcome in a complex aircraft compared to a simple one.

That doesn’t surprise me because when you suffer gyro failure in an Archer there’s not a lot to do other than keep it upright using the remaining instruments.

In a Bonanza there’s more stuff to fiddle with and more systems management to divert your attention from the primary task of keeping it upright.

The results are logical and predictable. The landing gear is PROB90 a red herring – it’s simply that complex aeroplanes with lots to fiddle with happen to have disappearing wheels and the simple aeroplanes don’t. Correlation without causality.

Or perhaps lowering the gear in a retractable might help instil a sense of emergency / abnormal operation and promote greater concentration on staying alive?

I’m very pleased we have a G5 in the TB10. Even though it replaced the directional gyro (in its traditional location, lower centre of six pack) when in instrument conditions I tend to have it set to PFD mode unless I’m actually on an instrument approach and really need the HSI mode. The second artificial horizon with a different power source is very reassuring and provides excellent protection against slow wind-down failure of the vacuum-driven AI, and in PFD mode it still gives you heading and track information anyway.

Last Edited by Graham at 16 Mar 12:20

flyingelephant wrote:

Correctly trimmed, dont all aircraft fly in a stable manner ?

All aircraft have lateral instability. Anything else would be impossible as the aircraft can’t “know” the difference between gravity and the centrifugal force in a turn. Thus an aircraft can’t right itself laterally. (If it could, we would not need gyro instruments for IFR.)

(To avoid sidetracking, I know that the centrifugal force is not a real force, but from the aircraft’s perspective it appears as if it were.)

Last Edited by Airborne_Again at 16 Mar 12:34
ESKC (Uppsala/Sundbro), Sweden

This video explains it well..more slippery airframe ….Lowering the gear in a V tail makes it much more stable much more drag and less chance to overspeed in case of LOC


Correctly trimmed, dont all aircraft fly in a stable manner ? I just dont get it.

Sure with trim the aircraft will remain stable real questions for how long? and under how much perturbation?

There is a whole chapter in Newton Physics (quantum field mechanics with Lagrangiens if you like) about stable states, these tend to require low energy and they are more stable if other states require more energy

So back to flying aircraft, if you set aircraft in speed ranges that require less power the aircraft will remain in those speeds hand-off for a long time as it needs power (usually in 1.3*VS-2.0*VS ranges)
If you drop the gear in those speed, you make aircraft stick to them as drag from gear is asymmetric: does nothing to slow speed flying but blocks high speed excursion from energy excess

The other thing in medium speed ranges with medium power you are deep inside flight and engine envelope for every control, even a monkey can fly the aircraft there without breaking it: you can bank and pitch +/-45 deg in IMC and nothing bad happens for 30 seconds, you can drop or raise flaps or gear, you can lean mixture, you can twist the propeller, you can fly in thunderstorm, you name it…

Flying fast near VNE on lot of power will be near edge of your trim, need precise flying (+/-2deg pitch and bank) not to bend it in spiral dive…same for flying very slow near VS on lot of power, risk of spins

If you have any doubt in SEP and you know you are likely to die, just fly 80kts-100kts speed range and keep wing level, it will be fine, remember power is your enemy when you lose AI and ability to fly attitudes with precision, drag is your friend

Feel free to let your hands go in IMC with kicks on controls at various speeds, powers and configs you will get an idea on things that work for your type

PS: we used to drop gear, open air-breaks and trim 1.3*VS when flying gliders in clouds (works in most types except slick high performance ones)

Last Edited by Ibra at 16 Mar 13:18
Paris/Essex, France/UK, United Kingdom

Referring to the original article I think this is a bad study.

A vacuum pump failure disables the attitude and heading indicator in most general aviation aircraft, leading to the possibility of spatial disorientation for pilots flying in instrument conditions. About three accidents a year are attributed to spatial disorientation due to a vacuum pump or instrument failure.

How many accidents are attributed to a “glass” failure?

Not many. The majority of “glass” flying is still under warranty so public reporting (forums etc) is much less common because most owners absolutely must look after their dealer relationship. I have lost count of the number of times I asked a pilot to post about something, and his reply was the “dealer relationship” angle! Plus keeping half an eye on selling their plane one day… who wants to post that their $50k refit has packed up 4 times?

Maybe glass is less reliable, maybe more reliable, but prob99 the failure of “glass” is a lot more obvious!

I have a collection of failed Aspen EFD1000 photos displaying a large cross, in solid IMC, which was not amusing to the pilot, but it was very obvious that he has just been deprived of his primary attitude instrument!

With a vacuum AI, unless you are monitoring other stuff (which most people don’t do) then a loss of vacuum will just quietly and gradually bring you into a roll, and eventually the “spiral of death”.

FWIW I do not buy that “glass” is more reliable than “steam”. Actually I think they are probably similar – unfashionable as this may seem. No way am I getting rid of my vacuum AI. It will work when everything else in the plane (except the engine) has died, blown up, etc. And I’ve been in electronics since the 1970s…

And retractables are more slippery, although the newer fixed gear are also slippery. An SR22 will be in the spiral of death in seconds if you let it go…

Shoreham EGKA, United Kingdom

Peter wrote:

With a vacuum AI, unless you are monitoring other stuff (which most people don’t do) then a loss of vacuum will just quietly and gradually bring you into a roll, and eventually the “spiral of death”.

@Peter, not just the loss of vacuum, but many other failures. A few months ago I was flying IFR OCAS in VMC (above clouds) and was about to enter IMC in about 10 min time as my passenger have noticed, that AI is not aligned with the reality quite substantially (10-15 degrees bank and pitch misalignment). Not a problem – we just changed our route to stay in VMC and descended to below the clouds, but the main things was that the vacuum was there and the DI was fine, just AI was screwed (it was checked OK on the ground before the takeoff). And no flags or other warnings!


flyingelephant wrote:

How does having fixed gear make an aircraft more stable than retractables ?

In ME flying there is the concept of ‘keel effect’, which comes into play in OEI ops. How marked it is depends on the airplane, but for example in the Tecnam it has an amazing effect – you can almost totally forget about banking into the good engine and crabbing, the airplane flies as if both donks were working. On the Baron, this is much less pronounced. I suspect this depends on the gear configuration: with or without doors. The doors act like fixed mini control surfaces that stabilize the airplane.

My SEP RG experience is almost exclusively in various Cessnas, almost all without gear doors. Here, the stabilizing effect is rather minimal, however the gear does act like a huge speed brake.

The regulators require training for limited panel but hardly any training for partial panel (pitot static/ADC) problems.

Yet most common cause is partial panel, especially with better reliability of modern electronic displays, where loss of attitude indicators is relatively rare.

Oxford (EGTK), United Kingdom

I had the impression limited pannel is loss of various instruments like pneumatic ASI, ALT, VSI…when partial pannel refers explicitly to the loss of gyro-like (AI/AH + DG/HSI)

One typically does both during training on stema gauges where things are “independent”, in glass cockpits things are well mixed up in the data computer ASI or GPS failure could toast AHRS outputs

What happens when someone departs with pitot covered in say DA40 with G1000 in low IMC?

Last Edited by Ibra at 17 Mar 12:20
Paris/Essex, France/UK, United Kingdom
15 Posts
Sign in to add your message

Back to Top