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Trusting instruments

Today I had good conditions for some VFR on top, a thin cloud cover with a few holes (7/8ths or so) and totally clear a few miles in one direction. I was just on a local flight but couldn’t resist climbing up on top for the view.

Whilst flying around on top, I noticed the artificial horizon was faulty. In a steady bank, after a few seconds or so the AI would revert back to displaying straight and level. Same for climb & descent. After levelling off the AI would display the last known attitude and slowly revert to displaying S&L again. Using the backup electric vacuum pump made no difference. This is a flying school owned PA28, but the owner of this school fixes every defect and nothing is left inop, in fact this AI was replaced this morning to replace the previous one that wouldn’t erect. My company obviously didn’t check it properly.

Due to the nature of this fault, I didn’t notice any problems previously as I was mainly looking out of the window.

In this particular situation I had two “outs” remaining, either descending through a hole or flying a few miles to where the cloud cover ended. Nevertheless, I’ve read enough Dick Collins articles on Air Facts to know that LOC in IMC is real and this experience spooked me slightly.

Having an instrument such as an AI, if one became stuck and had to descend through cloud on instruments, is a bit useless if it doesn’t work correctly. I’ve flown in a lot of IMC in a plane that is equipped with two mechanical AIs (one vacuum and one electric) plus an AHRS system but having triple redundancy is unusual and not to be expected in a rental spamcan.

That’s my first experience of such a fault but I notice many people fly IFR in planes that don’t have backup gyro instruments. Surely that’s a bad idea? How does this usually work, just hope the AI works I guess? I’m looking at getting one of those portable AHRS units for future flights.

United Kingdom

Ideally having a back up. I also pay a lot of attention to instruments on the ground, although that might not have helped in your specific case. This is why limited panel is taught and examined though, and is useful to have a look at now and again. The next bit is identifying that you have a limited panel. It is very, very worth knowing some performance numbers for an aircraft your fly – you will have heard of attitude + power = performance. If you know that (for example) 5 degrees nose up with full power gives a 90 knot climb, and all of a sudden 5 up and full power is causing no climb and 120 knots, then you can start to suspect something is wrong.

United Kingdom

IO390 wrote:

That’s my first experience of such a fault but I notice many people fly IFR in planes that don’t have backup gyro instruments. Surely that’s a bad idea? How does this usually work, just hope the AI works I guess? I’m looking at getting one of those portable AHRS units for future flights.

It’s illegal to fly IFR without backup gyro instruments, so I would be very surprised if many people do. The backup is usually the turn coordinator. Flying using the TC without the AI means a higher workload but is perfectly doable. (Indeed, you are trained and tested for that if you have an IR.) That’s also how most glider flying in clouds is done.

ESKC (Uppsala/Sundbro), Sweden

I had a similar experience with an arrow 4 whose vacuum pump was “on the way to leave”. It has been changed and solvedo the issue.
This been said, I wasn’t over the over the sea with a foggy horizon, and said to myself that I would have difficulties to believe in the instruments in such case.
Now in the ifr planes I am flying on, there are backups , vacuum +G5.

LFMD, France

I used to fly a school aircraft that was legal IFR but not ideal for practical IMC, the ADF/VOR needles keeps going left & right and AI sound is bad after you turn off the engine, one day I departed for two approach practice in IMC at Southend ILS from a nearby VFR airfield, I think vaccum reading was pretty green on power checks, right after takeoff I went into clouds at 800ft, the AI started to tilt & my airspeed start to increase with altitude decreasing, in a single moment I banked to fly runway heading and re-aligned my DI with compass, the AI was drunk that day so flew on electric TC

It was an 900ft cloudbase day, so I did not feel it was an issue to keep the practice going on partial panel on TC after all that what I was looking for, the 1st ILS was neat, the 2nd ILS was a fiasco that my hands were sweating on the go-around, maybe it has to do with getting tired (I was vectored all over the place and I should have let ATC, I was hand flying with no gyro), I can’t explain why things went that wrong that I bought a Dynon Pocket Pannel and started to be careful with renting an aircraft and fly it in IMC, that happened while I was bloody current in hand flying with more than 10h of IFR flying that month flying a simple C172…

You can fly a cloud break on heading compass & ASI/VSI, as A_A said gliders do this all the time (I have done lot of it myself even on dead battery for TC/AI but airframe has airbreaks and pilot has parachute ), the C172 has a well documented procedure in their POH with no AI/DI/TC, it’s done by flying low power setting and hands off the wheel, however, one has to be bloody careful chasing precision approach needles on partial pannel especially with gusty winds near the ground, it’s already tough on AI…

Last Edited by Ibra at 27 Aug 07:44
Paris/Essex, France/UK, United Kingdom

I’ve had an AI fail in night IMC, and indeed it takes a few seconds to spot it. But to do so you must be very attentive to other signals, the first of which is usually heading changes that don’t match bank angle. Then you have airspeed and altitude changes that don’t match, all of which are pretty clear indicators if you’re paying attention. If your AP is driven from the AI (as mine is), then you have to quickly move to hand flying off the TC, possibly also correcting an incorrect attitude in the process. It’s this initial identification and quick response that’s the most precarious period.

EHRD, Netherlands

Vacuum AI’s have gravity operated erecting vanes inside, which will slowly erect the AI to gravity during straight and level flight. These vanes operate in pairs, and act in pitch and roll. They are known to stick occasionally, and that will induce an indicating error. It won’t occur, nor resolve quickly, it’ll take several minutes, so is hard to notice, until it’s way out. This failure will not be indicated by a vacuum problem. In this case, the instrument needs to go for service.

I’ve also had a gyro bearing fail, this will cause a fairly sudden roll over and die there. This once happened to me at the moment I entered cloud. It was very disorienting. I started to follow it, but quickly reminded myself that there was no reason that the plane would suddenly roll, and no other instrument showed this. So, I reverted to partial panel, recognized an upside down AI, and continued out of the cloud with reference to the T&B/ASI/altitude.

Home runway, in central Ontario, Canada, Canada

IO390 wrote:

I notice many people fly IFR in planes that don’t have backup gyro instruments. Surely that’s a bad idea?

Yes its a bad idea. And its a small price to pay these days to get a backup. Ideally 3 attitude indicators to easily identify the faulty one. The reality is that flying with faulty instruments IMC can cause total confusion and loss of control quickly. Airfrance AF447 loss of control was basicly a misindicating airspeed reading to start the confusion. To identify the failure and which instruments to trust is the problem. And once control is lost its hard to get it back unless saved by a VMC breakout before hitting the ground or the plane fails under upset maneuvers.



THY
EKRK, Denmark

THY wrote:

Yes its a bad idea. And its a small price to pay these days to get a backup. Ideally 3 attitude indicators to easily identify the faulty one. The reality is that flying with faulty instruments IMC can cause total confusion and loss of control quickly

I know what many people think about flying on NAV(VOR/DME/GPS) data or cross-checking on moving maps but I think this is where they shine & become powerful: you can easily see that you are turning on dead horizon/gyro, or descent on clogged static, or going fast on clogged pitot

No need to have 3*PFD, just 2*PFD+1*MFD, the set (TRK, ALT, GS) from NAV/GPS is all one needs to know which one of their PFD went crazy, actually if you have stable (TRK, ALT, GS) from NAV/GPS with an upside down aircraft on 1*PFD then you are doing very well it’s unlikely to have both PFD & MFD going crazy at the same time, the physics involved is different

In IMC, it’s also easier to start understanding your situation with help of MFD before discussing which PFD instrument has failed…

Just like how a B747 will go full power when their RA or TAWS kicks in with 2000ft stuck in their baro altimeters

Last Edited by Ibra at 27 Aug 14:43
Paris/Essex, France/UK, United Kingdom

In IMC you can’t trust your senses, you have to trust your instruments. When one goes wrong the first problem is to identify that one has gone wrong and I was taught that this is the value of the scan. If the AI shows you are turning when you think you are straight and level at least one of the other 5 instruments will either agree or disagree with the AI.
An old RAF and airline pilot once told me that if you are flying along in IMC in your normal manner but something shows up as not quite right, turn the AP off and let go of everything for 10 seconds. He reckoned that the aircraft was a much better pilot than you are and that 10 seconds gives you chance to survey and to think.
I don’t know if it works or not, I’ve never tried it, but this guy had more than 30,000hours as ferry pilot, flew bombers during the war and was with various airlines both before and after it.
Mind he struggled to land a microlight because of the inverted commands, although he was in his eighties at the time and it was his first microlight attempt.:)

France
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