Trusting instruments

gallois wrote:

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.

You have to be in stable power/speed range, somewhere deep inside the G-V flight enveloppe with cruise power, then aircraft will revert back to that configuration with small corrections or even +/-45deg pitch/bank and let go without much issues, try trimming an aircraft for about 1.4*VS & 40% power and then fly it with rudder (as per C172 POH, I posed above), then handoff or pull it into +/-45deg pitch/bank unusual attitudes while letting go all the controls, it will fly back or give you 20s to catch it

Now try that with 75%-100% power then pull airspeed near VS or VNE, you need to fly +/-1deg pitch/bank or it will go mad in next 1s
If you pull it to +/-45deg pitch/bank in IMC it will be game over

Obviously, one has to know how much his aircraft can tolerate as unusual attitudes under that “stable config”? the above C172 technique does not work in some high performance aircrafts & gliders as they needs precise pitch control and is not guaranteed to work in 300T jets or helicopters?

But I agree on the reasoning: an aircraft will break in the hands of a disorientated pilot, quiker than if it’s left alone

Some light aircraft cannot be trimmed hands-off for long and will enter a turn if stick is let go for a minute or so. Even in calm air.
(Only elevator trim.)

To fly in IMC you do need a plane which you can depend on. Zero defect policy…

Partial panel is fine – I did practically the whole of my FAA IR on partial panel – but it is very hard work and even a very good pilot will only just hang together. It should not be a backup policy for the failure of a single gyro instrument.

And anyway you should be using the autopilot in IMC This is flying after all – not Dragon’s Den.

IO390 wrote:

Does anyone know if there is any difference in % of LOC in IMC accidents for aircraft with “steam gauges” and those with glass cockpits?

I’m beginning to distrust mechanical attitude indicators more and more given that some of the failure modes can induce a LOC way before you have time to go to partial panel.

I don’t know of any % difference, but my instincts about these things and my own experience since our avionics upgrade in the TB10 (we had the old DI replaced with a G5) has led to a change in my own methodology.

Our new G5 replaced the old DI in the standard (bottom middle of the six-pack) position, but the G5 can be switched between HSI and PFD modes. When established on course in the cruise, one does not actually need the full HSI display since the PFD mode still shows the heading, the track and the selected nav course. In any case, not having an autopilot and the aircraft being difficult to trim in roll, I tend to ensure adherence to desired track principally by comparing the DTK and TRK numbers on the GPS screen and nudging left/right as necessary. This leads to straighter SkyDemon breadcrumbs than choosing a wind-corrected heading and flying it.

So now in the cruise, and particularly in IMC, I tend to switch the G5 to PFD mode. This means I have two attitude indicators one above the other, and any discrepancy will be much more apparent than a discrepancy between the vacuum AI and the turn coordinator. If I saw a discrepancy I would tend to favour the G5 (vacuum failure being the most likely) and can verify with a check of the turn coordinator.

The one other thing I keep meaning to do is get one of those instrument-covering pieces of plastic/rubber and stick it in my flying bag. My gut feeling is that in IMC it might be fairly hard to disregard the AI while one can still see it.

IO390 wrote:

Does anyone know if there is any difference in % of LOC in IMC accidents for aircraft with “steam gauges” and those with glass cockpits?

It’s high likely glass cockpits have an AP which is your best friend for LOC in IMC, if you can hve that in steam, then the question is moot

For hand flying, I did hand flown both steam & glass in IMC, I think the size of the AI and it’s “pitch resolution” matters I find it tricky to keep -1deg pitch on high power cruise descent on steam AI gauge in turbulent clouds, it’s too small and wiggly, you have to watch it all the time and you may have to confirm power and ASI/VSI

In general, in SEP you aim to fly +/-20deg bank and +8/-3deg pitch that should cover any IMC flying you want outside unsual attitudes, this is better done in front of a big AI, I doubt they make A5 or A4 formats for mechanical AI?

Other choices of pitch & bank are available but they won’t make much diffrence aside from satisfying purists (the guy who told me it should be 18.5deg or 21.5deg not 20deg for Rate1 turns at 115KTAS has never been in convective clouds ) or take the aircraft too far away in AoA and will need further corrections latter on

Of course glass/steam, flying currency is king and everone is at the mercy of LSO/LOC

IO390 wrote:

Does anyone know if there is any difference in % of LOC in IMC accidents for aircraft with “steam gauges” and those with glass cockpits?

Good question :) I expect at least 95% plus of IFR traffic is using a glass display, which is less susceptible to failure than an old dry vacuum pump, at least where AHARS is concerned. AF447 was a partial panel event (pitot/static problem, not AI/DG which is a limited panel event), which is not part of any UK/EASA IR practical test standard, and arguably today the pitot/static is more prone to failure than the AI/DG.

Nevertheless, even with good glass cockpits pilots/crews are still having LOC events, which may, or may not be due to loss of AI/DG.

RobertL18C wrote:

AF447 was a partial panel event (pitot/static problem, not AI/DG which is a limited panel event), which is not part of any UK/EASA IR practical test standard, and arguably today the pitot/static is more prone to failure than the AI/DG.

Good point. As I understood the report the problem was a fixation on the (junk) airspeed and stall warnings, and a failure to fly power + attitude = performance. They had good gyros throughout, which is what everyone seems to fear losing the most.

Late on in my PPL training my instructor covered up the ASI during climb-out. I found it fairly un-nerving, which suggests it might be a good thing to practice.

Of course flying P+A=P in a jet at FL360 where the envelope is tight and pitch control must be precise is probably a good deal more demanding than settling a 1 ton SEP into a gentle climb at full power such that you see ~500fpm on the VSI.

RobertL18C wrote:

AF447 was a partial panel event (pitot/static problem, not AI/DG which is a limited panel event)

Yes, temporarily – that’s how it started – but during most of the accident sequence everything worked as intended.

I am not at all sure that “glass” is more reliable, in a strictly numerical sense, than “steam gauges”.

Lots and lots of people are fairly regularly paying out € thousands, via their “ever friendly Garmin dealer”. Without him you are completely screwed, which is why this stuff doesn’t end up much on forums, because he reads them too, and will make life very unpleasant for you, as I well know from my early days with the TB and Socata. I have lost count of things I’ve been told, with a requirement for keeping it confidential because the dealer must not find out.

And similar numbers are paying to overhaul individual instruments.

The difference is that for the former you have to go the official route. For the latter you can send the thing straight to a US overhaul shop, and you are free to talk about it. You might keep quiet if on an EASA-reg and using a US shop which is not EASA approved Preferably you have a shelf spare, which is very unlikely to be the case with “glass”, where a fault = “total aircraft downtime”.

With say a vacuum driven AI, yeah, when it goes it goes, or perhaps just gets a bit sticky, or perhaps it just doesn’t erect when you start the engine but runs fine afterwards (my KI256 is doing that right now, so I will swap it for the spare when I get a chance). But it is fairly obvious it has gone, and the key thing is that anybody with any brain can understand the concept of a vacuum pump driving the instrument, and how that system can fail. Whereas almost nobody will be familiar with the whole range of avionics system interconnections, dependencies, and single point failures in a “glass” PFD system, especially one with loads of extras like a G1000. Even most avionics installers won’t know all that – most just follow wiring diagrams. And IMHO the key AF447 takehome lesson is just that: the pilots had zero aircraft systems knowledge, so didn’t know that the pitch and roll come from the INS and the INS is not affected by unreliable airdata. It’s like the famous Aspen pitot problem which I believe was addressed only recently by introducing GPS data into the AHRS background erection loop, and which totally surprised everybody who got “the big cross”, often in IMC as my friend Stefan got a few times.

A year ago I had a weird AHRS failure which caused me to taxi 180 degrees the wrong way, to much ATC amusement, and then I had an “interesting 3hr flight” back home, with the AP holding altitude OK but with me steering laterally using the rudder pedals

It is easier to hold an attitude when it is a big display, with a brown bottom half and a blue top half, for sure. But if you get a failure, the plane will be very hard to fly using the backup instrument(s).