Altimeter Failure

[stefair]

Hi everyone,

Following scenario: Light twin, cleared for ILS approach, localizer has been captured and just before descent on GS failure of both altimeters. What to do?

Thank you. :ok:

[mad_jock]

Go down with the glide and and when you get to .1D start flaring if you haven't seen the ground yet.

If your a smart cookie you will have already worked out your ISA QNH height difference. So you you will be able to use the altitude off the mode C transponder.

Some people would proberly say GA and try and sort it all out but personally if I was on the loc and the DS bar was coming down I would go with it. Also if both Alt are out and you switch to alternate static source and they still don't work your mode C is likely to be out as well unless its got its own static port. And if the static system is shot your ASI will suspect as well. You could smash the VSI and hope what ever's blocked the static system isn't between the VSI and the alt's.

Easy option is just set your aircraft up and go with the slope if the controls start feeling sloppy put some power on and if they start feeling hard take some off while cross checking with the AH that you haven't got a stupid attitude.

[Tmbstory]

stefair:

Do as'mad jock' suggests. It will get you down ok.

If you are nowhere near the glideslope, then as one of the last ditch efforts, use your simple outside air temperature gauge as an indication of your height, if it stays the same, you are roughly level, if it increases you are losing height and if it is getting colder there is a fair chance you are climbing.

Tmb

[Pugilistic Animus]

I woulk suspect most twins have an alterate air---to save the VSI replacement cost;)

using OAT---never thought of that:ok:

[Clandestino]

So there you are, in a light twin, established on localizer, G/S is just about to get centered, when all of a sudden, both altimeters simultaneously decide to call it a day.

First pinch yourself. You are dreaming.

If it doesn't help, switch to alternate static source. Breaking the VSI to get static pressure back into lines is a procedure very unlikely to succeed without proper breaking tool (e.g. canopy axe) and even then potential for damage to adjacent instruments (HSI) is enormous.

Do not use ATC mode C readouts! On big expensive aeroplanes, readouts are taken from ADC, on cheaper ones there's usually mechanical encoder connected to altimeter. Whichever system is installed, if altimeter misbehaves, so does your C-mode altitude reporting. That was the final nail in the coffin of Aeroperu 603.

If both altimeters are inop, chances are you have lost static pressure and there's no reliable IAS indication. Follow the glide with typical approach power, accept that that there's no redundancy or crosschecking whether you're on the correct G/S (unless there's PAR on the approach runway, ready for use), fly your best and follow ILS to the ground, if need be. Very risky but then less risky than going around with no IAS or ALT.

And finally, ask the sadist that comes up with such implausible scenarios, what is the indication of altimeter failure on light twin, anyway.

[Pontius's Copilot]

If it happens at the point you describe, you're very very lucky - probably configured for the initial approach at least, with speed somewhere close to target for the approach.

Remember power-attitude-trim and all that, and the 'ballpark' power & attitude settings you learned: get the glidepath to the right place, set the 'ballpark' power for a 3 degree descent then fly the glidepath ... all the way down.