Loss of standby instrument during flight
 

If you lose a standby instrument during flight (like ISFD, RMI, etc.) would you land asap or continue to destination or return back to takeoff field ?

Hello there,

On my fleet of 737 the Left IRS powers the ISFD so youd have to lose the L IRS for this failure then go to appropriate checklist.
You may end up not RVSM approved and to fly manually a long way if the FAult doesnt clear.
I would return.

If you lose the RMI (ac stby bus powered) you have no back up for radio navigation if you end up with Both Gen failure (bad day huh).
I would continue if weather is reasonable at destination and you are under radar control all the time especially in descent/approach phase.


Both are NOGO items on the ground.

You have despatched so the QRH applies. What does your QRH say? The only further consideration might be the implications of landing at your destination with the problem, in so far as how it affects subsequent flight.

Personally, I have departed a USA airport for base airport and had the standby horizon fail on taxi out, we legally continued (I covered the defective instrument to prevent any possible confusion).

'de facto', L IRS input to the ISFD is only for heading information, the ISFD itself has the ability to measure and display the aircrafts attitude via onboard inertial sensors.

The stby attitude indicator in the B737 , or the modern ISFD, is powered by the battery bus, and has it`s own inertial sensors. It would not make sense to hook it up to the same inertial source as the left PFD.

Yes but it relies on an iru for heading information on many type aircraft.

So you can't use a mag compass for that anymore?

Kidding me, a pilot use a wet compass anymore?? (sarcasm)

Do not worry about the heading information. I can think of many pilots trying to work with the available attitude information. The thing has no FD, see?

- that seems to be ludicrous! Surely the whole point of a 'standby attitude indicator' is to provide attitude information when other systems fail, not to provide heading and not to be 'tied' to one of the systems that might fail. The answer is as cough says - you would have to use a mag compass for heading.

Which types are linked to one of the main IRUs?

Now - how many of you know how to let down through cloud using only pressure instruments and a standby compass?:)

Hi BOAC,

A320 series has an ISIS (integrated standby system), some types display heading information from No1 or No 3 ADIRU. The attitude is sensed from it's own internal gyro, but additional info is displayed in one neat unit.

Thanks rrat - neat and fine - as long as the AB wizardry CANNOT 'vote out' the whole unit with an IRU failure.

Depends on the system type to determine how independent it is. Some have their own magnetometers (also powered from the emergency battery), some use AHRS/IRS for their heading input only.
In the end it is down to 2nd and 3rd order failures to have a total loss of attitidue and/or heading........now that has never happened before ;)

Hi BOAC,

I don't know either - I'd need a Standby Artificial Horizon as well.
How do you do it?:confused:

Mmmmmm......I seem to remember that you have to do it on south otherwise the s/b compass is not steady enough - or was it north?!!!

I'm sure BOAC will let on soon!

Cheers
mcdhu

mcdhu wins today's prize - or not! Come'on you lot - work it out!

From what i hear from our maintenance the airbus ISIS and the boeing ISFD is basicly the same unit build by thales. However since the connectors are set up differently they are sadly not interchangeable between fleets which makes it necessary to stock both parts in my operation. Dunno about the airbus setup as i haven't been able to crosstrain yet but on the boeing 737 as mentioned above the heading is supplied by the left IRS only, however there is a note saying that you have to validate that heading information by crosschecking with the magnetic standby compass. Additionally it receives ILS information from NAV 1, altitute, airspeed and atttitude are computed internally.

Interesting enough the ISFD is powered by the battery bus (no surprise there) and cannot be put offline as it has no circuit breaker.

Hi BOAC,

Do you mean level wings with a descent in cloud? I thought you meant fly a published let down.

If you fly E or W, the compass needle will lie nearly horizontal around here with just a few degrees of dip. Maintaining the heading and the same "dip angle" relative to the aircraft will hold the wings level.

South.
 

in the northern hemisphere, flying south by the whiskey compass will give you the most accurate heading, as the compass reacts to heading change faster, try watching your wet compass: on a N heading, you can turn quite a bit before the compass shifts.
I know a complete failure of all heading info is unlikely, but the whiskey compass is not there for decoration only..

regards.

Well done OORW - restored my faith in this lot, you have. I occasionally demo'd it on AFIC courses to bring groundschool to life:)