Loss of standby instrument during flight

[cptpilot737]

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 ?

[de facto]

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.

[TopBunk]

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).

[I.A.W]

'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.

[latetonite]

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.

[grounded27]

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.

[Cough]

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

[SKS777FLYER]

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

[latetonite]

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?

[BOAC]

Yes but it relies on an iru for heading information on many type aircraft. - 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?:)

[rudderrudderrat]

Hi BOAC,

Which types are linked to one of the main IRUs?
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.

[BOAC]

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

[Nigd3]

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 ;)

[rudderrudderrat]

Hi BOAC,

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

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

[mcdhu]

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

[BOAC]

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

[Denti]

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.

[rudderrudderrat]

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.

[OutOfRunWay]

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.

[BOAC]

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

[de facto]

The Isfd instrument failure question as posted is bit vague.

I would think you have more chance to have a IRS failure in flight than a loss of all normal AC power(Dual gen failure).
For the "HDG part of the Isfd" to be inop you would need the L IRS (on my fleet) to fail.But the right IRS will provide correct heading info to the right PFD.
In that case as i mentioned the APs wont be available for the flight and if you are in for a long flight id return..

For the "STBY ATTITUDE part of Isfd" you d have to have no AC power at all..as it will work with battery power via the battery bus.(if both gens fail and dipatch with APU inop).
Then we all know we need to get on the ground asap.

If you lose the RMI (ac stby bus powered..), so you have no more AC/DC power, well yes you d need to fly with the magnetic compass for headings..

I would not take off with ANY standby instruments INOP.

As far as i remember flying with the mag compass, during the turn you need to overshoot South and undershoot north.
There are some acceleration errors but i forgot...(maybe flying east and west?)

[grounded27]

- 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.


One of our fleet aircraft does not have mel provisions for this function. Magis still there.

[DFC]

In many aircraft the standbys are there not just for the posibility of a failure in the main indicating systems but also to cope with the possible situation of total power failure, fire etc i.e. where possibly the crew shut all the main systems down to conserve power. For example it is not much use having 3 IRS, 3 ADCs etc etc working perfectly if all the screens are blank!!

The QRH will have the definitive pointers as to what should be done. Many issues also apply - weather, time of day, aircraft current status etc.

What I would compare it to is imagine during the cruise you discovered that all the fire extinguishers were empty except the one under your seat. There is no sign of fire......but what do you do if one starts?

If you can remove the posibility of ever needing the standby eg remaining in VMC then no problem.

There are now aircraft certified with no standby wet compass. However, they have a separate independent flux detector and indicating system for heading on the standby.

--------

The good old magnetic compass can be used as a last resort to provide wings level information in places where dip is obvious.

It is most noticeable on a heading of North or South because on these headings if the aircraft is banked the compass will swing even if the heading remains constant. On headings of east or west it will not work.

The reason why in the northern hemisphere "South" is the preferred heading for such a let-down is because when on a heading of South if the aircraft is banked left the compass will indicate an apparrent left turn. This makes it less likely to cause confusion.

On a heading of North when the aircraft is banked left, the compass indicates an apparrent right turn. Can be confusing but I am sure that one could cope if there were mountains to the south!!

On the (magnetic) equator - it isn't going to work so simply resort to the simple = trim out wings level in a slow descent and let go = rely on the aircraft's stability.

or, if cloud base permits-

Enter a sipn in your favourite direction and hold it until you break cloud then recover. Not much chance of overspeeding in IMC there!! :)

Usual disclaimers about aircraft approved for spinning etc etc etc :E :)

[OutOfRunWay]

an excellent explanation of the wet compass display errors.

regards.

[BOAC]

Enter a sipn in your favourite direction and hold it until you break cloud then recover. - memory is going but I THINK that was a technique proposed by Sir Francis Chichester? A lot going for it too - a stable manoeuvre. I would hate to think what my semi-circulars are 'reading' while I am in cloud:eek:

[Aviophage]

A 747 can fly on 3 engines from LAX to London, so I can definitely fly my A340 without stdby instruments.

[411A]

A 747 can fly on 3 engines from LAX to London...
Nope, sorry, it did not.:rolleyes:

Now, about this diatribe...
... so I can definitely fly my A340 without stdby instruments.

Maybe, maye not.
As with most things in aviation, it depends.
IF it all goes pear shaped (and yes I've had this happen with primary flight instruments...all of 'em) one had better have reliable standby instrumentation...otherwise, up the creek without a paddle might just be the case.:ooh::ooh:

[Aviophage]

BA 747 diverted to MAN after fuel emergency. It was destined for LHR however. Engine failure after takeoff from LAX and the crew elected to continue with the flight on the 3 remaining engines.

http://www.pprune.org/rumours-news/164208-ba-744-diversion-man-merged.html

[Nigd3]

Sorry Aviophage

I must be having a brain dump as I dont see the connection you are trying to make. You can fly any aircraft perfectly well without the standbys operating (albeit taking off as such, isnt exactly legal) provided, as 411A points out, the primary displays and sensors dont go u/s as well.
Please can you clarify the point you are making for us slower types

[TopBunk]

You can fly any aircraft perfectly well without the standbys operating (albeit taking off as such, isnt exactly legal)

Surely that depends on how your approved Ops Manuals are written?

For the B747-400, ours define despatch as the point at which the aircraft first moves under its own power with the intention of flight.

Anything thereafter you consult the firstly the QRH then the MEL to consider further implications.

[V1... Ooops]

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

I find that hard to believe. An aircraft as small as a new production Twin Otter uses a separate, dedicated magnetometer to provide the ESIS (Emergency Standby Instrument System) with heading information, and both the ESIS and the ESIS magnetometer are powered from their own battery, which has 6 hours endurance (same as the aircraft fuel tanks) and is independent of the aircraft electrical system.

[Denti]

Still true though, however that is usually an IRU which is powered by the standby bus in case of loss of generators and therefore continues to run until the batteries are dry.