CAT II & CAT III approches

[bobdazzle]

When shooting a CAT II or III approach, Why is it so important to discontinue the approach if the standby altimeter shows a flag?

[Microburst2002]

I think it is because if there is a CHECK ATT caution you have to go around refering to the STBY ATT. Otherwise you have to compare with the IR3 attitude by means of switching, finding which of the 1 and 2 is the "rogue" IR and go around referring to the healthy one.
I think.
Please can someone confirm?

[always half full]

The stby Altimieter is used as a reference datum should you loose either the left or right Altimeter. Therfore, if you loose the stby you have nothing to compare the remaining 2 against.

Eg. You are flying a Cat2 or 3 and the FO's Alt is different from the Capt's you should compare all three, the 2 which are reading the same are correct - the odd one out should not be followed.

Hope that makes sense!

[RAT 5]

Please specify type a/c and if this is your SOP or a manufacturer's instruction. On the a/c I've flown I have no idea why this should be the case. I've flown Boeing types, both Cat 2 basic, Cat 3A & B. These were both 3 A/P fail operational & 2 A/P fail passive a/c. There are monitors on all the relevant systems. On the Cat 3B a/c the autoland monitor would change Land 3 - Land 2. That was the trigger. A tiny flag which might not be spotted would not be a reason on its own to abort the approach; if memory serves me well.
More info please.

[bobdazzle]

Thanks for the replies.
I fly the 777's with 3 autopilot fail operational capability. I do not think that the autoland monitor would change the landing capability from Land 3 to Land 2 or even - No Autoland.for a flag on the standby altimeter.
In our SOP's it's a visual check of the standby alti at 1000' and if it shows a flag one is required to go around.
If the standby alti just serves the purpose of a standby datum incase of the primary altimeters failing- Why not just have a proceedure whereby the two pilots make a altimeter x check on noticing the flag on the standby and if both the primary altimeters match, then just go on with the approach?

[9.G]

Are you sure you're talking bout STBY ALT not bout SBY ATT indicator?
Cheers :ok:

[TopBunk]

From my experience in A and B aircraft, I can recall no such limitation in flight.

Think it through, you have both primary altimeters working and you have established (via DME, radar, GPS, whatever) that you at a given point on the approach. You then have multiple Rad Alts that confirm everything is ok and are in agreement both with each other and DME, FMC, GPS etc. Why would you seriously go around for a flag on a standby altimeter:rolleyes::ugh:

OK, your ops manual must be followed first and foremost, I just don't quite understand the logic.

[Edited to add: crossed with 9G above. I agree about the requirement for the Stby Attitude indicator]

[9.G]

A/C is dispatchable with STBY ALT inop however it's limited to VMC only for A 320 at least. The question of failure in flight is a bit far fetched in my eyes but then again maybe it's more easy going on the Boeing, who knows. Talking bout STBY ATT indicator, dependable on operator's approval it can be ignored once the approach has been commenced. 777 has got 3 autopilots or channels quite amazing indeed.
Cheers :ok:

[adverse-bump]

not sure i would go around with that

assuming your well into the approach (ie rad alt displayed) then why not continue and land.

if you do end up throwing the approach away (bcoz you dont get the lights) then climb and maintain what ever height on the two working altimeters.

same goes if its the attitude indicator (did a similar thing in last sim with this) are stby att has nothing to do with cat II capabilities.

[TopBunk]

9G

In my view, you dispatch according to the MEL/CDL, if there are stipulations about the STBY ATT/STBY ALT in the MEL/CDL, then you comply.

If, in flight, with everything serviceable at dispatch, then the QRH governs.

Hence, if at dispatch, there is no restriction on the STBY ALT, and the QRH does not require it for Cat 2/3 (as is the case for A320/B747-400, to my knowledge), the even with a failure/flag in the STBY ALT system, you can perform a Cat 2/3 approach and landing.

[Thunderbug]

For the 320 series you require a Standby Horizon for CAT II or CAT III approaches.

The MEL allows dispatch with an inop AH, but only in VMC conditions. It also specifies that CAT1 is the max approach capability.

As for the standby horizon failing during a low vis approach - it is not one of the items that calls for the approach to discontinued below 1000'.

[9.G]

TopBunk, as mentioned before MEL restricts the dispatch of the aircraft with this failure to VMC flight ONLY. Once the aircraft dispatched MEL still can be consulted if deemed necessary by commander in case he/she suspects significant airworthiness degradation. I must say I'm completely caught off guard with this unusual question indeed. Never before have I heard about the requirement of checking ALT red flag on the STBY ISIS. ATT yes, but not ALT.
BTW QRH 5.04 required equipment for CAT II III is part of MEL. Neither does this table contains anything bout STBY ALT nor does volume 4 mentions anything bout it in the section 4.5.70 FAILURES AND ASSOCIATED ACTIONS ABOVE 1000 FT FOR CAT II or CAT III. I'm not entirely sure the question is relevant one but stand to be corrected.
Cheers:ok:

[Denti]

Interesting subject. On the 73 there is no requirement to have the standby altimeter for the approach that i know of, however on the fail operational ones you need the standby attitude as it is the third attitude source for the fail operational system, thus it gets monitored and you will get a LAND 2 if it fails (above 200' RA that is).

[BOAC]

I'm not entirely sure the question is relevant one but stand to be corrected. In our SOP's it's a visual check of the standby alti at 1000' and if it shows a flag one is required to go around. - it is relevant

[clevlandHD]

I betcha that your current employer has in the past operated A306/A310. It was required to have a functional stby alt on them for CAT2/3.

[411A]

When shooting a CAT II or III approach, Why is it so important to discontinue the approach if the standby altimeter shows a flag?

Type, L1011, standby altimeter is not required for CATII/III.
Likewise, standby ATT indicator is not required for CATII/III.

However, for flight dispatch requirements, both must be serviceable for planned IMC flight. Failure in flight is just that, a failure, neither affects the landing capability.

777 has got 3 autopilots or channels quite amazing indeed.
Cheers

Even more amazing is the L1011, two dual channel autopilots (IE; dual/dual ), for fail operational performance.
An old aeroplane, yes, but quite capable, even today, if maintained properly.
Fully CATIII certified, out the door, circa 1972.

[Microburst2002]

Bobdazzle
Sorry, did not read carefully your question.

In 320 only stby ATTitude is required for cat 2 and 3. Not so stby Altimeter.