A320 Bank Angle Problem

[captainpaddy]

Hi Everyone,

Just wondering if anyone can shed any light on this for me.

A SID from CFU (can't remember the SID name) from the southerly runway (can't remember the runway either!) requires a max bank angle of 15 degrees for a left turn that effectively brings you back overhead at a minimum altitude of about 3,000 feet or something like that. The SID plate shows a track to overhead being in the region of 12 or 14 miles. When the SID is loaded into the FMGS, it draws a tight circle for the turn with a track of maybe 4 or 5 miles to the overhead. On departure, the AP rolled all the way over to 30 degrees in an attempt to keep itself following this green track line, giving us a really tight turn which left us passing the height constraint at 1500 feet!

Question:

I can't begin to understand the logic of the FMGS in this case, but I am suprised that it did not limit itself to 15 degrees for a SID. Am I right in assuming that it was simply attempting to follow it's own green line which it drew earlier and that it will use any bank angle it can to achieve it (up to 30 degrees)

Also, if I'm right there are limits to the AP bank angle depending on the margin over V2 or VLS or something like that? Would this fact be helpful in any way to limit the bank angle used?

Finally, would HDG mode have resulted in the use of a different bank angle? Or is it simply a case of havingto hand fly this departure next time, ignoring the FD?

Any ideas would be very helpful!

Thanks!

CP

[Wino]

10 years in Airbuses...

Don't like what its doing? HAND FLY IT!

managed climb won't help you either if you were thinking of going there...
Expidite might have (would have pinned you at green dot), I suspect you were at 250 in which case it was trying to perform a standard rate turn. A slower airspeed would yeild a lower bank angle inorder to maintain that standard rate turn.

you could have also put a speed in the climb page (V2 for example) that would keep you there and maintain that speed till you were over your crossing restriction. For a lower crossing restriction of 2500 or 3000 feet you could just change the acceleration altitude on the takeoff page...

Furthermore when you built the SID if you take out the constraints you will get an accurate view of how the precedure will actually be flown by the computers (remember the aircraft will not pitch up and sacrifice speed for altitude to make a climb restriction, it will try at whatever speed it defaulted to and fail, the only thing a managed climb will do is STOP a climb, not increase rate over "open climb."

Cheers
Wino

[captainpaddy]

Wino,

Entire SID was completed at approx V2+10. I had said that we passed the constraint at 1500 feet in the climb. Therefore, acceleration had not yet begun. Managed climb in this case would make no difference whatsoever as all constraints were "at or above".

No combination of preselected speeds and/or accel altitudes would recover the 1000 or 1500 feet or so needed over such a short track distance. The bank angle used is in my opinion the only reason why the constraint was missed as it would have at least halved the available track miles available for the climb to that point.

[Bearcat]

a320 fmgc sids are not gospel....you are the capt....intervene. good example is nice off 4R....@ 400 ft turn right 140 ....it turns if you leave it at 1000ft rattling the rich neighbors chandeleirs. Green line line junkie or not...its the capts call.

[captainpaddy]

I agree with you both regarding intervention if things don't go the way you've intended.

However, I'm trying to understand more about the way the aircraft operates and what techniques can be used to ensure the aircraft behaves the way you want it to.

Without the slightest bit of offense intended, it is a bit of a cop out to simply say intervene when things don't go as expected. I think it's better to say something didn't go right, I've intervened this time round, now I want to know how to stop that happening again. That way I'll recognise the potential for error when I get in a similar situation in the future.

Sometimes of course, there is no other option, as in your case Bearcat. Perhaps this is the case for my scenario too?

[Soga]

In fact to comply with a Sid turn and a constraint the best option was mentioned; ( selected speed during climb and or forced in climb page . and comparing what we can expect from P-Rnav and approvals requested one is about the bank angle 15º and speed constraints to overfly the waypoints, and correct me if i wrong, the standard FMGC need to be update ( hardware) to comply with this
Soga

[Dixons Cider]

Deaparture with config 1+F will produce a 15 degree bank angle cap'n paddy.

[arba]

:confused: Don't clean up too soon ! Maybe ?

[captainpaddy]

Dixons Cider,

Ah ha! I feel as though a light just went on in my noggin! We use flap 2 or 3 out of CFU, never flap 1+F. That may well be it. Do you know what bank angles are related to the other configs?

Also, I presume that the 15 degrees you mention is only at V2+10??

[Office Pest]

I feel you may be barking up the wrong tree. I believe the 15 degree limit may only apply to flap 1+F and greater in the case of an engine failure. I must look at those books more often.

Regards,
Office Pest

[Office Pest]

To add to my cryptic answer from a few moments ago the fifteen degree limit applies to the engine out case when the aircraft speed is below the maneuvring speed for the flap selected at the time. I believe that apart from that it will not respect any bank angle limits (apart from the usual) as I dont know of any way to input them into the system.

Office Pest

[Ag2A320]

The Same question was raised during my last sim session at Airbus Miami, After Engine Failure At V1 , ( Working out of JFK 31L Canarsie Climb) My sim partner was alarmed to see the Sim Banked over @ 30 degrees during a EFTO, AP engaged, The instructor promptly said u engaged the AP at a speed higher than V2+10 hence it respected the SID Nav constraint and since u had a speed higher than V2+10 did away with the 15 degree limit. Ah AIRBOOS LOGIQUE. after 5 yrs still have to ask is it doing what i want it to?

We had an incident where the SID calls for left turn and climb to 4000 to get away from the terrain, had light A320@ 115Klbs on a ferry to MTC base, used TOGA as per the MEL , rocketed off like a bat out of hell and once the AP engaged does it follow the FMS/SID no it promptly turns into the terrain, disconnect and hand fly the DP, on landing call Flight Ops/Training who then call the Airbus Rep, his reply was the FMS Calculated that due to the high Rate of climb that the terrain restriction would be met and so went for the shortest distance to the next waypoint hence a right turn instead of the left turn In the SID. Ah joy!