BAW492 diversion at Gibraltar
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The other “heard from a mate in BA...” story is clearly nonsense.
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Gibralter have recently introduced RNP(AR) approaches (last year?). Boeing and Naverus got down to 0.11nm minima on the 737. Maybe Airbus tried to outdo them and went for a 0.1nm minima. To achieve this it seems they used some software intended for tracking an engine failure/engine out. Bank angle limits on say an ILS are single figures, but on an RNP(AR) approach it is 30 degrees - but that should be a constant bank to achieve an RF leg. Somehow it got lost in translation and Airbus software thinks you can now roll thru a 60 degree arc. A320 roll rates are 30 degrees/second clean, 25 deg/sec dirty and only 15 deg/sec in manual flight.
The RNP(AR) software “operates outside normal flight control laws” and it seems roll rates are unlimited. The software gets stuck in a “loop” rolling thru a 60 degree arc. It will do the same in the SIM if you put in a spot wind of 40|45 knots (crosswind) and mod/sev turbulence - on an RNP(AR) approach. It is too sensitive and too reactive.
It’s a bit like the Airbus Rudder Limiter. It seems to have got lost in translation and software also limits rudder pedal movement. As little as 1 inch of rudder pedal travel will achieve full rudder deflection for the given speed, as Air Canada A319 ACA 190 encountered 10 Jan 2008. Explained in in the report under “Rudder Control System”. (TSB GC CA a08w0007 2008)
Should a bank angle limit become a roll limit (60 degree arc), should a Rudder Limiter limit rudder pedal travel or just rudder deflection?
The crew in Gibralter most likely experienced some Spatial Disorientation and what was perceived/reported as “turbulence” was in fact as a result of g loading and spoiler deflection/buffet. Recommendation for spatial disorientation is to keep the autopilot engaged - crew did everything by the book. Info plus some accident history on site code7700.com under spatial disorientation.
The RNP(AR) software “operates outside normal flight control laws” and it seems roll rates are unlimited. The software gets stuck in a “loop” rolling thru a 60 degree arc. It will do the same in the SIM if you put in a spot wind of 40|45 knots (crosswind) and mod/sev turbulence - on an RNP(AR) approach. It is too sensitive and too reactive.
It’s a bit like the Airbus Rudder Limiter. It seems to have got lost in translation and software also limits rudder pedal movement. As little as 1 inch of rudder pedal travel will achieve full rudder deflection for the given speed, as Air Canada A319 ACA 190 encountered 10 Jan 2008. Explained in in the report under “Rudder Control System”. (TSB GC CA a08w0007 2008)
Should a bank angle limit become a roll limit (60 degree arc), should a Rudder Limiter limit rudder pedal travel or just rudder deflection?
The crew in Gibralter most likely experienced some Spatial Disorientation and what was perceived/reported as “turbulence” was in fact as a result of g loading and spoiler deflection/buffet. Recommendation for spatial disorientation is to keep the autopilot engaged - crew did everything by the book. Info plus some accident history on site code7700.com under spatial disorientation.
Last edited by BlueUpBrownDown; 3rd Mar 2019 at 03:09.
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Auto Pilot/FG Architecture.
The FG has two loops – essentially two software logic circuits. - the Inner Loop - the Guidance loop The inner loop is the more reactive one and gives short-term commands while the guidance loop has more of a mid-term action/role. The two loops are controlled by two different cards/processors. When in approach, the gain of the inner loop is different from cruise so it is more reactive to any perturbation that could make the A/C deviate from its path. With FMS release 1A when using the RNP AR mode the gain laws have been tightened so the rate of change to guidance commands is enhanced and will mean more rapid adjustment if necessary to maintain desired flight path accuracy.
Airbus Findings
The aircraft was subjected to a lateral gust of wind which was the root cause of the commencement of oscillations. The Auto Pilot inner loop, being more reactive in RNP approach mode, was the cause of the abrupt roll. It was confirmed in simulator tests that these dedicated laws can, in a few cases, generate lateral oscillations when associated with some specific strong lateral gusts. These oscillations did not exceed the RNP AR limits and oscillations remained contained.
Summary
Abrupt roll oscillations may be experienced on an RNP AR approach in gusty conditions. This is most likely associated with the A/P FG architecture. The aircraft should stay within its RNP XX boundaries and the A/P should remain engaged.
The FG has two loops – essentially two software logic circuits. - the Inner Loop - the Guidance loop The inner loop is the more reactive one and gives short-term commands while the guidance loop has more of a mid-term action/role. The two loops are controlled by two different cards/processors. When in approach, the gain of the inner loop is different from cruise so it is more reactive to any perturbation that could make the A/C deviate from its path. With FMS release 1A when using the RNP AR mode the gain laws have been tightened so the rate of change to guidance commands is enhanced and will mean more rapid adjustment if necessary to maintain desired flight path accuracy.
Airbus Findings
The aircraft was subjected to a lateral gust of wind which was the root cause of the commencement of oscillations. The Auto Pilot inner loop, being more reactive in RNP approach mode, was the cause of the abrupt roll. It was confirmed in simulator tests that these dedicated laws can, in a few cases, generate lateral oscillations when associated with some specific strong lateral gusts. These oscillations did not exceed the RNP AR limits and oscillations remained contained.
Summary
Abrupt roll oscillations may be experienced on an RNP AR approach in gusty conditions. This is most likely associated with the A/P FG architecture. The aircraft should stay within its RNP XX boundaries and the A/P should remain engaged.
Auto Pilot/FG Architecture.
The FG has two loops – essentially two software logic circuits. - the Inner Loop - the Guidance loop The inner loop is the more reactive one and gives short-term commands while the guidance loop has more of a mid-term action/role. The two loops are controlled by two different cards/processors. When in approach, the gain of the inner loop is different from cruise so it is more reactive to any perturbation that could make the A/C deviate from its path. With FMS release 1A when using the RNP AR mode the gain laws have been tightened so the rate of change to guidance commands is enhanced and will mean more rapid adjustment if necessary to maintain desired flight path accuracy.
Airbus Findings
The aircraft was subjected to a lateral gust of wind which was the root cause of the commencement of oscillations. The Auto Pilot inner loop, being more reactive in RNP approach mode, was the cause of the abrupt roll. It was confirmed in simulator tests that these dedicated laws can, in a few cases, generate lateral oscillations when associated with some specific strong lateral gusts. These oscillations did not exceed the RNP AR limits and oscillations remained contained.
Summary
Abrupt roll oscillations may be experienced on an RNP AR approach in gusty conditions. This is most likely associated with the A/P FG architecture. The aircraft should stay within its RNP XX boundaries and the A/P should remain engaged.
The FG has two loops – essentially two software logic circuits. - the Inner Loop - the Guidance loop The inner loop is the more reactive one and gives short-term commands while the guidance loop has more of a mid-term action/role. The two loops are controlled by two different cards/processors. When in approach, the gain of the inner loop is different from cruise so it is more reactive to any perturbation that could make the A/C deviate from its path. With FMS release 1A when using the RNP AR mode the gain laws have been tightened so the rate of change to guidance commands is enhanced and will mean more rapid adjustment if necessary to maintain desired flight path accuracy.
Airbus Findings
The aircraft was subjected to a lateral gust of wind which was the root cause of the commencement of oscillations. The Auto Pilot inner loop, being more reactive in RNP approach mode, was the cause of the abrupt roll. It was confirmed in simulator tests that these dedicated laws can, in a few cases, generate lateral oscillations when associated with some specific strong lateral gusts. These oscillations did not exceed the RNP AR limits and oscillations remained contained.
Summary
Abrupt roll oscillations may be experienced on an RNP AR approach in gusty conditions. This is most likely associated with the A/P FG architecture. The aircraft should stay within its RNP XX boundaries and the A/P should remain engaged.
In case it’s not clear, the above quote is related to a different roll oscillation incident than Gibraltar. Note also that the recommendation is to leave the A/P engaged.
Originally Posted by Doug E Style
If you can say what your actions would have been after having a bit of a think about it having watched a couple of posted videos, rather than the information available to the crew, not to mention the considerable “startle-factor”, then that’s a bit worrying.
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Now ask yourself: what would the average TRE in your airline say you should do on an RNP approach in IMC with a sudden a/p failure? My bet would be most will say to g/a and ask for another type of approach.
This is the mentality I've found most common during my time with 2 of Europe's largest carriers. So I can clearly see why MANY flight crews would sit and watch the a/p misbehave on an RNP approach and do nothing, even in vmc.
The same many are usually great at quoting details from the fcoms though.
It’s fairly likely the idea of disconnecting the AP was thought of and discussed during the incident; after all, the oscillations went on for some time.
You’re sat in the pilot’s seat of a fully FBW aeroplane, which develops an uncomfortable but contained roll oscillation with AP engaged. You’re climbing away from the ground and the wobbles appear to be getting neither better nor worse, giving time for some diagnosis/options/action. One possible scenario could be that there is a severe flight control issue which the AP and/or envelope protection is only just dealing with and going “manual” might make the situation worse or lead to LOC. There is always the pressure to DO SOMETHING but what if the cure is worse than the disease? The AP might readily disengage but not want to go back in...
I’m pretty sure that if the rolling had carried on after config. changes, at some point they’d have agreed to take the AP out. After all, landing in that condition is not going to be fun.
You’re sat in the pilot’s seat of a fully FBW aeroplane, which develops an uncomfortable but contained roll oscillation with AP engaged. You’re climbing away from the ground and the wobbles appear to be getting neither better nor worse, giving time for some diagnosis/options/action. One possible scenario could be that there is a severe flight control issue which the AP and/or envelope protection is only just dealing with and going “manual” might make the situation worse or lead to LOC. There is always the pressure to DO SOMETHING but what if the cure is worse than the disease? The AP might readily disengage but not want to go back in...
I’m pretty sure that if the rolling had carried on after config. changes, at some point they’d have agreed to take the AP out. After all, landing in that condition is not going to be fun.
Non disengagement of the autopilot
Originated in BEA where some of the bomber boys could not cope with the Trident especially on approach way behind the drag curve and was the reason their monitored approach was developed. It was also the reason that Hamble was taken over by the corporations in 1960 when there were thousands of ex military pilots available.
Key facts from our Airbus Flight Tech Manager:
Autopilot remained on throughout. Conf Full with Gear down in FINAL APP mode. Windshear at 700ft followed by GPWS "sink rate". TOGA with configuration unchanged as per windshear SOPs. After selecting Conf 3 roll reduced, accelerated and cleaned up normally.
Autopilot remained on throughout. Conf Full with Gear down in FINAL APP mode. Windshear at 700ft followed by GPWS "sink rate". TOGA with configuration unchanged as per windshear SOPs. After selecting Conf 3 roll reduced, accelerated and cleaned up normally.
Blind Pew, you obviously have an axe to grind. Whatever your motives are it’s not very edifying and suggests bitterness and a lack of factual knowledge.
For your your education the monitored approach was developed to combat the three or so seconds it takes the human brain to switch cognitive tasks, in this case the transition from instrument flying to visual flying at CAT 1 (or in past times CAT 2) minima. The science showed (shows) that a pilot already processing visual references as soon as they became available made (makes) a better job of the final approach, flare and landing. Some types / companies still legitimately perform manual landings off a CAT 2 approach, BA doesn’t.
FWIW as a recent product of the above training department it is top notch from the top down.
I’d suggest this event was a surprise for everybody involved, was well handled (others may have chosen to do it differently and that may well have resulted in just as good an outcome), and you can bet the manufacturer is working on it as a matter of urgency.
LD
Oh dear Locked Door you seem to have swallowed the fly to swerve mantra. Blindpew, who I have absolute respect for, unfortunately knows a lot more about this than you ever will.
Please promise me that you will not sneer at the rest of us in the UK flying similar types but the corporation really is not the panacea and superlative you think it is.That might be quite hard for you to understand and appreciate having swallowed the company mantra.
The aviators of the 1960s and 1970s were pilots and they were flying difficult and poorly designed aircraft.They were great at handflying. A Captain who had been used to a Lancaster or a Halifax in his youth would struggle in a Trident with its BEA spec flight system , a system that the rest of the world rejected!
Let the boy fly the approach for you but do the landing yourself, however then have the boy pull reverse for you on landing. It sounds stupid and it was . Make sure that you wear your hat though!
Please promise me that you will not sneer at the rest of us in the UK flying similar types but the corporation really is not the panacea and superlative you think it is.That might be quite hard for you to understand and appreciate having swallowed the company mantra.
The aviators of the 1960s and 1970s were pilots and they were flying difficult and poorly designed aircraft.They were great at handflying. A Captain who had been used to a Lancaster or a Halifax in his youth would struggle in a Trident with its BEA spec flight system , a system that the rest of the world rejected!
Let the boy fly the approach for you but do the landing yourself, however then have the boy pull reverse for you on landing. It sounds stupid and it was . Make sure that you wear your hat though!
LD?
I went from BEA monitored approach with the "decide...change hands" to the BOAC monitored approach with the PM continuing to monitor the aircraft (experimental cross transfer after BA was formed ) and then onto the Swiss monitored approach which was a cross between the two.
The most foolish part of the lot was changing who was operating the throttles.
Recently at a guild knees up one of my first managers admitted that with hindsight they had got it wrong (we bent eight aircraft in my six years).
I learnt that passenger comfort and allaying their fears was not only part of the job but deferentiated between professionals and cowboys; this included not doing emergency descents with reverse, explaining (truthfully) to the punters what was going on, not planting the aircraft on the aiming point and not letting the automatics do silly things scaring the #### out of everyone.
I took these skills into light aircraft and glider instructing were you can see the fear in the punters first hand rather hiding behind a locked door.
In SR we were expected to greet then thank the pax when disembarking. .try it sometime as it teaches you humility.
There are still some that show that courtesy to pax.
Without doubt many aviators would not have left the autopilot doing unpleasant gyrations..there are times in your career that you will realise that company procedures do not cater for every contingency and you have to act on your own initiative.
It appears that the crew excepted 40 degrees of bank..how far would it have gone before they intervened and what could have been the consequences?
The most foolish part of the lot was changing who was operating the throttles.
Recently at a guild knees up one of my first managers admitted that with hindsight they had got it wrong (we bent eight aircraft in my six years).
I learnt that passenger comfort and allaying their fears was not only part of the job but deferentiated between professionals and cowboys; this included not doing emergency descents with reverse, explaining (truthfully) to the punters what was going on, not planting the aircraft on the aiming point and not letting the automatics do silly things scaring the #### out of everyone.
I took these skills into light aircraft and glider instructing were you can see the fear in the punters first hand rather hiding behind a locked door.
In SR we were expected to greet then thank the pax when disembarking. .try it sometime as it teaches you humility.
There are still some that show that courtesy to pax.
Without doubt many aviators would not have left the autopilot doing unpleasant gyrations..there are times in your career that you will realise that company procedures do not cater for every contingency and you have to act on your own initiative.
It appears that the crew excepted 40 degrees of bank..how far would it have gone before they intervened and what could have been the consequences?