Kaitak From Ireland, joined Aug 1999, 7354 posts, RR: 18 Posted (2 years 2 weeks 4 days 15 hours ago) and read 2449 times:
I just came across this report of an Airbus windshear incident at HKG a few years ago (it's not from an official report):
"I have some written report of an a340 windshear incident in HKG, didn't realize it was Virgin.
What I remember is that there was a typhoon some 200nm away, and windshear alearts were present, extra fuel uplifted.
The first approach was abandoned because of a windshear encounter at some 500'.
After some time in the hold, a second approach was started.
The tower issued a windshear alert of -25kt at 3 miles.
Now you have to know the LLWAS system in order to understand what happened: the systems gives out first-encouter-max-strength alert, which means if you have a medium shear at 3 miles and one more severe at 1 mile, you will get the alert that the crew got.
Sure enough, the encountered some shear at 3 miles, but it was moderate and the approach continued, believing they had passed the reported shear. Then at some 300', they encountered the big one.
The captain, who was the PF, executed a recovery maneuver (as far as i know a340, its just pullng all the way back and opening max power), and they recovered at 80' AGL, and diverted to their alternate."
80' is when you tend to get a tiny bit concerned that perhaps you should have packed your swimming togs into your hand baggage. However, that's not really what I wanted to ask ...
I was always under the impression (wrongly, it would appear from the above) that in the event of windshear, the aircraft itself would engage go around power, rather than it being a crew decision; the above suggests that it's not - they decided to continue the approach. Of course, if the aircraft did "decide" itself to go around and the crew tried to override, you'd probably end up in a China Airlines/Interflug type situation, where the aircraft just goes all over the place. (They were A300s/A310s, but can the same happen on the FBW types?)
Any Airbus pilots care to clarify this?
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Pihero From France, joined Jan 2005, 1557 posts, RR: 42 Reply 1, posted (2 years 2 weeks 4 days 15 hours ago) and read 2428 times:
You are confusing a lot of issues, here, so let's take them one by one :
Quoting Kaitak (Thread starter): The captain, who was the PF, executed a recovery maneuver (as far as i know a340, its just pullng all the way back and opening max power),
an emergency procedure called Windshear TOGA. Normally, the pilot would, as you justly said, open max power -in our case,push the throttles forward into the TOGA detent-. That action triggers on the primary flight display the move of the pitch command bar to Speed Reference mode :you just follow it as it would give you the best climb/speed performance of your airplane.(that means that you'd still fine tune your pitch inputs to "fly" the command bar}.
In your case where obviously the major concern was CFIT,your maneuver becomes a "Pull up TOGA" in which you would ask the airplane to prioritize arresting the sink rate : Full aft stick input and keep it there ! until you have exited the shear and recovered some safe altitude. That maneuver would give you the maximum lift available {if you're interested, we could go into deeper technicalities on AoAs and protections).
Quoting Kaitak (Thread starter): was always under the impression (wrongly, it would appear from the above) that in the event of windshear, the aircraft itself would engage go around power, rather than it being a crew decision
No, it won't happen unless you were fast asleep during that approach. This is one of the protections I alluded to above : if the airplane approaches a stall situation, an automatic max thrust is triggered, keeping you within a safe margin from the actual stall.
Quoting Kaitak (Thread starter): Of course, if the aircraft did "decide" itself to go around and the crew tried to override, you'd probably end up in a China Airlines/Interflug type situation, where the aircraft just goes all over the place.
This is a set-up common to a few airplanes (the 744 has it in the same conditions ) whereby once the crew has selected a decision altitude on a non-precision approach, flying below that altitude with at least an item of the flight guidance system selected (an autopilot or one flight director), the system initiates a go-around -by applying max thrust. One gets out of that situation not by fighting the aircraft -as the CA crew attempted- but by deselecting the A/P or FD that should not be "ON" anymore at that point. The reason for that flight without AP or FD lies in the "visual" part of the landing where the FD commands could-and generally do- disagree wiyh the perceived visual path, and AP minimum altitude limitations
Zeke From Hong Kong SAR, PRC, joined Dec 2006, 3648 posts, RR: 49 Reply 2, posted (2 years 2 weeks 3 days 17 hours ago) and read 2162 times:
Quoting Kaitak (Thread starter): I just came across this report of an Airbus windshear incident at HKG a few years ago (it's not from an official report):
You sure it was not a few months back, I landed at HKG the day VS had their tailstrike, it was nasty.
Quoting Kaitak (Thread starter): I was always under the impression (wrongly, it would appear from the above) that in the event of windshear, the aircraft itself would engage go around power, rather than it being a crew decision; the above suggests that it's not - they decided to continue the approach. Of course, if the aircraft did "decide" itself to go around and the crew tried to override, you'd probably end up in a China Airlines/Interflug type situation, where the aircraft just goes all over the place. (They were A300s/A310s, but can the same happen on the FBW types?)
Its the pilot.
The airbus has predictive windshear (part of the weather radar system) and reactive (part of the FBW system), if you get a windshear caution/waning its the pilot that starts the recovery.
Most people go around in those conditions not from a windshear warning/caution, they go around because the approach becomes unstable, i.e. go beyond predefined glide slope, localiser, vertical speed, air speed, bank or pitch tolerances for the final part of an approach.
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Starlionblue From Greenland, joined Feb 2004, 12750 posts, RR: 57 Reply 3, posted (2 years 2 weeks 3 days 17 hours ago) and read 2141 times:
First of all thx to Pihero and Zeke for their usual cogent and detailed answers. I have finally learned why the FD needs to be off during a non precision approach.
Quoting Kaitak (Thread starter): 80' is when you tend to get a tiny bit concerned that perhaps you should have packed your swimming togs into your hand baggage.
Go arounds can in some conditions have the wheels on the runway at some point. That is, even if you go to TOGA thrust, the aircraft is still descending and will not start climbing until after altitude 0.
[Edited 2006-09-28 22:26:31]
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Kaitak From Ireland, joined Aug 1999, 7354 posts, RR: 18 Reply 6, posted (2 years 2 weeks 3 days 15 hours ago) and read 2071 times:
Thanks very much everyone; very informative and educational!
So, that means that the A320 (or any Airbus - or indeed any other - FBW aircraft) is just as susceptible to being brought down by windshear as any other aircraft, if the crew doesn't react in time or properly?
"What are we going to do tonight, Brain?" "Same thing we do every night, Pinky. Try to take over the world".
Starlionblue From Greenland, joined Feb 2004, 12750 posts, RR: 57 Reply 7, posted (2 years 2 weeks 3 days 14 hours ago) and read 2048 times:
Quoting Kaitak (Reply 6): So, that means that the A320 (or any Airbus - or indeed any other - FBW aircraft) is just as susceptible to being brought down by windshear as any other aircraft, if the crew doesn't react in time or properly?
Yes and no. There is alpha floor functionality involved, which makes the aircraft well nigh impossible to stall. However, there are considerations with flare mode that I shall let the experts get into.
In any case, FBW Airbi may have more protection, but that doesn't mean you can't crash them as has been unfortunately proven multiple times.
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Kaddyuk From Wallis And Futuna Islands, joined Nov 2001, 4124 posts, RR: 27 Reply 8, posted (2 years 2 weeks 3 days 13 hours ago) and read 2025 times:
Quoting Starlionblue (Reply 7): In any case, FBW Airbi may have more protection, but that doesn't mean you can't crash them as has been unfortunately proven multiple times.
The problem with the -600 is its length... it tends to sink into the rotation instead of floating on the ground effect, it just tends to pivot and then strike further forward that airbus designed it to... That is why the software onboard the aircraft doesnt function correctly at times, when the AC sinks, the strike indicator moves, but it moves too far because the sensor is in the wrong place, because its in the wrong place, the pilot over compensates or not enough and strike...
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Jben From Australia, joined Aug 2006, 71 posts, RR: 2 Reply 9, posted (2 years 2 weeks 3 days 11 hours ago) and read 1975 times:
I'm interested in a couple of things...
Firstly, are the protections the same on all airbus aircraft? Is the software upgraded often, and has airbus ever changed the flight laws?
Secondly, an airbus normally flies in normal law... is it possible to get the plane into alternate law or lesser mode of operation? I'm just wondering what happens if the flight control software does something obviously dangerous, or refuses to do something necessary to avoid a collision (which would involve taking it outside the envelope protection)?
One other thing, with things like the alpha floor, bank protection, etc... are they hard limits, in that the plane will not let you go over them, or do they have some softness in them to let you exceed them, but then force you back to the hard limit?
Starlionblue From Greenland, joined Feb 2004, 12750 posts, RR: 57 Reply 10, posted (2 years 2 weeks 3 days 10 hours ago) and read 1952 times:
Quoting Jben (Reply 9): Firstly, are the protections the same on all airbus aircraft? Is the software upgraded often, and has airbus ever changed the flight laws?
All Airbus FBW aircraft (A318-321, 330, 340, 380) have the same protection system. 350 will also have the protection system. 300 and 310 do not have it.
Quoting Jben (Reply 9):
Secondly, an airbus normally flies in normal law... is it possible to get the plane into alternate law or lesser mode of operation? I'm just wondering what happens if the flight control software does something obviously dangerous, or refuses to do something necessary to avoid a collision (which would involve taking it outside the envelope protection)?
"Refuses to do something necessary to avoid a collision" is the antithesis of what the hard limits allow. By allowing maximum safe maneuvers the pilots can fly closer to the edge of the envelope without worrying that the wings will snap or the plane will stall. So if you are about to crash into a hill, envelope protection will allow the tightest pull-up, balancing on a razor edge with maximum turn rate, but without either stall or structural failure.
As for situations where normal law would be a bad idea, there is unusual attitude recovery. For example, if the aircraft is upside down, the aircraft enters "Abnormal Alternate Law", allowing recovery.
As for disconnecting normal law manually, I don't know if there is such a mechanism. I assume there is. I guess you could start pulling circuit breakers.
Quoting Jben (Reply 9): One other thing, with things like the alpha floor, bank protection, etc... are they hard limits, in that the plane will not let you go over them, or do they have some softness in them to let you exceed them, but then force you back to the hard limit?
They are hard limits. No softness allowed. As I said before this allows a pilot to pull as hard as he wants and get the absolute max the plane can take. The "Pull up TOGA" scenario described by Piherio is a good example. By simply pulling back on the stick to the max and throttles to TOGA, the aircraft will fly with maximum available lift at that speed. In a "conventional" aircraft, the pilot would have to worry about stalling the plane, and thus would have to fly further from the edge of the envelope, getting less lift.
Zeke From Hong Kong SAR, PRC, joined Dec 2006, 3648 posts, RR: 49 Reply 11, posted (2 years 2 weeks 3 days 10 hours ago) and read 1951 times:
Quoting Kaddyuk (Reply 8): The problem with the -600 is its length... it tends to sink into the rotation instead of floating on the ground effect, it just tends to pivot and then strike further forward that airbus designed it to... That is why the software onboard the aircraft doesnt function correctly at times, when the AC sinks, the strike indicator moves, but it moves too far because the sensor is in the wrong place, because its in the wrong place, the pilot over compensates or not enough and strike...
This is incorrect.
Quoting Jben (Reply 9): Firstly, are the protections the same on all airbus aircraft? Is the software upgraded often, and has airbus ever changed the flight laws?
Not the laws per se, minor changes withing the laws have changed, however the laws remain, i.e. direct, normal, alternate, flare etc. The A380 has a slightly different yaw law than previous types.
The software does change from time to time, new upgrades brought about from new models tend to filter their way across all fleets. The actual changes are unnoticeable to the flight crew.
Quoting Jben (Reply 9): Secondly, an airbus normally flies in normal law... is it possible to get the plane into alternate law or lesser mode of operation? I'm just wondering what happens if the flight control software does something obviously dangerous, or refuses to do something necessary to avoid a collision (which would involve taking it outside the envelope protection)?
The laws will actually help prevent a collision. IFALPA have done CFIT studies and find that hard protections leads to more consistent and better responses.
Quoting Jben (Reply 9): One other thing, with things like the alpha floor, bank protection, etc... are they hard limits, in that the plane will not let you go over them, or do they have some softness in them to let you exceed them, but then force you back to the hard limit?
The hard limits correspond to certified design limits. If for example that aircraft was in an unusual attitude for whatever reason these hard limits are taken away.
The laws change with the phase of flight, for the very last part of the landing, and for takeoff, the laws transition from a direct law to a normal law.
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