DATE: December 10, 2014 AD #: 2014-25-51 Emergency Airworthiness Directive (AD) 2014-25-51 is sent to owners and operators of Airbus Model A318, A319, A320, and A321 series airplanes. Background The European Aviation Safety Agency (EASA), which is the Technical Agent for the Member States of the European Community, has issued Emergency Airworthiness Directive 2014-0266-E, dated December 9, 2014 (referred to after this as the Mandatory Continuing Airworthiness Information, or the MCAI), to correct an unsafe condition on all Model A318, A319, A320, and A321 series airplanes. The MCAI states: An occurrence was reported where an Airbus A321 aeroplane encountered a blockage of two Angle of Attack (AoA) probes during climb, leading to activation of the Alpha Protection (Alpha Prot) while the Mach number increased. The flightcrew managed to regain full control and the flight landed uneventfully. When Alpha Prot is activated due to blocked AoA probes, the flight control laws order a continuous nose down pitch rate that, in a worst case scenario, cannot be stopped with backward sidestick inputs, even in the full backward position. If the Mach number increases during a nose down order, the AoA value of the Alpha Prot will continue to decrease. As a result, the flight control laws will continue to order a nose down pitch rate, even if the speed is above minimum selectable speed, known as VLS. This condition, if not corrected, could result in loss of control of the aeroplane. To address this unsafe condition, Airbus *** [has] developed a specific Aircraft Flight Manual (AFM) procedure, which has been published in AFM Temporary Revision (TR) No. 502. For the reasons described above, this AD requires amendment of the applicable AFM [to advise the flightcrew of emergency procedures for abnormal Alpha Prot].
I'm not well informed on Air Bus instrumentation and I wonder what the AoA probe configuration is. I remember the AoA Indicator on the Boeings is a vane on a swivel. I guess that without heat, like the pitot probe, it could be frozen in place. I'll have to edify myself by talking with my old gang. This AD looks a bit spooky.
Well there's your problem.... ....damn thing plummets to earth on its own accord despite what the "Pilot in Command" asks it to do.
Complimentary FAA Emergency AD http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgAD.nsf/0/a9d1d41e719a5c8b86257daa007d72ef/$FILE/2014-25-52_Emergency.pdf
or the wings fall off due to failed fasteners http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgAD.nsf/0/d89947f24ddb126786257db0007325db/$FILE/2014-26-53_Emergency.pdf
Of course AD's are issued quite often. Both Boeing and Airbus have had more than a handful in the last 30 days. Airworthiness Directives
The only problem I've ever had with an AOA probe is when a gate agent smashed into it with a jet bridge. I don't understand how one could clog, as it's kind of like a small air foil. Maybe the AOA/probe heat was inop and they iced up?
I assume the Airbus has the airfoil style of AOA probe? Some AOA probes look like a little cone sticking out the side of the airplane with a slot on them. Those I could see clogging.
Don- Nearly all of those AoA (and yaw) probes like the one you described are heated and it takes quite a bit for one of them to freeze or clog. We tested the heat (crew chief) before take-off. You did not have to touch it to feel the heat.
Pitot tubes are heated as well... and they still ice over (at least on Airbusses). Also, the slot could be clogged by bugs or debris. Apparently the A320 actually uses the little airfoil thing, and also apparently at one point Thales was using an incorrect lubricating oil, which could cause them to bind up in cold temperatures. So it doesn't necessarily even require ice to fail.
If a pitot tube is taped over during maintenance and the tape is not removed, something similar could happen; it did happen to at least one Boeing 757. Considering that the AirAsia flight is now believed to have flown into an area of heavy icing, one could envision this being a cause of the recent crash. I hope that the recorders are found soon.....
It's not a giant leap to think that Airbus have already determined what they believe to be the cause of this recent crash based on data they have... I recall reading quite a bit of press about the data transmission back to "home base" in the Malaysian Airlines disappearance. I wonder if the AirAsia plane was outfitted with the same sort of data telemetry that has already been analyzed.
One of at least two...I'm thinking the probe heat failed or it was selected off for some reason. I'll ask the next time hitch a ride on the bus. Image Unavailable, Please Login
They may have made that connection, I don't know. However, this AD was issued before that accident took place, so unless they can see into the future, it's not directly related.
Incidentally, here are accident reports on two 757s that were done in by pitot or static port problems, but both were due to maintenance issues. Birgenair Flight 301 - Wikipedia, the free encyclopedia - in this case, it was believed that the pitots were left uncovered for a long period while the airplane was not in use, resulting in blockage caused by debris, including the possibility of a wasp nest! Aeroperú Flight 603 - Wikipedia, the free encyclopedia - here, it was determined that duct tape had been left over the static ports during maintenance and not removed.
A similar incident occurred at Boeing in 1960 on the final assembly line. Pressure test on the center section of TWA's first -320 was in progress and the supervisor that was in charge of the test bench that was controlling the pressure left for lunch while pressure was being applied to the center section to check for leaks. As I was walking down a transportation aisle near it there was an explosion. It was the center section. An investigation determined that the pressure sensing port on the test bench was taped over and could not read the over pressure that blew up the center section. The test bench had just been overhauled and repainted and the tape had not been removed from the port opening. Not as disastrous as the afore mentioned incident but a costly repair. Man causes more failures than fate, as mentioned in Ernie Gann's "Fate Is The Hunter".
I've said many times here that the programming of the A300 series flight control system isn't robust. That is, the failure of sensors that can happen routinely can cause the flight control system to lose control of the aircraft. This happened in the A320 crash in 2008 and had to do with the same AOA sensor. Here is the post mortem on that crash. Sensor icing caught out A320 crew in Perpignan crash - 9/16/2010 - Flight Global This AD is another example of how the loss of two sensors can cause the whole flight control system to think that one thing is happening and have it all go to hell in a hand basket. The previously noted crash was percipitated by a stall event, and this AD is concerning overspeed, but the bottom line is that the software is in control and it can be fooled. If you're going to have that level of automation in the aircraft, the software has to be sufficiently robust that any combination of failures doesn't fool it, or that when it throws up it's hands, it gives control back to the pilot with the level of sophistication of a 172. The current FAA standard for software is that it must be able to sustain control when you have two failures. That is double redundancy. The reality is that often you have a failure, or you might have a failure prone sensor, and at that point you really on have single redundancy. While the results often aren't an issue since most operations aren't in horrible weather or at envelope extremes, when it happens (as it eventually does) that you lose that key third sensor, in these aircraft there seems to be little margin for error and the result is predictable. In this instance the control system is actually not in compliance with FAA standards if the loss of only the two AOA sensors can cause loss of control. Like I've been saying, this isn't a very robust flight control system.
I'm not a fan of the Airbus flight control system logic. They should have definitely had a probe heat fail/off message present. With that message, probe heat selected off, or any pitot/AOA probe failure, the flight control logic should (IMO) automatically inhibit or be able to be switched off.
