Airbus: Potential Problems

[Lu Zuckerman]

TO THE READER: The following material was transmitted to the FAA,TRANSPORT CANADA AND THE DGCA


RMS Engineering Fax Cover Sheet

From:S L Zuckerman
To: Norm Martenson
Date of transmission : 21 December 1999
No. of pages including cover sheet: 2

Dear Mr. Martenson, Note to the reader: After sending this material to the FAA and the Canadian MOT everything got real silent. I also sent the same material to the DGCA of France and they never replied to any of my messages.

I appreciate your rapid response to my request for the A-310 AD. Now, I have an additional request. I need more information. What I am trying to do is to reconcile the differences between the Canadian MOT AD on the inspection of the Vespel Bushing and the FAA AD on the same subject. The MOT AD indicates that if the Vespel Bushings are inspected in according with the Airbus Service bulletin, and are found to be damaged, or, are found to have electrical continuity, they are to be replaced. The FAA AD 94-01-12 does not reference an electrical continuity check of the Vespel Bushes although it does reference an electrical continuity check of the flap system.

What I find difficult to understand is that the MOT AD states that if the Vespel Bushes have electrical continuity, they should be replaced. When the flap and slat drive systems were designed, the Vespel Bushes were Made by an English firm and they were referred to as Rose Bushes (named after the firm) and the Rose Bushes were impregnated with a conductive carbon compound that according to design was electrically conductive. This ,according to design, would tie the systems together providing bonding to the airframe.(THIS WAS NEVER PROVED IN PRACTICE) The only places on both the flap and slat systems that were bonded directly to the airframe were the power control units which were at the beginning of the drive system, and the Position Pick Off Units which were at the wing tips. All other elements of the system were floating in reference to ground. This was discovered when the flap/slat "Iron Birds" were checked. The Iron Bird installation is exactly like the aircraft installation. My problem is, did the MOT AD which was translated directly from French get it right, or if the Vespel bushes are to be replaced if found to be electrically conductive then what Airbus Service Bulletin changed the design.

During the final stages of the design of the A-310 wing, it was discovered that the flap and slat systems were floating in relation to airframe ground. This could result in a catastrophic event, if the partially extended slat was struck by lightning,it would result in in the wing being blown off, due to ignition of the fuel in the wing tank. It could also cause interference problems with the electronic systems on the air craft.

In the case of the flap, a static charge of 800 to 1200 volts would build up when the flaps were extended. When the flaps were retracted, this high level charge would discharge into the upper wing skin or the rear spar causing electrostatic discharge pitting in those areas resulting in the weakening of the structure.

So, getting back to my original request, I need copies of the Airbus Service Bulletins that caused the ADs to be created.

If you are interested in the following, I will be happy to provide you with any necessary information. I should tell you, this same information was provided to your office in 1985. It was sent to a Mr. Waterman. Your office took action and two people that worked in a German firm were fired, but, nothing was done to change the design.

1) The Flap system was never fully tested and therefore, should not have been certificated.
2) The same goes for the Flap/Slat Computer.
3) The power control units have a potential failure mode that could result in an uncommanded extension or retraction of the flaps or slats, which could not be stopped by the computer. An uncommanded extension of the slats would not pose a problem as the air loads would prevent extension. However, an uncommanded retraction of the slats could cause a major problem. An uncommanded but unarrested flap extension could result in the loss of the aircraft
4) An A-320 in Canada has experienced an uncommanded retraction of the flaps during take-off and the pilots almost lost it. They had to apply full power to carry them through.
5) All Airbus aircraft share a common hydraulic system architecture and are subject to these same failure modes.

The systems safety requirements set up by the FAA JAR, state that the uncommanded extension or retraction of a flap or slat system should occur no more frequently than one time in a billion hours for a given fleet of aircraft.
If this is true, this should statisticaly never occur on an A-320 or its’ derivatives in our lifetime or the lifetimes of the next twenty generations of our collective families

Very truly yours,

S L Zuckerman RMS Engineering

The following is included for pilots of Airbus aircraft.

Airbus Industries does not recognize the uncommanded actuation of either the flaps or slats as it is their contention that the flap/slat computer will shut the system down. In reality, the computer can not stop what it didn’t start. Therefore, the procedure is not taught in the training syllabus. Most if not all Airbus aircraft share a common hydraulic systems architecture. Slats powered by Green and Blue. Flaps powered by Green and Yellow. If you suffer a runaway in flight, shut down both systems powering the effected control system. Turn one on. If the controls move, turn that system off and turn the other system on. Or, if when you turn the first system on and the controls don’t move leave the other system in the off position. It has to be done quickly or you can lose it.

I have to say one thing and that this is based on my personal knowledge as a senior R&M engineer working on the system as it was being designed. Things may have been modified but based on no correcting ADs having been issued I doubt it seriously.

If you have any questions please contact me.

Lu Zuckerman
[email protected]



------------------
The Cat

[N2]

Lu,

If I understand your post, Your concern is that a static build up / discharge could cause an uncommanded movement of the flap / slat system on the A310.

There is a solenoid pack on each flap drive motor which needs to be energized to port fluid to drive the system. Could you pls explain further how this discharge could cause these solenoids to fail,porting fluid to drive the system. A final thought, in the event this did occur, would there not now be a difference between flap handle CSU position and drive system IPPU / APPU position now signaling a symetrical runaway to the SFCC's tripping the wing tip brakes?

N2

[Lu Zuckerman]

The static build up on the flaps is allowed by the fact that the flaps as well as the slats are free floating relative to the airframe. This problem can lead to serious concequences if a partially deployed slat were to be struck by lightning. By serious I mean that the wing at the least could catch fire and at the worst, the outer panel of the wing could be blown off. This problem has nothing to do with an uncommanded deployment of the flaps and slats. You are correct about the solenoids that control speed and direction but in this case it is an internal leak that bypasses the solenoids and goes directly to the hydraulic motor on the effected hydraulic module on the PCU. It is true, the PPUs would indicate movement but if the CSU is in the flaps and slats retracted position the computer is out of the loop and can't stop the action. This was demonstrated on the slat Iron Bird during testing. The manufacturer of the PCU was supposed to notify Airbus about the runaway but they didn't. Instead they reworked 17 ship sets and contacted Lufthansa and Swiss Air telling them that they had improved the design and would swap out the reworked PCUs for the PCUs on the respective aircraft. This was in total conflict with the contract and it reflected poor judgement on the part of the company officers. After I notified the FAA the Vice President and Cheif program manager were fired but the lack of bonding and the design of the PCUs were never changed. I have contacted many US operators of Airbus aircraft telling them of the potential for runaway and that they should incorporate procedures in their training syllabus in how to counter the runaway. After about a year in doing this I was contacted by US Airbus telling me that I was defaming their product. I told them I was only interested in safety. They told me to stop and I told them that I would if I could talk to one of their engineers. They told me they would set it up. That was over five years ago and I am still waiting for the call. That is why I posted my material on this forum. If you are interested in helicopter safety please read my postings on ROTORHEADS IN THIS FORUM.

More recently I flew in an Air Canada A320 from Toronto to Chicago.. I was up front on the flight deck telling the pilots about the potential for runaway on the flaps and slats due to the hydraulic system architecture which is the same on all Airbus aircraft. While I was still talking, the First Officer who was also Air Canadas' director of flight safety, handed me a fax. It indicated that two days earlier an Air Canada A320 had an uncommanded retraction of the flaps on take off. In this case, the CSU was in the flap slat takeoff position and the computer didn't catch it. The pilots almost lost the aircraft and their only salvation was to apply full throttle and fight the aircraft until it was in a stable flight regime.

A similar internal leak caused the thrust reverser to deploy in flight on a Lauda Air 767 causing it to crash and it may be eventually proved that an internal leak caused the 737 rudder problems.
That is my opinion.