============================================== A just-released report reveals that a popular upper-range business jet had a potentially fatal design flaw which could disable both channels of a pitch control system simultaneously, with no reversion system.
The UK Air Accident Investigation Branch (AAIB) report describes the crew's loss of control over the horizontal stabiliser in a Bombardier Challenger 604 business jet (VP-BJM) en-route from Lagos, Nigeria to Farnborough, UK on 11 November 2005.
It was diverted to London Heathrow because Farnborough's runway was not long enough for a flapless approach, and carried out a safe landing there.
The cause, says the report, was the ingress of moisture - apparently condensation from normal ground-level humidity - into the single horizontal stabiliser trim control unit (HSTCU) box which contained the control circuitry for both the "independent" channels that controlled the two separate stabiliser actuators.
The AAIB concluded: "In the absence of a mechanical backup system or sufficient physical separation of the control channels, there was insufficient protection within the design of the HSTCU against the effects of environmental contamination."
The Board adds: "The airworthiness requirements relating to the design and installation of electronic components did not sufficiently address the specific effects of fluid and moisture contamination as a source of common-cause failures."
The report notes that both the aircraft and system manufacturers were aware that there had been similar occurrences previously.
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There seems to be two separate issues here....insufficient seals against the elements, and a single point of failure (the HSTCU). If design problems are found on the Challenger, it would seem prudent to also examine the CRJ series for similar issues.
In this case, the moisture contamination issue seems to be common between the two families of aircraft. Is the CRJ series also susceptible to (as I understand it) this single point of failure with regard to the stabilizer actuators?
CptSpeaking From United States of America, joined Apr 2006, 639 posts, RR: 1 Reply 1, posted (5 years 4 months 1 week 4 days ago) and read 3445 times:
Ya know...I was just reading a thread on APC the other day about somebody who had this problem in the CRJ 200...their QRH called for a flaps 20 landing instead of flaps 0 though...apparently the failure is more common than you'd think, but normally the trim can be reconnected in-flight.
Tdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 81 Reply 2, posted (5 years 4 months 1 week 4 days ago) and read 3420 times:
Quoting 2H4 (Thread starter): There seems to be two separate issues here....insufficient seals against the elements, and a single point of failure (the HSTCU).
I'm not sure this is really a single point of failure issue...I certainly agree about the seals.
If the cause is really environmental moisture, having one box or two shouldn't make too much difference since both boxes live in the same environment and you could have a common mode failure.
DC8FriendShip From United States of America, joined Apr 2005, 242 posts, RR: 3 Reply 3, posted (5 years 4 months 1 week 1 day 20 hours ago) and read 3231 times:
The CRJ's big design flaw is in it's flap system. The plaps are literally operated by large speedometer cables(flexshafts). These have a high amount of wear, and are subject to "springing" as an old speedometer cable can. Also there is a problem with moisture entering the system, which several AD's and SB's have not apparently resolved yet. Good thing they went with torque tubes on the -700 and up....
Flyf15 From , joined Dec 1969, posts, RR: Reply 4, posted (5 years 4 months 1 week 1 day 13 hours ago) and read 3179 times:
Very interesting to read this... The CRJ's horizontal stab trim control system is another one of the systems on the plane that is a complete piece of junk. Although, from my knowledge, when it fails it more commonly fails in a runaway condition rather than a pure shutdown of the system.
I know of one pilot who had a stab trim runaway on him... ended up throwing his back out and went on indefinite medical leave due to the force that was required to fight the location the stab trim broke itself at.
2H4 From United States of America, joined Oct 2004, 8950 posts, RR: 62 Reply 5, posted (5 years 4 months 1 week 1 day 13 hours ago) and read 3175 times:
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Quoting Flyf15 (Reply 4): I know of one pilot who had a stab trim runaway on him... ended up throwing his back out and went on indefinite medical leave
I always wondered how badly it would hurt to be kneecapped by a runaway trim wheel in a 737....while the crank handle is folded outward. Ouch.
Ilikeyyc From United States of America, joined Dec 2003, 1373 posts, RR: 23 Reply 6, posted (5 years 4 months 1 week 1 day 13 hours ago) and read 3168 times:
Quoting 2H4 (Thread starter): The AAIB concluded: "In the absence of a mechanical backup system or sufficient physical separation of the control channels, there was insufficient protection within the design of the HSTCU against the effects of environmental contamination."
Bombardier has long been aware of this problem, there have been instances of trim run-away in the past.
The real prblem with the HSTCU is that the box is located directly under the main door opening. That means that when a plane sits on the ground for any amount of time with the main door open in a rain storm, all the water that gets in the plane (and it can be a lot!) finds its way below the floorboards to all the electronics (including the HSTCU) underneath. Rain water in the cabin is also the primary cause of corrosion on the FS280 frame.
There have been several upgrades to the HSTCU box over the years, with the latest part a dash 10. Currently our fleet has speed tape on top of the HSTCU as a small form of protection from water.
One rainy night, I had a captain ask me to inspect below the door to make sure the boxes were dry- and they were. Also I had a PA amplifier short out on me after lots of water made its way into the cabin, ran into the cockpit and dripped onto the PA amp, shorting it out.
FlyASAGuy2005 From United States of America, joined Sep 2007, 6506 posts, RR: 11 Reply 7, posted (5 years 4 months 1 week 1 day 8 hours ago) and read 3128 times:
Quoting Tdscanuck (Reply 2): I'm not sure this is really a single point of failure issue...I certainly agree about the seals.
If the cause is really environmental moisture, having one box or two shouldn't make too much difference since both boxes live in the same environment and you could have a common mode failure.
I would think so too, unless they decide to move the systems to a totally different area, unless they figure out a why to pretect it from the elements. But from what I understand, the CRJ is rather cramped under the cabin and there is virtually no space to fit anything else.
Quoting Flyf15 (Reply 4): I know of one pilot who had a stab trim runaway on him... ended up throwing his back out and went on indefinite medical leave due to the force that was required to fight the location the stab trim broke itself at.
Quoting 2H4 (Reply 5): I always wondered how badly it would hurt to be kneecapped by a runaway trim wheel in a 737....while the crank handle is folded outward. Ouch.
Could you guys explain this too me? What is a stab trim run-away. And how exactly can you get hurt?
Quoting Ilikeyyc (Reply 6): I can't wait to work on real airplanes...
IYHO, would you say the 700/900 is a much better airplane?
CAM2:"Lightning coming out of that one." CAM1: "What?"
Tdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 81 Reply 8, posted (5 years 4 months 1 week 1 day 2 hours ago) and read 3070 times:
Quoting FlyASAGuy2005 (Reply 7): Quoting Flyf15 (Reply 4):
I know of one pilot who had a stab trim runaway on him... ended up throwing his back out and went on indefinite medical leave due to the force that was required to fight the location the stab trim broke itself at.
Quoting 2H4 (Reply 5):
I always wondered how badly it would hurt to be kneecapped by a runaway trim wheel in a 737....while the crank handle is folded outward. Ouch.
Could you guys explain this too me? What is a stab trim run-away. And how exactly can you get hurt?
The horizontal stabilizer is a completely movable surface (separate from moving the elevators). It's not used as a moment-to-moment flight control because it's slow to move (and very powerful) but it's adjusted to trim the airplane for whatever CG/weight/speed combination you're at at the time. On many aircraft, like the 737, the actuator is a motor driving a jackscrew. If there's a system malfuction somewhere, the motor can start running and not stop, which drives the stabilizer way out of the trimmed range. The stab has "run away" from the flight crew. Keeping the aircraft under control when the stabilizer is way off requires large elevator inputs; that's what FlyASAGuy2005 is talking about.
2H4 is talking about something different...on a 737, the horizontal stabilizer trim wheel is on the sides of the control pedestal (where the throttles are) about even with your knee. When the auto-trim function runs, the wheels spin, just like if the flight crew was moving them by hand. There's a crank handle in the trim wheel that's usually folded in but, if it's out, it would smack you right in the kneecap when the autotrim kicked in.
DC8FriendShip From United States of America, joined Apr 2005, 242 posts, RR: 3 Reply 9, posted (5 years 4 months 1 week 18 hours ago) and read 2992 times:
Quoting FlyASAGuy2005 (Reply 7): IYHO, would you say the 700/900 is a much better airplane?
head to toe, mostly. there are a few quirks.
Quoting Ilikeyyc (Reply 6): I can't wait to work on real airplanes...
Lots of real world experience on RJ's. You just have to look for it.All airliners are basically the same, with a few differences. Alot can be transferred from one to another.
UAL Bagsmasher From United States of America, joined Sep 1999, 2130 posts, RR: 11 Reply 10, posted (5 years 4 months 1 week 16 hours ago) and read 2972 times:
You have to love the "Loss of Balls" task for checking the flap actuators on the CRJ-200. I'm not making this up. There is a task requiring checking for loss of ball bearings in the actuator. They could have chosen a better name for the task though
BR715-A1-30 From , joined Dec 1969, posts, RR: Reply 12, posted (5 years 4 months 6 days 20 hours ago) and read 2849 times:
Well, you must be talking about the CRJ-100/200... If you want to get technical with some things, the CRJ-700/900/1000 are in a class all by themselves. New wing, New Horizontal Stabilizer, New Engines, Same Flight Deck. The -100/200 are the problem children of the series now, in that nobody likes to fly on them, they are gas guzzlers, and they are constantly having some sort of problem whether it be with flaps, or electrical.
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