Flight Controls, Maintenance, and Fresh Warm Air
What could go wrong?
Reprinted with permission from FAA Aviation News Magazine
As the spring and summer flying season starts to get rolling, we need to be aware of certain potential hazards that could cause us great harm, or worse, if left unchecked. Take the flight control check, for example. If you have done it enough times, it becomes repetitive, so are you really paying attention to what you are doing?
There are two possible hazards looming that come to mind. The first is unavoidable: control cables contracting and expanding. This is a function of temperature. Its basic physics, when the temperature gets hotter things expand, and when it gets colder things contract. This principle applies to your flight controls too. So those cables you finally got balanced and tensioned just right last fall, are probably anything but now. Changes in temperature can change the length of these cables leading to slack (or lack of slack) in the cables resulting in “sloppy” controls or in extreme cases the possible loss of control effectiveness.
The other hazard is the chance that something could go wrong during maintenance. Sometimes a mechanic simply makes a mistake, but as one mechanic said, mechanics don’t make small mistakes. One small thing left undone can have tragic consequences, so perform a very detailed preflight. The most dangerous time to fly an airplane is probably when it is fresh out of maintenance.
While there are many flight control accidents to choose from, these two 2003 accidents involve Beech 1900D aircraft and are remarkably similar. These accidents do not reflect any defect or inherent safety problem in the aircraft itself, but rather are just two similar accidents that happened to occur in the same kind of aircraft. All information regarding these accidents was taken directly from the NTSB reports.
Air Midwest Flight 5481
On January 8, 2003, Air Midwest Flight 5481 suffered a loss of pitch control on takeoff from Charlotte-Douglas International Airport. The Beechcraft 1900D aircraft crashed shortly after takeoff, killing 19 passengers, two crew members, and one person on the ground. The aircraft crashed into a US Airways maintenance hangar and came to rest about 1,650 feet east of the runway.
The aircraft had been in for maintenance just prior to the accident, from the night of January 6 to the next morning. Specifically, the maintenance completed was the “detail 6” (D6) check which covers the aft fuselage and empennage including the pitch control systems. The airplane completed nine flights after the D6 check before the accident on the tenth flight. The aircraft managed to fly normal service after the D6 check and was handed off to the accident crew with no reports of any problems. The previous first officer during the hand off told the new first officer that “everything was normal” and “it was a good flying airplane.”(AAR-04/01, p1)
The NTSB concluded that the probable cause of the accident was: “…the airplane’s loss of pitch control during takeoff. The loss of pitch control resulted from the incorrect rigging of the elevator control system compounded by the airplane’s aft center of gravity, which was substantially aft of the certified aft limit.”(AAR-04/01, p131)
When the elevator control cables were adjusted during the inspection, they were incorrectly rigged to provide a maximum of seven degrees Aircraft Nose Down (AND), which is about half of the downward travel specified by the manufacturer (AAR-04/01, p128). The NTSB calculated that an input of 9.5 degrees AND would have been required to recover from the initial upset. As this was not possible, the flight was doomed as soon as it took off. In this case, it was a combination of an excessively aft center of gravity (CG) and limited control authority that lead to this loss. As the circumstances around this flight show, the aircraft was flyable with a CG within limits. It flew fine on the previous nine flights following maintenance. These flights included ones flown by the accident crew and at least one other crew. Neither of these crews discovered the incorrectly rigged pitch controls, and clearly from the comments cited above, there was no sense of any abnormal situation with regard to the aircraft. The loss of control would not have occurred without the aft CG, but the aircraft would have likely been controllable with the aft CG had the crew been able to exercise full authority over the downward travel of the elevator. It’s possible the crew might have been able to recover from the upset and return to the field. But the fatal combination of aft CG and reduced control travel left the crew with no ability to avert disaster.
Colgan Air Flight 9446
On August 26, 2003, Colgan Air Flight 9446 attempted to take off on a flight from Barnstable Municipal Airport in Hyannis, Massachusetts, to Albany International Airport in Albany, New York. During the takeoff roll, the pitch trim system began to move in the downward direction. The initial movement speed from 1.5 degrees AND to 3.0 degrees AND was consistent with the electric pitch trim motor. Four seconds later the pitch trim movement increased from 3.0 degrees AND to 7.0 degrees AND at a speed that was beyond the capacity of the electric motor. The crew declared an emergency, reporting runway trim, and attempted to return to the airport. The flight reached 1,100 feet while attempting to return to the airport. “Witnesses observed the airplane in a left turn, with a nose-up attitude. The airplane then pitched nose-down and impacted the water ‘nose first.’”(NYC03MA183) Only the flight crew was on board the aircraft, both were killed.
The accident aircraft had been brought in for its D6 check on August 23. On the morning of August 24 the check was interrupted and the remaining work was deferred. Ten revenue flights were conducted. The evening of August 24th the aircraft was returned for completion of the D6 check which was concluded on the 26th. Extensive work was done on the pitch trim tabs and trim control system (Check the report for specifics). After takeoff the crew reported the runway trim and manually selected Aircraft Nose Up (ANU) trim settings, but the aircraft trimmed full AND. The control column force was measured at 250 pounds, but the crew was unable to maintain control.
“The Safety Board performed a miss-rigging demonstration on an exemplar airplane, which reversed the elevator trim system. An operational check on that airplane revealed that when the electric trim motor was activated in one direction, the elevator trim tabs moved in the correct direction, but the trim wheel moved opposite of the corresponding correct direction. When the manual trim wheel was moved in one direction, the elevator trim tabs moved opposite of the corresponding correct direction.” (NYC03MA183).
The NTSB determined that the probable cause of the accident was: “The improper replacement of the forward elevator trim cable, and subsequent inadequate functional check of the maintenance performed, which resulted in a reversal of the elevator trim system and a loss of control in-flight.” (NYC03MA183).
Because the crew did not perform a first flight of the day check, as company policy and manuals required, they were unaware of any problem. The replacement of the elevator trim cable was noted in the aircraft release paperwork, but no mention of that was made by the crew. Several non-pertinent conversations were noted in the transcript of the Cockpit Voice Recorder (CVR). With the flight being empty it is possible the crew may not have been as conscientious as they would be if there were passengers on board. The NTSB cited “…the flight crew’s failure to follow the checklist procedures…” as a factor in this accident.
The point of the preceding examples is flight control malfunctions/maintenance errors happen. Airlines have professional maintenance organizations that usually specialize in, or have specialized training on, the make or model of aircraft they are servicing. Almost all GA mechanics are very professional and have extensive training, but in most cases they are more generalistic. This is the only practical system. While they have probably worked on most models, they may not have worked on your particular version or model. Most airports can’t support specialized mechanics only for Cessna, Piper, Cirrus, Beechcraft, Mooney, and several other manufacturers – not to mention all of the different avionics and engine suppliers. So if you fly a Cessna 172, almost any mechanic probably knows it front to back, but if you fly a Socata Tampico (or any of a number of less popular aircraft) you might be out of luck. As a practical matter, these generalist mechanics are a perfect solution in realistic terms. Title 14 Code of Federal Regulations (CFR) section 43.13 (a), which covers performance rules for mechanics, states:
“Each person performing maintenance, alteration, or preventive maintenance on an aircraft, engine, propeller, or appliance shall use the methods, techniques, and practices prescribed in the current manufacturer’s maintenance manual or Instructions for Continued Airworthiness prepared by its manufacturer, or other methods, techniques, and practices acceptable to the Administrator…”
Title 14 CFR section 65.81(a) tells us mechanics are required to have performed any maintenance task successfully, under supervision, before they can do it on their own. Most mechanics probably do not get a chance to practice every procedure, on every type of aircraft, on a regular basis. But with strong general skills and guidance provided by the manufacturers, GA mechanics do a superb job keeping the fleet running. The point is, if a crew with extensive specialized training can make mistakes, as was the case in both of these accidents, then there is the chance that any mechanic could do the same.
On the other side of the coin, professional, trained flight crews failed to notice any problem until it was too late. Again, these were flight crew trained to Airline Transport Pilot standards (at least the captains) which are far more rigorous, with greater frequency requirements, than most GA pilots face. Most airline pilots also fly more frequently than many GA pilots, leading to greater proficiency. They also have access to higher quality simulators to practice emergency procedures with far greater fidelity. Yet in these accidents they failed to detect any problem. In the first case, the problem may have been more difficult to detect. In the second case, the failure to do a thorough preflight and before takeoff check eliminated any chance for detection. What makes the second case especially troubling is that the aircraft came directly out of maintenance with notations about what had been done. Every sign was there to alert the crew to the potential for a problem with the elevator trim. So they should have been paying attention and done a much more thorough check of the pitch control and pitch trim systems. Would you be able to recognize a problem if you saw one? At least three airline crews didn’t (one crew for the Colgan accident and at least two crews for the Air Midwest accident).
In GA, our airplanes may be less complicated, but the question remains: Would we even know if things weren’t as they should be? We can generally determine whether or not things are pointing in the right directions. But would we notice a more subtle defect in the rigging? In the case of the Air Midwest accident, it was an incorrect rigging that likely would have been hard to detect. The error cost the crew just enough to make the accident inevitable. While a confluence of factors had to come together to make this happen, it is a situation that most GA pilots can probably envision for themselves. You’re getting ready for a big trip. You called your local Fixed Base Operator (FBO) to arrange for an aircraft rental (I specify a rental here because that would be more akin to the environment of an airline where pilots don’t have a single airplane they always fly, but the same situation is possible with an aircraft owner) for the next few days to visit friends or relatives a few hundred miles away. As a favor to you, just trying to be a good service provider, the FBO assigns you an aircraft fresh out of maintenance with a clean annual/100 hour inspection. You carefully work out the weight and balance for you and your three compatriots, bags, and other items. At the last minute, one of your passengers loads a bag in the rear that they forgot to mention to you while you were inside taking care of one thing or another. Many passengers probably wouldn’t have an understanding of CG or Max Gross Takeoff weights. Now you’ve just found yourself in the same situation as the Air Midwest crew, a hopeless one if you take off.
The old analogy, accidents happen when the holes in the Swiss cheese of our safety nets line up precisely, is likely to remain correct regardless of our efforts. Our goal is to reduce the size of the holes and improve our chances of survival. While we can’t say with any certainty that any preflight’s done were improper or inadequate, the NTSB report in one case suggests that the crew did not follow the operating checklists very closely. In the case of the Air Midwest accident, it may not have helped at all. In the case of the Colgan accident, the crew was not as careful or thorough as they should have been considering that the aircraft just rolled out of maintenance. According to the report, the captain noted the inoperative Digital Flight Data Recorder (DFDR) but made no mention of the freshly replaced elevator trim cable that was listed in the same paperwork.
This time of the year you are probably thinking about what to do on your summer vacation. What we can learn from these deadly accidents is the need to be especially vigilant with aircraft coming out of maintenance. Maybe we should do a thorough control preflight when we know the rigging is right to get a mental picture of what full deflection should be. One extra step you might consider taking is reviewing the Type Certificate Data Sheet (TCDS) for the aircraft you normally fly. These documents contain the proper deflection angles for any aircraft. They are available for free from the FAA at <http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgMakeModel.nsf/MainFrame?OpenFrameSet>. This information, combined with a simple grade school protractor, would allow you to insure your flight controls are as they should be. Insuring our own safety, and that of our passengers, is about making every possible effort to gain maximum advantage. This stresses the need for a greater understanding of how flight controls (or any other critical system) really work and how they might be affected by maintenance or changes in conditions. This understanding, along with a professional dedication to do more than a cursory preflight, will help us reduce the risks we naturally face in our pursuit of flying.