MRO
Perhaps many of you can relate to my “retirement goal.” I hoped to enjoy flying a simple taildragger out of an uncontrolled field. For four years, two able partners and I worked to restore an Aeronca Champ. I couldn’t have asked for better partners. Both were retired captains from a major airline and had extensive lifetimes of experience in general, in military and commercial aviation. Partner No. 1 had flown a variety of aircraft from Stearmans to helicopters to the F-4 Phantom and had been a leading member of the pilot safety committee at his major airline. Partner No. 2 had a wall full of trophies from the Reno Air Races, where he competed successfully in the T-6 category. Besides having FAA waivers for air shows in a variety of aircraft including the MiG 15, he was an A&I and had instructed at the U.S. Air Force’s Test Pilot School. Between the three of us we probably had in excess of 10,000 flight hours specifically in taildraggers.
There was no question of who would do the first flight when the much-anticipated day arrived for our return-to-service test flight. All of those trophies from the Reno Air Races clearly spoke to Partner No. 2’s expertise flying taildraggers. We watched and listened as Partner No. 2 proceeded through the various stages of checks. Finally it was time for the initial landing. We excitedly watched our partner set up for a nice touchdown, but became concerned as we saw him aggressively working the rudder during the touchdown and roll-out. This surprised us because the Champ has a nice reputation for having rather forgiving ground-handling characteristics for a taildragger. Upon exiting the cockpit our partner announced that something was askew. It then took us about four months of painstaking analysis to determine the cause. With the use of lasers we eventually found that the main landing gear was slightly asymmetric. This caused an unacceptable instability in the aircraft that required exceptional pilot skill to keep from turning into an accident. Fortunately our post-maintenance flight didn’t end up in the NTSB records. Others haven’t been as fortunate.
I originally intended to summarize notable lessons after reviewing the most-recent 125 NASA ASRS reports and 75 NTSB accident reports involving post-maintenance test flights. However, that plan changed because of a recurring problem with post-maintenance test flights that led to the NTSB issuing a Safety Alert titled “Pilots: Perform Advanced Preflight After Maintenance.” The NTSB found common safety issues including maintenance personnel who serviced or checked the systems not recognizing that the control or trim surfaces were moving in the wrong direction, as well as pilots failing to detect the control anomalies during their preflight checks. These failures often ended in fatal injuries.
While the maintenance field has a system of double checks to make certain that critical tasks are inspected by a second source, lapses in human performance occasionally miss these items. On July 25, 2006, a Spectrum Aeronautical LLC 33 (prototype experimental light jet) departed Utah’s Spanish Fork Airport (KSPK) on a local maintenance test flight. Witnesses reported that the airplane entered a right roll almost immediately after liftoff. The roll continued to about 90 deg. right wing down, at which point the right wingtip impacted the ground, killing both pilots and destroying the aircraft.
While examining the wreckage, NTSB found that the aileron control system was connected in such a way that the airplane rolled in the opposite direction to that commanded in the cockpit. The maintenance performed on the airplane before the accident flight required the disconnection of a portion of the aileron control system and the subsequent reconnection of that system resulted in reversed movement of the ailerons. None of the mechanics who performed the work could recall if the position of the ailerons in relation to the position of the control stick was checked. The NTSB report points out that such a position check, if it had been performed by either the mechanics after the maintenance or by the flight crew during the preflight checks, would assuredly have indicated that the system was installed incorrectly. The NTSB determined the probable cause was the incorrect installation by company maintenance personnel of the aft upper torque tube bell crank, resulting in roll control that was opposite to that commanded in the cockpit. Contributing factors included the failure of maintenance personnel and the flight crew to check the position of the control stick relative to the ailerons after the maintenance and during the preflight checks.
There are many other NTSB reports of post-maintenance test flights that involved incorrect installation of flight controls and/or trim systems. On Oct. 16, 2003, the CommutAir flight crew of a Beech 1900D picked up the aircraft in Albany, New York, after it had been “signed off” by maintenance technicians for work involving removal and reinstallation of the elevator trim system. The incident flight was the first flight after maintenance. During the takeoff roll, the flight crew was unable to rotate the airplane. Fortunately there was enough runway remaining for the crew to successfully abort the takeoff. This is particularly worth a mention because the aircraft’s rotation speed occurs after the critical go/no-go speed. Discovering a problem with an aircraft’s flight control system at rotation speed may leave the flight crew in a precarious position with too little runway left to safely abort.
Examination of this Beech 1900D revealed that when the elevator trim wheel in the cockpit was positioned to neutral, the elevator trim tab was actually in the full nose-down position. The elevator trim wheel could not be physically moved lower than three units of nose-up trim. The maintenance technician did not index the trim wheel when he removed it, and then reinstalled it incorrectly. In addition, the maintenance manual did not contain a procedure to remove and reinstall the elevator trim wheel. Following the maintenance, no functional check of the elevator trim system was performed. When the captain performed a preflight inspection of the airplane, he did not set the elevator trim wheel to the setting prescribed on the preflight checklist, and he failed to detect the error. The NTSB determined the probable causes of this incident included the maintenance technician's improper maintenance performed on the airplane and his failure to perform a functional check, which resulted in a restricted movement of the elevator trim wheel. Factors were the captain's inadequate preflight inspection and insufficient information in the manufacturer's maintenance manual. In this case, the industry and the two flight crewmembers (the only persons on board) avoided a serious accident.
Unfortunately this accident is not an isolated event. Other similar examples illustrate the vulnerability of Non-Routine Flight Operations (NRFO) flight crews when maintenance has been done improperly to flight control systems, and/or flight crews have not adequately understood nor properly preflighted an aircraft after flight control maintenance. On Aug. 26, 2003, a flight crew picked up a Beech 1900D in Yarmouth, Massachusetts. This was the first flight after maintenance personnel replaced the forward elevator trim cable. When the flight crew received the airplane, the captain did not address the recent cable change noted on his maintenance release. He also did not perform a first flight of the day checklist, which included an elevator trim check. Shortly after takeoff, the flight crew reported a runway trim, and manually selected nose-up trim. However, the elevator trim then traveled to the full nose-down position. The control column forces subsequently increased to 250 lb., and the flight crew was unable to maintain control of the airplane. While attempting to return to the airport, the plane pitched nose-down and impacted the water at an approximate 30-deg. angle, killing both pilots.
During the replacement of the elevator trim cable, the maintenance personnel skipped a step in the manufacturer's aircraft maintenance manual (AMM). They did not use a lead wire to assist with cable orientation. In addition, the AMM incorrectly depicted the elevator trim drum, and the depiction of the orientation of the cable around the drum was ambiguous. The maintenance personnel stated that they had completed an operational check of the airplane after maintenance. The NTSB performed a mis-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.
The NTSB determined the probable causes of this accident included 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. Factors were the flight crew's failure to follow checklist procedures, and the aircraft manufacturer's erroneous depiction of the elevator trim drum in the maintenance manual.
The NTSB Safety Alert recommends that pilots check systems more thoroughly than the normal preflight checklist implies after maintenance. For example, if a preflight checklist states, “trim-set takeoff,” you should verify not only the trim setting but also the proper direction of travel. If you suspect a problem with a flight control or trim system, ask qualified maintenance technicians to inspect the aircraft.
It was an invaluable lesson to get hands-on experience (under the direct supervision of an A&I) with the inner workings of a conventional cable-and-pulley flight control system. Removing worn components and reinstalling new ones provided great insight into the many possible failure modes of wear and fatigue that could eventually lead to flight control malfunction. Credit: Patrick Veillette
CHECK FLIGHT CONTROL MOVEMENT
From our very first flight lessons we were taught to check for “full and free correct movement” of flight controls. Every preflight, whether it is for a normal flight or a post-maintenance test flight, must ensure that the flight controls exhibit full and free correct movement without any possibility of binding. This isn’t rocket science, and yet the NTSB files contain events in which this simple concept wasn’t followed.
A Messerschmitt-Bolkow-Blohm BO 105LS A-3 was conducting a maintenance test flight on April 13, 2006, in Green Bay, Wisconsin. Following takeoff, the helicopter began spinning around its vertical axis to a height of approximately 200-300 ft. and descended without directional control, impacting terrain. The rotorcraft was substantially damaged and one fatality resulted. The copilot's anti-torque control pedals were found in their full forward position with a safety wire installation that was contrary to specifications cited in the field approval for the pedal cover. The NTSB determined the probable cause was the pilot's inadequate preflight check of the flight controls prior to takeoff and the directional control not possible by the pilot. Additional causes were the improper installation of the anti-torque pedal cover by company personnel, which blocked the flight control system.
Any possible restriction to a flight control’s full and free correct movement must be corrected prior to flight. A pilot and a mechanic were flying a second maintenance flight check of a Eurocopter AS 350-B2 on July 1, 2005, at Scottsdale, Arizona, to check the rotor tracking. During a previous maintenance test flight they encountered a restriction in the collective’s movement when the collective down lock inadvertently engaged. They entered a descent at approximately 1,200 ft. AGL and prepared to level off at approximately 700 ft. AGL. When the pilot tried to pull up on the collective, it would not move and was observed to be latched by the collective down lock. They tried to unlatch the collective from the down lock but did not have enough time before the pilot had to flare the helicopter for landing. With the collective stuck in flat pitch, they landed hard and with forward speed. The flight crew evacuated the AS 350 once it came to rest. An ensuing post-accident fire destroyed the helicopter.
The investigation noted that a new avionics control panel had been installed and the collective down lock, which is secured to the panel, was adjusted prior to the flight. This was the second known accident where the collective lock had inadvertently engaged in flight with this particular aftermarket avionics panel installed. The NTSB determined the probable cause was inadvertent inflight engagement of the collective down lock, which resulted in an uncontrolled descent and ground impact. The collective down-lock engagement was caused by the improper installation and/or adjustment of the collective locking system, which reduced the clearance between the locking plate and the collective control.
Know ‘Normal’ Movement
The NTSB Safety Alert titled “Pilots: Perform Advanced Preflight After Maintenance” recommends that pilots become familiar with the normal movement of the aircraft’s flight controls and trim surfaces before it undergoes maintenance. It is easier to recognize abnormal movement if you already are familiar with what normal looks like. I would add “feel” and “hear” to this statement. During a preflight on a cable-and-pulley flight control system, as you move the flight controls and trim surfaces to their full deflection, not only are you checking for the correct direction of movement (on larger aircraft it will be necessary to have someone outside the aircraft directly communicating with you), but you should feel for any restriction as well as carefully listen for any hint that the control cables are possibly rubbing against other surfaces or falling off pulleys.
This saved my bacon during a preflight for a maintenance test flight when the yoke didn’t seem to move normally. The amount of force required to move it seemed slightly abnormal. There was some small but definite binding in the movement. Additionally, I could hear a faint but abnormal “rubbing” noise as I moved the yoke. The maintenance controller uttered a long sigh when I described this over the phone. Troubleshooting required ripping up the flooring to examine the flight control cables. A few days later I received feedback that the cables had become displaced from a pulley and were actually moving in between the pulley and its mounting bracket. Not only were the cables becoming frayed, but it was possible that they could have become completely jammed.
When a cable-and-pulley flight control is reinstalled it is necessary to properly tension the control cables. During the 10 years in which I was a non-routine flight operations captain at a fractional operator, I performed many post-maintenance flights checking the flight controls. It wasn’t until liftoff during one of those flights that we discovered the difference in flight control movement when the cables didn’t meet the required tension. The co-captain rotated for takeoff and the aircraft uncharacteristically begun a significant roll to the right. The co-captain was using every bit of his upper body strength to try keeping the aircraft upright. I distinctly remember his strained words, “Paaaaaaat, I caaaan baaaarely hold it.” The two of us had a hard time keeping the wings level and I had just enough mental reserve to quickly tell ATC that we had an emergency with a flight control problem and needed some airspace to work out the problem and formulate a recovery plan. ATC was great. It was a handful, no pun intended, to fly an aircraft that wasn’t handling in a normal manner. We landed, with considerable difficulty but without further incident, and pulled into the company ramp thanking ATC and the fire department for its escort. During the post-flight I re-checked the ailerons to see if I could feel any difference in the amount of force to manually deflect them up and down. I was not able to discern any noticeable difference from “normal.” Nor was it possible during our post-flight investigation to feel a difference when moving the yoke. The difference became noticeable only when there was an aerodynamic load on the flight control.
Flight Control ‘Free Play’
A destructive form of fatigue called limit cycle oscillation (LCO) is caused by excessive free play within the flight control surfaces and associated components. This condition generally becomes worse at higher speeds and altitudes. An example of this affected some Hawker 800XP and 850XP aircraft that experienced wing/aileron oscillations at altitudes above 33,000 ft. and speeds over Mach 0.73. When the speed was reduced and the airplane was at an altitude below 30,000 ft., the oscillations ceased. Investigation of the incidents revealed missing aileron bushings, low cable tensions and improperly installed brackets. If the aileron system, including cable tension, is not properly maintained, wing oscillations could develop into divergent flutter, thereby causing severe damage to the structure. When corrective maintenance brought the aircraft into compliance with the type design configuration, the oscillations did not recur.
The FAA issued Special Airworthiness Information Bulletin NM-14-05, dated Nov. 27, 2013, recommending a one-time maintenance check to verify all the bushings in the aileron and aileron tab assemblies are correctly installed, that the free play is within limits, and to ensure that the hinge brackets are properly installed and the cable tensions correct.
This type of oversight mistake has happened to other business aircraft as well. On March 19, 2004, the flight crew of a BAe 1000 had to declare an emergency to return to Palm Beach International Airport (KPBI) due to a control problem after experiencing severe longitudinal oscillations. The yoke oscillated left and right rapidly and the wings were flexing 4-6 in. Following inspection by a technician, a maintenance check flight was conducted from KPBI to Tampa International Airport (KTPA) on March 21, 2004, during which similar symptoms were exhibited. Re-inspection of the aircraft found that three aileron hinge bushings had not been installed at the previous maintenance.
During a preflight of cable-driven flight controls, pilots should check the free play by gently moving the flight control. The exact amount of free play (i.e., the amount you can jiggle the flight control without restriction) should be stipulated in the aircraft maintenance manual. If excessive free play is found during a preflight, the proper action is to note the discrepancy in the aircraft logbook and not fly the aircraft until the condition is corrected by maintenance technicians.
A pilot conducting a post-maintenance test flight should be attentive to the maintenance corrections made as well as know that subtle vibrations at relatively low altitudes will likely be exacerbated at high altitudes where true airspeeds increase. Clearly some knowledge of aeroelasticity is necessary for flight crews who conduct post-maintenance test flights, especially of flight control systems. Troubleshooting vibrations will often require changes of airspeed, changes of throttles and configuration, and possibly changes of altitude to get a trend. It will be important to observe whether the vibrations were high frequency or low frequency. For example, the flight crew of a business aircraft experienced divergent flutter at approximately 2,000-3,000 ft. accelerating through 170 KIAS. The flight crew decreased to 150 KIAS, whereupon the flutter went away. They declared an emergency and landed uneventfully. They subsequently discovered the port side trim disconnected from the control arm that was recently out of maintenance at the FBO (ASRS 700868, June 2006).
PAINT EFFECTS
Any time you change a flight control’s mass, it negatively affects the speed at which the flight control can flutter. The mass of a flight control can change due to drain holes getting plugged as well as bird nests. Even a change caused by a single additional layer of paint can create a previously non-existent flutter mode in a flight control surface.
On Aug. 24, 1996, a Burkhart Grob G-115D experienced an inflight breakup over Dupuis Reserve near Indiantown, Florida, during a local aerobatic instructional flight. Both flight instructors were fatally injured. Parts of the aircraft were widely scattered, which indicated the inflight breakup. The first item in the line was the top of the rudder; the lower portion of the rudder was never found. The stabilizer and both elevators were found 900 ft. from the main wreckage. The only part of the airplane left intact was the left aileron.
Maintenance logs revealed that the airplane had been repainted 96 flight hours before the flight, but the flight control surfaces had not been rebalanced. Grob specifications permitted a hinge moment range of -0.22 foot-pounds (meaning that the aileron was leading-edge heavy) to 0.074 foot-pounds. The hinge moment of the retrieved aileron was between 0.138 and 0.200 foot-pounds, considerably “tail-heavy” and outside of factory specifications.
Hawker 800. Credit: Nigel Howarth
Samples of the exterior skin were examined for paint thickness to evaluate the balance and residual hinge moments for the remaining flight control surfaces. The test determined that all control surfaces were not in compliance with Grob’s specifications. The NTSB determined the probable cause of the accident was “failure of maintenance personnel to rebalance the flight controls after the airplane had been repainted, which resulted in rudder flutter and inflight breakup of the airplane.”
Getting Outside of the Aircraft’s Envelope
A review of the NASA ASRS and NTSB reports found other maintenance test flights in which the aircraft proceeded outside of its normal flight envelope while the crew was checking flight controls. During a test flight at Salina, Kansas, on June 12, 2001, a Learjet 25D encountered an elevator system oscillation while in a high-speed dive. The aft elevator sector clevis fractured due to reverse bending fatigue caused by vibration, resulting in a complete loss of elevator control. The flight crew reported that pitch control was established by using horizontal stabilizer pitch trim. They stated that during final approach the aircraft's nose began to drop and the flying pilot was unable to raise it using a combination of horizontal stabilizer trim and engine power. The Learjet landed short of the runway, striking an airport perimeter fence and a berm. The aircraft was destroyed and the two pilots were seriously injured. The NTSB determined the probable cause was the pilot in command's delayed remedial action during the elevator system oscillation, resulting in the failure of the aft elevator sector clevis due to reverse bending fatigue caused by vibration, and subsequent loss of elevator control.
One of the most dramatic departures from the aircraft’s flight envelope during an NRFO flight occurred on May 4, 2006, to a Hawker 800A during a maintenance test flight northwest of Lincoln, Nebraska. The aircraft had just come out of extensive maintenance and refurbishment. On board the flight were two Raytheon test pilots and four passengers that Raytheon considered crucial for the test flight. The first maneuver to be performed was a clean stall. Prior to the flight the crew calculated the stick shaker activation speed to be 115 kt., pusher speed at 107.5 kt., and that aerodynamic buffet speed would occur at 105.5 kt. The aircraft was level at 17,000 ft. MSL with the autopilot engaged in altitude hold and heading hold modes. As it slowed to approximately 126 kt., the right wing suddenly stalled, the nose dropped through the horizon and the aircraft continued to roll to the right in a near-vertical descent. The Hawker entered a cloud layer below them and, due to the attitude of the aircraft, the gyros tumbled. The crew was unable to determine the attitude of the aircraft until they exited the cloud layer. The aircraft continued to roll to the right, about three turns in total, when it experienced a rapid roll reversal to the left. It rolled about two to three turns to the left. When they exited below the base of the cloud layer, the captain saw only ground through the windshield and immediately pulled back on the yoke and regained control at approximately 7,000 ft. MSL. The crew returned to Lincoln Airport (KLNK), declared an emergency and made a no-flap landing on Runway 36.
The subsequent investigation discovered that the crew had difficulty locating an area that was in visual meteorological conditions to perform their stall tests. Two passenger statements mentioned that they saw ice on the leading edge of the wings. During their interview the crew stated that they never activated the TKS anti-icing system. Due to the absence of any malfunction with the aircraft systems, any abnormal flight characteristics after test flight, and the addition of statements from passengers and another pilot, it is possible that the wing of this aircraft was contaminated with ice during the stall. The aircraft flight manual (AFM) states that the clouds should be at least 10,000 ft. below the aircraft prior to stalls, the autopilot disengaged, and that stalls not to be made in icing conditions.
There were other inconsistencies in the prescribed procedures. The AFM stated that intentional stalls were to be performed with the autopilot off. However, company maintenance test flight procedures required it be engaged in order to verify autopilot disconnect at stick shaker prior to approving the aircraft for return to service. The AFM also specifically noted that all external airframe surfaces must be free of ice when performing intentional stalls. Afterward, Raytheon issued a stall training syllabus that outlined operational considerations for stall testing and clarified approved recovery procedures. In addition, they discontinued the practice of approaching intentional stalls with the autopilot connected for in-service aircraft until the stall characteristics of the aircraft have been ascertained.
These events lead to the question, “Should NRFO pilots be placed in a situation in which they are flying an aircraft so close to the limits of the aircraft envelope?” The FAA’s Information for Operations #08032, “Non-Routine Flight Operations,” explicitly says the NRFO is not a test flight and the crew are not qualified test pilots. With that said, sometimes the margins in the aircraft envelope can be razor-thin. If so, shouldn’t they receive extra training in the handling issues associated with operations close to the aircraft’s flight envelope, how to detect the deterioration in the sometimes razor-thin margins prior to an excursion beyond the limits of the aircraft envelope, and how to properly recover the aircraft to a stable condition in a deteriorating situation?
One suggested solution would be extra training in the simulator in preparation for flight so close to the margins of the aircraft envelope. However, that has created false impressions in pilots regarding the reaction of the aircraft. The NTSB determined that a contributing cause of an Airborne Express DC-8 accident on Dec. 22, 1996, was the flight training simulator's inadequate fidelity in reproducing the airplane's stall characteristics. Most modern simulators don’t have the fidelity to replicate the aircraft’s handling that close to the edge of the envelope. This has been one of the central issues in the debate over loss-of-control prevention training. Additionally, the Airborne Express example is but one of several in which control techniques learned in the simulator have actually made the situation deteriorate worse in the aircraft.
Airbus is applying the unique expertise of its pilots who test new and pre-delivery aircraft to develop a training course for hand-picked pilots who will perform airworthiness flight checks. The course lasts five days and comprises three modules. The first module is a full two-day ground school covering crew responsibilities, flight preparation, preflight briefings, recording of inflight parameters to engineering standards, risk management, aircraft type specifics and use of special system checklists. The second module includes two 4-hr. full flight simulator sessions that are evidence-based and specific to each aircraft’s unique handling characteristics. The third module is an in-aircraft 4-hr. flight to train key procedures.
The NASA Aviation Safety Reporting Systems (ASRS) and NTSB databases have plenty of other important lessons for all involved in post-maintenance flying. Some of you have contributed great reports to the ASRS about the difficulties you encountered during post-maintenance test flights. Your well-written narratives discussed the extra workload, the need for low workload airspace because you are focused on the aircraft, or missing an ATC radio call while you were performing these abnormal test procedures. Any in-depth discussion of the proper training, procedures and risk management required to safely conduct post-maintenance test flights needs to address these important topics.
