The fatal crash of a Gulfstream IV attempting to take off from Hanscom Field, Bedford, Mass. on the night of May 31 last year is a standout among business aviation mishaps for numerous reasons, all of which were up for discussion and speculation in the immediate aftermath of the tragedy: flight-control problems; pilot judgment; their apparent failure to adhere to required procedures; and the design effectiveness of a cockpit safeguard intended to prevent such a disaster.
The NTSB has yet to issue its final report on the probable cause of the accident but in the meantime, on July 31, GIV manufacturer Gulfstream Aerospace sent a 58-page “party submission” to the NTSB’s evidence docket–ERA14MA271–that Board members will consider in the process of arriving at their final finding. Gulfstream’s contribution to the docket makes for sobering reading.
The accident claimed the lives of the two pilots, the flight attendant and all four passengers. The captain was the pilot flying when the aircraft failed to leave the ground and went off the end of Runway 11 at speed after the attempt to take off at BED was aborted.
In its submission Gulfstream says, “This accident occurred because the flight crew deviated from approved procedures and training that left the gust lock engaged and attempted to continue takeoff with a recognized flight control anomaly, rather than promptly executing a coordinated abort procedure.”
The company points to the crew’s failure to follow the Airplane Flight Manual by not disengaging the gust lock before engine start, not checking for freedom of flight control movement after engine start and not confirming elevator freedom of movement at 60 knots during the takeoff roll. The 60-knot point is when there is sufficient airflow for the elevator to begin floating, sending important flight-control feedback to the pilot holding the control wheel.
Gulfstream says the crew never discussed the abnormal position of the gust lock handle at any point after engine start until the aircraft achieved rotation speed. There were no crew communications related to any aircraft pre-flight or checklist activity while loading passengers, departing the parking position and taxiing to the runway. As was their custom, the crew did not perform the required flight control sweeps to confirm the controls were free to move at any time before rotation speed, according to the report, which cited recorded data.
The crew also disregarded a rudder limit CAS message despite spending 10 seconds discussing the warning. They did not discontinue the takeoff despite being unable to push the thrust levers to their normal full-power position on the quadrant, another warning that the gust lock was engaged. The crew continued the takeoff, engaging the autothrottles, and somehow managed to bypass the interlock. Gulfstream said the lock moved on its own during the takeoff roll, “either through pilot awareness and deliberate movement, or through a gust lock detent pin failure within the handle.”
Design Defect Discovered
In its July 31 submission, the OEM also acknowledges that after the accident it discovered that the gust-lock interlock system used on the GIV and GIII does not meet the required throttle movement limitations. During its post-accident investigation, Gulfstream discovered a design defect in the gust lock system on the GIV that, “while limiting throttle movement when engaged, did not achieve its stated six-degree throttle movement limitation.” Gulfstream said that although this defect was never noticed on more than 500 aircraft delivered over 25 years, those aircraft still safely completed more than two million takeoffs.
Despite the defect, Gulfstream’s investigation, as well as the NTSB’s, “demonstrates that the interlock as it existed on the accident aircraft achieved its safety intent of limiting the operation of the aircraft, which was recognized by the flight crew early in the takeoff roll.” Gulfstream said the flight crew responded improperly, “attempting to disengage the gust lock while continuing with the takeoff instead of immediately aborting the takeoff and following proper unlock and engine start procedures.”
At approximately 60 knots, the pilot not flying noticed the gust lock was not stowed and attempted to release it, an action that also caused a reduction in takeoff thrust. Once the crew realized the gust lock was engaged and the elevator movement was abnormal at the 80-knot callout, they apparently did not react, an omission that Gulfstream attributes to lack of situational awareness. The crew “incorrectly assumed they had resolved the gust lock issue…and continued the takeoff roll, without comment, through decision speed V1 (119 knots) and Vr (125 knots). At this speed, and with the rudder trim hydraulic preload, the aerodynamic forces acting on the elevator surface were now contributing to the preload on the gust-lock hooks, preventing full disengagement.” Gulfstream said the two pilots exhibited “a loss of situational awareness as the throttles and engine power are reduced and remain reduced for 13 seconds before Vr.”
At rotation, the first time the captain attempted to move the control column, the physical restriction of the gust lock became obvious. He said “lock is on” seven times over the next 12 seconds. Gulfstream said, “Still fixated on accomplishing an immediate takeoff, the crew’s initial reaction to this unexpected condition was for the pilot not flying to continue troubleshooting the gust lock system instead of immediately beginning a rejected takeoff. When the abort did begin, the brakes were engaged 11 seconds past Vr and the throttles were not brought to idle for another four seconds as the thrust reversers were deployed.”
Gulfstream asserts that if, at the moment the pilot flying first announced that the gust lock was engaged, or even 2.5 seconds after his sixth callout, the takeoff rejection had been accomplished in a coordinated manner–full brakes and power levers to idle simultaneously–the aircraft would have stopped on the paved runway.
GIV Owner Portrays Bedford Crash in Different Light
In an 11-page “non-party submission” sent to the NTSB’s docket on June 15 this year, the owner of the Gulfstream IV that crashed attempting to take off from Hanscom Field on May 31 last year asserts that it cannot be determined whether the gust-lock handle was in the up/on, down/off or an intermediate position when the pilots started the engines, when they lowered the flaps to 20 degrees or when they began their takeoff roll.
The submission is signed by two managers of SK Travel, named as the owner of the GIV, N121JM. The NTSB chairman’s factual report on the accident, published on February 12 this year, shows the operator of the jet as Arizin Ventures.
While Gulfstream maintains that “this accident occurred because the flight crew deviated from approved procedures and training and left the gust lock engaged and attempted to continue takeoff with a recognized flight control anomaly, rather than promptly executing a coordinated abort procedure,” the owner lays the blame primarily on what it asserts is a defective gust lock system.
“The probable cause of the accident,” the owner’s submission proposes, “was the failure of the gust lock mechanical power lever interlock to restrict the movement of the power levers to a maximum 6° +/-1° above ground idle with the gust lock system engaged, which allowed the engines to produce enough power to accelerate the aircraft to rotation speed (VR) without ‘unmistakable warning’ to the flight crew that the gust lock was engaged.” The owner also cites these contributing factors:
- The lack of any GIV crew alerting system indications (warning or caution lights or aural warnings) that the gust lock was on with the engines running.
- The failure of the accident flight crew to perform a proper flight control check after engine start.
- The lack of a gust lock mechanical interlock override system (similar to the GII and GIII systems).
- The flight crew’s attempt to abort the takeoff with insufficient runway remaining after following the GIV Airplane Flight Manual (AFM) emergency procedure for an “Immovable Flight Control, Elevator Control (Pitch)” and guidance in the GIV AFM’s line-up checklist regarding the expected delay in rotation if the flight power shutoff handle is pulled at rotation due to a flight control problem.
The aircraft owner further notes that “an informal technical inspection on nine in-service GIVs found that with the gust lock in the up/on position, the forward throttle movement varied from 18.2 degrees to 24.2 degrees from the throttle levers’ full-aft position.”
Addressing the issue of whether the GIV met the design requirements of Part 25 certification, the owner continued, “The GIV gust lock system was required to comply with 14 CFR §25.679, which mandates that, if the gust lock system, when engaged, prevents normal operation of the control surfaces by the pilot, then it must either ‘automatically disengage when the pilot operates the primary flight controls in a normal manner’ (§25.679(a)) or ‘limit the operation of the airplane so that the pilot receives unmistakable warning at the start of the takeoff.’ (§25.679(b)).
“The accident aircraft did not either automatically disengage the gust lock system when the accident flight crew moved the primary flight controls or provide the accident flight crew ‘unmistakable warning at the start of the takeoff’ that the gust lock system was engaged.” From this, the owner infers that “the accident aircraft did not comply with the requirements” of FAR Part 25.679.
The owner’s “non-party submission” can be found at http://dms.ntsb.gov/public/57000-57499/57175/578150.pdf
As is customary in the aftermath of a tragedy such as this, the Members of the NTSB are now tasked with weighing all the evidence and all the submissions to the docket and arriving at their own conclusions in a final report, in which they will make recommendations that they hope will, if acted upon by the FAA, prevent a recurrence.