Description

On 8 March 2017, a Boeing MD83 (N786TW) being operated by Ameristar Charters on a non-scheduled passenger flight from Ypsilanti to Washington Dulles being flown by a type rated Captain undergoing supervised type-variant differences training could not be rotated during a daylight takeoff in normal visibility and the unavoidable high speed rejected takeoff led to an overrun of 350 metres. An emergency evacuation of the 116 occupants was successfully accomplished with just one passenger sustaining a minor injury but the damage to the aircraft was substantial.

Investigation

An Investigation into the accident was carried out by the NTSB. The FDR and CVR were removed from the aircraft and their data successfully downloaded. It was noted that the 54 year-old Captain under supervision who had been acting as PF and occupying the left hand pilot seat had a total of 15,518 flying hours which included 8,495 hours on DC 9 variants. He had been employed by Ameristar just over a year earlier as a DC9 First Officer and had been given a command a month later. The 41 year-old MD83 Company Check Pilot who was in command of the accident flight had a total of 9,660 flying hours which included 2,462 hours on DC 9 variants. He had been employed by Ameristar since 2004 and had received his initial DC9 type rating in 2007, subsequently upgrading to the MD80 variant in 2011.

What Happened

It was established that after being parked at Ypsilanti for two days prior to the accident takeoff, the aircraft crew had been obliged to follow their Company’s procedures for non-towered airports for their departure after the ATC TWR service had been withdrawn following an interruption to electrical power supplies caused by high winds. The Captain under supervision carried out the pre flight external check with no untoward findings and the two pilots agreed the applicable takeoff speeds V1 / Vr / V2 as 139 / 142 / 150 KIAS respectively. Given the gusty conditions, wind forecast to be from 250° at 32-48 knots, it was agreed that for the takeoff from the 2300 metre-long runway 23L, rotation would be delayed. Control checks whilst taxiing out were normal and the takeoff was commenced but when Vr was called, the PF reported having been unable to get the expected pitch up response to his aft movement of the control column. In response to the reject call, the supervising pilot was recorded as responding “no, not above…” and then “…don’t abort above V1 like that,” with the Captain replying “it wasn’t flying” but thereafter supported rejected takeoff procedure being followed with both pilots applying maximum braking. FDR data showed that the maximum speed reached was 173 KCAS and integrated groundspeed data showed that brake application to reject the takeoff had commenced with about 480 metres of runway remaining.

The aircraft reached the end of the runway, continued across the 61 metres paved blast pad and, having left this surface at 100 KCAS, then began to gently deviate to the left of the extended centreline across the grassed part of the runway safety area before exiting the airport perimeter through a fence and crossing a slightly elevated road and coming to a stop just beyond it with the tail resting on the road - see the reconstruction of the ground track followed below.

An emergency evacuation was immediately ordered by the Check Pilot and was accomplished using just four of the eight emergency exits. Two of the 4 overwing exits were not used, the evacuation slide at door 1R did not inflate and the tailcone exit door could not be opened fully because a seatbelt buckle “became wedged under it”. By the time the seatbelt buckle had been removed by one of the cabin crew, everyone had evacuated through the other available exits.

Damage to the aircraft was considerable and included all the landing gear assemblies being “bent, fractured and displaced aft” and lower fuselage damage consequent on gear collapse with lower skin panel assemblies showing severe buckling and deformation with some missing sections and damaged internal structure evident in places.

Why it happened

It was quickly obvious on an initial examination of the aircraft that the right elevator was jammed with its trailing edge down and could not be moved by hand as should have been possible. FDR data showed that after being powered up prior to departing on the accident flight, the left elevator had moved in the wind between approximately 16° trailing edge down and 27° trailing edge up but the right elevator had not moved at all from approximately 16° trailing edge down. An examination of relevant maintenance records found no evidence of any prior issues with either of the aircraft elevators.

The elevators on this aircraft type are primarily moved indirectly by the action of the control column on the control tabs and there is no direct link between the movement of the control column and the position of the elevators. The elevators are attached to the rear spar of the horizontal stabiliser by hinges and equipped with control, geared and anti-float tabs at their trailing edges. Elevator travel is possible in the range 27° up to 16.5° down limited in each case by mechanical stops. Each elevator also has a damper to prevent elevator flutter during flight and to dampen rapid movement of the elevator during gusting winds when the aircraft is on the ground. This means that when the aircraft is parked, each elevator is free to move independently if acted upon by an external force such as the wind or during maintenance activity and there is no gust lock and no connection between the left and right elevators.

Further examination of the aircraft showed that control column movement produced the expected response in the control tab positions although structural damage caused during the overrun was preventing the cables linking the two from moving smoothly. However, inspection of the elevators then found that the actuating mechanism controlling the movement of the right elevator had been forced beyond its normal range of travel and had become locked in an over-centre position thus effectively jamming the elevator in a full trailing edge down position so that it would not respond to any control tab movement. The possible effect of gusting winds on this elevator mechanism whilst the aircraft had been on the ground prior to the accident flight became the focus of attention.

Airworthiness standards applicable at the time of the accident were found to require that “flight control systems and surfaces of transport-category airplanes must be designed for the limit loads generated when the airplane is subjected to a 65 knot horizontal ground gust from any direction while parked and taxiing”. The aircraft type involved had, as the Douglas DC-9, been originally certified with a ground gust limit load equivalent to approximately 52 knots, and revised to 65 knots in 1997. Boeing advised the Investigation that they had not received any previous notifications of an elevator jam on an aircraft exposed to ground gusts above this revised limit.

No evidence that the prevailing wind once the aircraft had been powered up for the accident departure had been outside the certification limits was found and so available information on the wind conditions which had prevailed whilst the accident aircraft had been parked in the lee of a very large hanger at Ypsilanti was assembled. The open space recorded wind speeds were found to be well within the 65 knot ground gust limit load but it was suspected that this may not have been true of air movements around the aircraft tailplane because of localised turbulence generated in the lee of the adjacent large hangar. A virtual reality 3-D scale model of the hangar structure and its environs in relation to the position and profile of the parked aircraft was therefore created using data gathered using a sUAS and this and the available wind data were then passed to Boeing in order to populate a wind simulation model based on the principles of computational fluid dynamics (CFD). The results of this CFD modelling were then applied to a test rig of the MD83 horizontal stabiliser and elevator system and it was possible to show that wind eddy effects were indeed likely to have led to vertical wind forces in excess of the type certification limit being intermittently exerted on the right hand elevator (only) and that these would have been capable of causing the damage found to its actuator mechanism which had led to it being locked trailing edge down.

Other Safety Matters

In the course of the main thrust of the Investigation, other safety matters considered included the following:

• On an aircraft with this type of elevator control system, it was impossible for the flight crew to detect the jammed right elevator during either the external pre-flight inspection or the “full and free” control column check whilst taxiing out for takeoff.
• The decision of the flight crew to increase Vr for the accident takeoff by about 5 knots because of the gusty wind conditions was within Ameristar SOPs and considered to have been appropriate by the Investigation.
• Although Ameristar SOPs in the case of a rejected takeoff clearly established that the responsibility for the go/no-go decision was exclusively that of the aircraft commander, in this case the Check Pilot and in the accident takeoff this had created potential for confusion, it was noted that the Check Pilot had not taken control of the aircraft but having observed that the PF had initiated a rejected takeoff, had then assisted this. It was considered that had the Check Pilot assumed control of the aircraft after the Captain decided to reject the takeoff, “the results could have been catastrophic”. It was concluded that the Check Pilot had “demonstrated disciplined restraint in a challenging situation”.
• In respect of the malfunctioning evacuation slide at door 1R, it was concluded on the evidence available that it was “the incorrect installation of the valve release cable in the valve assembly” that had led to the failure of the slide to inflate. It was noted that the OEM had revised the CMM to include more description of the valve testing procedures intended to prevent improper cable installation.
• Although it had no direct bearing on the accident or its cause, it was found that prior to an evacuation of the building where he was based, the duty meteorological observer at Ypsilanti had failed to change the designation of subsequent METARs to AUTO during an unscheduled absence and that there was no FAA procedure to require this.

As a result of the Investigation, six Safety Issues were formally identified as follows:

• Lack of a means to enable flight crews of Boeing DC-9/MD-80 series and 717 model airplanes to verify before takeoff that the elevators are not jammed.
• Need for lower ground gust criteria for elevator physical inspections and operational checks by maintenance personnel for Boeing DC-9/MD-80 series and 717 model airplanes.
• Potential inadequacy of ground gust limit loads for the certification of transport category airplanes.
• Lack of procedures for operators of Boeing DC-9/MD-80 series and 717 model airplanes to monitor the wind that affects parked airplanes.
• Lack of procedures for weather observers related to sign off and backup augmentation responsibilities during a facility evacuation.
• Evacuation slide malfunction.

The Probable Cause of the accident was determined to be “the jammed condition of the airplane’s right elevator, which resulted from exposure to localized, dynamic wind while the airplane was parked and rendered the airplane unable to rotate during takeoff”.

Two Contributory Factors were also identified as follows:

1. the effect of a large structure on the gusting surface wind at the airplane’s parked location, which led to turbulent gust loads on the right elevator sufficient to jam it, even though the horizontal surface wind speed was below the certification design limit and maintenance inspection criteria for the airplane.
2. the lack of a means to enable the flight crew to detect a jammed elevator during preflight checks for the Boeing MD-83 airplane.

It was additionally found that the survivability of the accident had been aided by:

• the Captain’s timely and appropriate decision to reject the takeoff
• the Check Pilot’s disciplined adherence to standard operating procedures after the Captain called for the rejected takeoff
• the dimensionally compliant runway safety area where the overrun occurred.

Safety Action taken whilst the Investigation was in progress was noted as having included but not necessarily been limited to the following:

• Boeing began developing a SB to modify the elevator structure of DC-9/MD-80 series and 717 aircraft by attaching a secondary travel stop that would prevent the excessive elevator trailing edge down travel that could result in the elevator actuation mechanism becoming inoperative.
• Boeing began developing an AMM revision for DC-9/MD-80 series and 717 aircraft to add new elevator wind damage inspection procedures which would also include a lower recorded wind speed threshold for such an inspection. These new inspection procedures would not apply to any aircraft which had the secondary stop introduced in the above SB incorporated.
• Ameristar prepared and obtained FAA support for a Bulletin directed at their flight following personnel which required a more robust process of monitoring and assessment of available meteorological data at locations where company DC9/MD-80 series aircraft were parked out of service between flights.

A total of six Safety Recommendations were made as a result of the Investigation as follows:

• that the Boeing Company complete the development of a modification for Boeing DC-9/MD-80 series and 717 model airplanes that will prevent the possibility of elevator jamming due to ground wind exposure. [A-19-1]
• that the Boeing Company develop new preflight procedures or other mitigations for Boeing DC-9/MD-80 series and 717 model airplanes that will enable a flight crew to verify before takeoff that the elevators are not jammed. [A-19-2]
• that the Boeing Company, until the actions in Safety Recommendations A-19-1 and -2 are completed, revise the Aircraft Operating Manual and Aircraft Maintenance Manual (AMM) for Boeing DC-9/MD-80 series and 717 model airplanes to lower the ground gust criteria that will require physical inspections and operational checks of the elevators by maintenance personnel. [A-19-3]
• that the Federal Aviation Administration determine if the ground gust limit loads contained in Title 14 Code of Federal Regulations 25.415 adequately ensure that critical flight control systems are protected from hazards introduced by ground gusts that contain dynamic, vertical wind components. [A-19-4]
• that the Federal Aviation Administration ensure the operators of Boeing DC-9/MD-80 series and 717 model airplanes have procedures that define who is responsible for monitoring the wind that affects parked airplanes and for notifying maintenance personnel when conditions could meet or exceed the ground gust criteria specified in the Aircraft Maintenance Manual. [A-19-5]
• that the Federal Aviation Administration revise Order JO 7900.5D, Surface Weather Observing, Change 1, to specify sign-off procedures and backup augmentation responsibilities for all types of weather-observing personnel when they are unable to perform their prescribed duties from their normal duty station during normal duty hours. [A-19-6]

In accordance with the ICAO guidance for reporting of accident investigations under the heading “effective new investigation techniques” the use of an sUAS to obtain the imagery necessary to produce an accurate, photogrammetric 3-D model of the hangar, other buildings, and terrain near the area where the aircraft had been parked was highlighted.

The Final Report of the Investigation was adopted on 14 February 2019 and published on 7 March 2019.

Further Reading

• Runway Excursion
• Flight Controls
• Flight Crew Pre Flight External Check
• Elevator
• Trailing Edge
• Wind