The Weekly Reflektion 34/2022

When a design weakness has been identified, compensating measures have to be agreed and implemented.

The tail elevators showing the right one locked down in the wrong position for take off

Are your compensating measures adequate?

Thanks to Neil Sultan, one of our subscribers for bringing this incident to our attention. On 8th March 2017, an MD83 aircraft, a variant of the DC-9 from Boeing McDonnell Douglas was taking off from Willow Run airport in Michigan, US with a destination of Washington, Dulles. The pilot did not have many hours on the MD83 aircraft but was very experienced with the DC-9. There were, however, differences between theaircraft types, and a second senior pilot was in the cockpit as a ‘Check Airman’ to ensure the differences between the aircraft were clear. Winds at the time of take-off were high and gusting. As a precaution the pilot and ‘Check Airman’ in the role of first officer agreed to accelerate for six seconds after the V1 speed, the speed where abortion of the take-off was not possible. These six seconds would give a safety margin for potential reduction in aircraft lift caused by the gusting wind.

Upon reaching the V1 speed plus the agreed six seconds, the pilot attempted to take-off. It became quickly apparent that the aircraft was not lifting from the runway, and the pilot took the decision to abort the take-off, even though the plane would certainly overshoot the runway. The first officer, also a senior pilot initially protested that the take-off should not be aborted, but as his role was first officer, he complied with the pilot’s decision. The plane overshot the runway, through the perimeter fence, and over a road, shearing off the landing gear before coming to a stop. The plane was evacuated successfully with no injuries.

The resulting investigation revealed that a known weakness in design had caused the accident. As the post-accident photograph shows, the left rear elevator was in the correct position to start the take-off, but the right elevator was locked in the wrong position preventing the take off. During high winds, specified to over 65 knots, Boeing required a manual check to ensure that the elevators were free, as a ‘control tab’ and ‘geared tab’ that were directly connected to the aircraft control stick, and activated the elevators, were known to be able to move past their intended stop point and lock-up when exposed to wind. As the winds had only been measured at 58 knots, no check was made. The investigation found that the plane, parked near a hangar, was exposed to vortexes as the wind accelerated over the building. Modelling followed by physical testing confirmed that these vortexes could cause forces on the plane’s tail which could result in a locked elevator.

The Check Airman’s first impulse was to continue the take offas the speed was greater than the calculated V1 speed, but, as discussed in the crew resource management meeting before take-off, he chose to support the pilot’s decision to abort, even though he knew the plane would crash. Both the pilot and the Check Airman were praised for taking the decisions they didand preventing a bigger catastrophe during take-off. The crew resource management techniques where roles are clarified before take-off was a key factor in preventing a disaster.

When a design weakness has been identified, how effective are your compensating measures?

Reflekt AS