Saturday, March 25, 2017


The MD-83 turning onto Runway 23L at Willow Run International Airport (KYIP) would never be able to takeoff, but no one on board knew that. The right elevator was jammed in the down position, and the pilots had no chance of ever being able to raise the nose enough to lift off.

Designing and flying a safe airplane is about delicately balancing huge forces. Gravity's remorseless tug must be balanced by lift; thrust is balanced by drag. If you do this right, you get steady level flight. To turn, you have to slightly perturb this arrangement. The ailerons on the wings bank the airplane (this is called the roll axis). The rudder rotates the plane left or right (yaw). The elevator, meanwhile, rotates the nose up or down (pitch).

The tail (or the empennage, if you want to sound fancy) on most conventional airplanes consists of a vertical stabilizer, sticking up like a shark fin and housing the rudder, while the horizontal stabilizer sprouts from either side of the tail, each containing half the elevator.

With the right elevator jammed in the down position, the most that the pilots would be able to do would be to cancel it out by pulling back on the control yolk until the left elevator was in the full up position. Even by doing that, they could only get back to zero net effect on pitch. They couldn't overcome it and raise the nose to takeoff.

What's more, they wouldn't be able to figure out there was a problem until they were already at a high enough speed to takeoff. On the DC-9/MD-80/MD-90 family, the full elevator isn't controlled by the yolk. Instead, only a small servo tab at the trailing edge of the elevator is actually controlled. Once the tab is deflected into the airstream, the airstream creates lift on the tab. Since it has a lever arm relative to the rest of the elevator, it uses this torque to pull the rest of the elevator in its direction and into the position desired by the pilot. This GIF is for a trim tab, but it works on the same principle.

The upside is that this significantly reduces the force needed for the pilot to move the big elevator without requiring hydraulic assistance. This directly translates into a weight savings. In aviation, weight is everything. A pound of extra weight is a pound of load you can't carry. Worsey, you also have to buy fuel to haul that extra pound of dead weight around with you.

The downside is this exact scenario. If the elevator is jammed but the servo tab is free to move, it's hard to tell that anything's wrong. The pilots would have no idea: I can move the controls back and forward with no problem.

The big question is one that I can't answer: Should the pilots have known that the elevator was jammed prior to beginning their takeoff roll? I don't know. I'm not a pilot, let alone an ATP (Airline Transport Pilot) with a type certificate for the MD-80. I don't know if part of their pre-flight inspection is verifying that the elevators are both in a neutral position / can freely move. It's possible that those big, gusty winds jammed the elevator during taxi. The NTSB's prelimary report lays the blame on damage to "right elevator geared tab inboard pushrod linkage". I can imagine a scenario where it was already fatigued and a gust of wind on taxi or while parked on the ground finished it off.

I will, however, contend that the pilots did everything right once they began their takeoff. The captain pulled back on the controls at 152 kts. Nothing happened. The speed rose to 166 kts, when the crew decided to abort takeoff. At this point, they had to know that they were above a speed known as V1. V1 is the maximum speed at which you can abort your takeoff and have enough runway left to safely stop without runnning off the end. They knew they didn't have enough room to stop, but they also correctly decided that they had a better chance of staying alive even if they ran off the end of the runway. They reached this conclusion probably less than 3 seconds after first trying to pull back on the control yoke. Between the decision to abort and braking / activating the thrust reversers, the plain gained another 7 kts of airspeed, but the speed dropped quickly as they approached the end of the runway. Without accidentally rolling the airplane, they also managed to veer left and avoid the metal structures of the runway lighting and instrument landing systems as they came to rest balanced over a ditch. Everyone walked away, which is the best thing you can say about a plane crash.

  • The pilots were some grizzled veterans. The captain for this flight "had accumulated 15,518 hours total and 8,495 hours on DC-9 type aircraft" and his co-pilot was the charter company's chief pilot (9,960 hours total, 2,462 hours on DC-9s). The captain had spent almost an entire year of his life aloft in DC-9s alone. Combined they'd spent almost 3 years in the air.
  • The wind might have damaged the aircraft. But it probably helped them stop in time. Airspeed is the measure of how fast the wind is going over your wings. With the winds reported at KYIP at that time, they had an effective 30-43 kt headwind, meaning they could be going 30-43 kts slower relative to the ground when they tried to take off. So that 173 kt max speed turns into about 140 kts of ground speed, which may have saved them a critical amount of stopping distance before the trees and ravine ahead of the plane.
  • Pulling the yolk all the way back would only cancel out the effect of the jammed elevator on pitch. The elevator would also have smaller but notable effects on roll and yaw that would have to be canceled by movements of the ailerons and rudder.

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