On March 5, 2015 Delta Flight 1086, an MD-80 en route from Atlanta to New York, skidded off Runway 13 at LaGuardia airport, coming to rest on a dike alongside the runway with its nose hanging over the waters of Flushing Bay.

29 of the 127 passengers and crew on board sustained minor injuries and the aircraft was substantially damaged. Both cockpit crew were highly experienced MD-80 pilots with thousands of hours logged in the aircraft, and the captain had made many landings at LaGuardia under winter conditions.

How did this accident happen?

The National Transportation Safety Board (NTSB) accident report[1] states that the crew was concerned about the available landing distance given the conditions at LaGuardia and spent considerable time en route analyzing the aircraft’s stopping performance, consulted the aircraft’s operating manual and requested braking action reports starting 45 minutes before landing, but none were available at the time. They determined that they could not land unless braking was reported as “good.” This and uncertainty about the runway condition, which was closed for snow removal, added to their status.

After the runway re-opened, two aircraft landed and reported that braking action was good. Reported weather conditions were ¼ mile visibility in snow and freezing fog, indefinite ceiling 900 feet, with an 11 knot left crosswind. The runways were wet and had been de-iced with chemicals.

When the plane emerged from the overcast on final approach the runway appeared snow covered, contrary to what the crew expected. The NTSB stated, “The snowier-than-expected runway, along with its relatively short length and the presence of Flushing Bay directly off the departure end of the runway, most likely increased the captain’s concerns about his ability to stop the airplane within the available runway distance, which exacerbated his situational stress.”

One second after touching down the captain used “relatively aggressive reverse thrust.” The aircraft began sliding to the left, the captain ceased reverse thrust, and attempted to steer to the right with rudder, nose wheel and braking, but the aircraft departed the runway. It continued to slide, its left wing stuck the dike at the water’s edge, and it came to rest with its nose over the water.

Because the engines of the MD-80 series aircraft are rear-mounted, near the tail and rudder, the hot, turbulent exhaust gases created by reverse thrust reduce the rudder’s steering ability (“rudder blanking”). As a result, the manufacturer’s maximum recommended reverse thrust value is 1.3 EPR[2] on “contaminated” (wet or snowy) runways. The NTSB stated, “Both pilots were aware that 1.3 EPR was the target setting for contaminated runways.” It found that Flight 1086’s maximum reverse thrust exceeded 2.0 EPR on the left engine and 1.9 on the right, and that these high values caused rudder blanking and loss of directional control. The NTSB concluded that the probable cause of the accident was “the captain’s inability to maintain directional control of the airplane due to his application of excessive reverse thrust, which degraded the effectiveness of the rudder in controlling the airplane’s heading.”

NTSB Board member Robert Sumwalt added a statement to the accident report that the crew was aware of the challenges they faced, carefully assessed landing distances and alternatives, and that the captain initiated reverse thrust promptly and aggressively “presumably because of his concern for stopping on this relatively short, snow-covered runway. He went on to say, “As a former airline pilot for over 20 years, I’m confident that having to limit reverse thrust on a relatively short, slippery runway is counter-intuitive: When you need it the most, you have to use it the least.” In other words, MD-80 pilots landing on short, slippery runways face a Catch-22 dilemma: risk overrunning the runway, or risk losing control of the aircraft.

Mr. Sumwalt further noted that the NTSB reviewed 80 Delta MD-80 landings and found that all of them exceeded the maximum recommended reverse thrust on wet runwaysi.e., that the actions of Flight 1086’s captain were far from exceptional. And he pointed out that Delta’s procedures referred to the reverse thrust figure as a “target” value, rather than a maximum, saying that sometimes a target is met and sometimes it isn’t, but a maximum value “is one pilots know not to exceed.” The Sumwalt statement was signed by the NTSB Chairman and the other participating Board members.