Accident at Cleveland, Ohio, 17 Feb 1991

Accident narrative

About 0019 on February 17, 1991, Ryan International Airlines flight 590, a DC-9-15, crashed while taking off from Cleveland-Hopkins International Airport in Cleveland, Ohio. The flight was contracted to carry mail for the U.S. Postal Service. Both pilots were fatally injured, and the airplane was destroyed in the crash and postimpact fire.

### The flight

Flight 590 originated at Greater Buffalo International Airport in New York, with a final destination of Indianapolis International Airport in Indiana, and an intermediate stop in Cleveland.

The flightcrew consisted of two pilots. The captain had accumulated approximately 10,505 total flight hours, of which 505 were in the DC-9 series 10. He had previously flown the DC-9-30 in the U.S. Air Force. The first officer had approximately 3,820 total flying hours, with 510 in the DC-9. Most of his experience was in DC-9 series 30 airplanes at USAir; he had about 30 hours in the DC-9-10 with Ryan. The crew was on their sixth successive night flying the Buffalo-Cleveland-Indianapolis route.

### Events preceding takeoff

About 55 minutes before flight 590 landed at Cleveland, another DC-9 flightcrew reported moderate turbulence and rime icing from 3,000 to 7,000 feet. Approach control relayed this and a second pilot report of moderate rime icing from 7,000 feet to the surface. The flightcrew of Ryan 590 acknowledged receiving this information during their descent.

The airplane landed at 2344 and taxied to the mail ramp. During the approximately 35 minutes the airplane was on the ground, dry and blowing snow fell continuously. The temperature was 23°F and the dewpoint was 20°F. Neither Ryan 590 nor any other flight that departed Cleveland during that time requested deicing service. Witnesses indicated that the pilots remained in the cockpit during the stop.

### The takeoff sequence

The tower cleared flight 590 for takeoff on runway 23 left at 0018:17.

The cockpit voice recorder captured the captain calling "Vee one" at 0018:44, "rotate" at 0018:45, "Vee two" at 0018:48, "plus ten" at 0018:49, and "positive rate" at 0018:50.

At 0018:51, the captain warned the first officer to "watch out" three times in quick succession. Immediately after, the recorder captured sounds similar to engine compressor surges, followed at 0018:53 by the sound of the stick shaker stall warning. The sound of the first impact occurred at 0018:57.

Witnesses and the tower controller observed the airplane lift off, roll sharply to the left and right, pitch up, and roll past 90 degrees to an inverted position before crashing. Flames or a fireball were seen coming from the rear or the left engine during the attitude changes.

### What the investigation found

The airplane's left wing struck the ground to the right of the runway. The wreckage path extended about 1,600 feet, and the fuselage came to rest inverted.

Investigators found no evidence of preexisting faults in the airplane's structure, systems, or engines. The flaps were set at approximately 20 degrees, the spoilers were retracted, and the horizontal stabilizer was trimmed 3.4 degrees nose up, all of which were proper for takeoff. The Board concluded that windshear was not a factor.

Damage to the left engine indicated high rotor speed at impact. The Board concluded that the engine compressor surges heard on the recorder were an effect of disrupted airflow from an aerodynamic stall, not a causal factor.

A performance study determined that the airplane lifted off at about 148 knots, slightly higher than normal, and reached a maximum normal acceleration of 1.17 g before abruptly decreasing. The stick shaker activated at an airspeed of about 150 knots and an angle of attack of 12 degrees. Under normal conditions, the airplane should have had more than 20 knots of stall margin at liftoff. The Board concluded the lift coefficient for the wing was nearly 30 percent less than theoretical values.

The DC-9 series 10 lacks the wing leading edge devices found on later DC-9 and MD-80 models. The manufacturer had previously noted that a minute amount of ice contamination—as thin as 0.014 inch, equivalent to medium-grit sandpaper—could reduce maximum lift by 25 percent and cause the wing to stall before the stick shaker activated. The Board noted four previous accidents since 1968 involving DC-9 series 10 airplanes losing control on takeoff due to wing ice contamination.

The Board believed the most likely explanation for the ice formation was that the crew used the wing anti-ice system during the descent, and the blowing dry snow melted on the heated wings after landing and refroze as the wings cooled.

Ryan International Airlines' DC-9 operations manual did not require an exterior walkaround inspection at intermediate stops, unlike its manual for the Boeing 727. The Board concluded the crew did not violate written company policy by remaining in the airplane, but that failing to conduct a walkaround was contrary to good practices. The Board noted that from the cockpit or cargo door, detecting a minute amount of ice would have been unlikely. The Board also noted that the crew's previous experience in DC-9 series 30 airplanes, which are less vulnerable to minute contamination, might have lessened their concern about the hazard.

The Board concluded that efforts by the FAA and the manufacturer to educate line pilots about the DC-9 series 10's vulnerability to minute ice contamination had been inadequate, and that the information should have been included in the approved Airplane Flight Manual.

### Probable cause

The National Transportation Safety Board determined that the probable cause of this accident was "the failure of the flightcrew to detect and remove ice contamination on the airplane’s wings, which was largely a result of a lack of appropriate response by the Federal Aviation Administration, Douglas Aircraft Company, and Ryan International Airlines to the known critical effect that a minute amount of contamination has on the stall characteristics of the DC-9 series 10 airplane. The ice contamination led to wing stall and loss of control during the attempted takeoff."