Case Study: Air Florida 90

Figure: Flight path and witness locations, from NTSB Aircraft Accident Report, Figure 1.
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[NTSB Aircraft Accident Report, AAR-82-8, §1.1]

Flight 90 was scheduled to depart Washington National Airport at 1415 e.s.t. . . . Snow was falling throughout the early afternoon.
About 1420, American Airlines maintenance personnel began deicing the left side of the fuselage. . . . Fluid had been applied to an area of about 10 feet when the captain terminated the operation because the airport was not going to reopen at 1430.
Between 1445 and 1450, the captain requested that the deicing operation be resumed. The left side of the aircraft was deiced first.
Between 1445 and 1500 the operator of the deicing vehicle was relieved from this deicing task, and he told his relief operator, a mechanic, that the left side of the aircraft had been deiced. The relief operator proceeded to deice the right side of the aircraft.

The aircraft was deiced about 50 minutes prior to its eventual takeoff. The report, page 56, notes: "The Safety Board is concerned that pilots may erroneously believe that there is a positive protection provided for a period following the application of deicing/anti-icing solution which eliminates the need to closely monitor the aircraft for contaminants during ground and takeoff operations. The Safety Board concludes that the only way to assure that the deicing process has been effective and that the aircraft is clean of adhering ice, snow, or frost, is by observation by the flight crew just before takeoff roll." Of course we have made a lot of progress since this mishap, but don't think you can always use the holdover tables or that they remove the obligation for a pretakeoff contamination check.

More about this: Cold Weather Operations.

A tug was standing by to push Flight 90 from gate 12. . . . At 1525, the tug attempted to push Flight 90 back. However, a combination of ice, snow, and glycol on the ramp and a slight incline prevented the tug, which was not equipped with chains, from moving the aircraft. When a flight crewmember suggested to the tug operator that the aircraft's engine reverse thrust be used to push the aircraft back, the operator advised the crewmember that this was contrary to the policy of American Airlines. According to the tug operator, the aircraft's engines were started and both reverser's were deployed. He then advised the flightcrew to use only "idle power."

Boeing warns against the use of reverse thrust in this situation because of occurrences of takeoff pitch control anomalies with the B-737 after ground operation in freezing conditions. This warning was incorporated into Air Florida flight manuals.

Witnesses estimated that both engines were operated in reverse thrust for a period of 30 to 90 seconds. During this time, several Air Florida and American Airlines personnel observed snow and/or slush being blown toward the front of the aircraft. One witness stated that he saw water swirling at the base of the left (No. 1) engine inlet. Several Air Florida personnel stated that they saw an area of snow on the ground melted around the left engine for a radius of 6 to 15 feet. No one observed a similar melted area under the right (No. 2) engine.
When the use of reverse thrust proved unsuccessful in moving the aircraft back, the engines were shut down with the reversers deployed. The same American Airlines mechanic that had inspected both engine intakes upon completion of the deicing/anti-icing operation performed another general examination of both engines. He stated that he saw no ice or snow at that time.
[After a second tug was hooked up] At 1535, Flight 90 was pushed back without further difficulty.
No witnesses saw the flight crew leave the aircraft to inspect for snow/ice accumulations while at the gate. Departing and arriving flight crews and others who saw Flight 90 before and during takeoff stated the aircraft had an unusually heave accumulation of snow or ice on it.
At 1538:16 while accomplishing the after-start checklist items, the captain responded "off" to the first officer's callout of checklist item "anti-ice." [The aircraft was cleared to taxi behind a New York Air DC-9, call sign Apple 4]

The flight manual required the use of engine anti-ice under these circumstances and there were no restrictions against its use. Furthermore, there was no significant performance penalty for its use during takeoff and climb.

At 1540:42, the first officer continued to say, "it's been a while since we've been deiced." At 1546:21, the captain said, "Tell you what, my windshield will be deiced, don't know about my wings." The first officer then commented, "well--all we need is the inside of the wings anyway, the wingtips are gonna speed up on eight anyway, they shuck all that other stuff." At 1547:32, the captain commented, "(Gonna) get your wing now." Five seconds later, the first officer asked, "D'they get yours? Did they get your wingtip over 'er?" The captain replied, "I got a little on mine." The first officer then said, "A little, this one's got about a quarter to half an inch on it all the way."

It appears the captain was attempting to use the jet blast from the aircraft in front of him to deice his airplane, varying his position in the taxiway to get various portions of his aircraft.

Figure: JT-8D Exhaust pattern, from NTSB Aircraft Accident Report, AAR-82-8, figure 7.
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Of course taxiing closely behind another aircraft in these conditions would have the opposite effect from what was intended. From page 61: "The Safety Board believes the heat of the exhaust gases may have turned snow, which otherwise might have blown off during takeoff, into a slushy mixture. The mixture then froze on the wing leading edges and the engine inlet nose cone. The flight manual states: "Maintain a greater distance than normal between airplanes when taxiing on ice or snow covered areas. Engine exhaust may form ice on the ramp and takeoff areas of the runway and blow snow and slush which freezes on surfaces it contacts.

Figure: Engine sensor locations, from NTSB Aircraft Accident Report, Figure 5.
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At 1548:59, the first officer asked, "See this difference in that left engine and the right one?" The captain replied, "Yeah." The first officer then commented, "I don't know why that's different — unless it's hot air going into that right one, that must be it — from his exhaust — it was doing that at the chocks awhile ago. . . .ah." At 1551:54, the captain said, "Don't do that — Apple, I need to get the other wing done."

Post accident simulator tests led the investigators to conclude that the left engine Pt₂ engine probe was blocked, causing the anomaly the first officer spotted.

...the tower transmitted the takeoff clearance and the pilot acknowledge, "Palm 90 cleared for takeoff."
The CVR indicated that the pretakeoff checklist was completed at 1559:22. At 1559:45, as the aircraft was turning to the runway heading, the captain said, "Your throttles." At 1559:46, the sound of engine spool up was recorded, and the captain stated, "Holler if you need the wipers." At 1559:56, the captain commented, "Real cold, real cold," and at 1559:58, "the first officer remarked, "God, look at that thing, that don't seem right; does it?"

Most of the engine instruments were damaged from the impact or the water, but simulator tests show the flight path could be approximated with an engine EPR setting of 1.70, versus the 2.04 target. Tests of the engine EPR system also showed that with a blocked Pt₂ probe, the probe senses vent pressure and will indicate 2.04 EPR with the engine actually operating at 1.70. Had they activated the engine anti-ice system, they would have "immediately notice a substantial drop in the indicated EPR."

Between 1600:05 and 1600:10, the first officer stated, "...that's not right...," to which the captain responded, "Yes it is, there's eighty." The first officer added, "...maybe it is," but then 2 seconds later, after the captain called, "hundred and twenty," the first officer said, "I don't know."

The aircraft pitched higher than normal during the rotation because of contamination on the wing.

Eight seconds after the captain called "Vee one" and 2 seconds after he called "Vee two," the sound of the stick shaker was recorded. At 1600:45, the captain continued, "Come on, forward, forward, just barely climb." At 1610:00, the first officer said, "Larry, we're going down, Larry," to which the captain responded, "I know it."

Simulator tests verify that had they increased thrust at any point from after liftoff until about 15 seconds, they could have pulled out of the descent and continued the climb.

About 1601, the aircraft struck the heavily congested northbound span of the 14th Street Bridge, which connect the District of Columbia with Arlington County, Virginia, and plunged into the ice-covered Potomac River.

[NTSB Aircraft Accident Report, AAR-82-8, ¶2.3]

The Safety Board concludes that neither the low thrust used during the takeoff nor the presence of snow or ice on the aircraft, alone, would likely have led to the crash. In most other reported incidents in which B-737's have pitched up during takeoff, the flightcrews had sufficient control authority with forward control column force and stabilizer trim to overcome the pitching moment, reduce the pitch attitude, accelerate to a lower angle of attack,and climb out successfully. The Safety Board believes that if the proper thrust level (that for 2.04 EPR) had been used for the takeoff this flightcrew could have recovered from any difficulties caused by the contamination — induced aerodynamic performance penalties.
Furthermore, based upon the engineering simulation, the Safety Board concludes that even with the low thrust during the takeoff roll and the aerodynamic penalty of the snow or ice contamination, the accident was not inevitable as the aircraft lifted off. However, both immediate recognition of the situation and positive effective actions by the flightcrew to both counter the noseup pitching moment and add thrust were required. With these actions, the aircraft should have been capable of continued acceleration and achieved a sufficient performance margin for climbout.

[NTSB Aircraft Accident Report, AAR-82-8, page ii.] The National Transportation Safety Board determines that the probable cause of this accident was the flightcrew's failure to use engine anti-ice during ground operation and takeoff, their decision to take off with snow/ice on the airfoil surfaces of the aircraft, and the captain’s failure to reject the takeoff during the early stage when his attention was called to anomalous engine instrument readings. Contributing to the accident were the prolonged ground delay between de-icing and the receipt of ATC takeoff clearance during which the airplane was exposed to continual precipitation, the known inherent pitch up characteristics of the B-737 aircraft when the leading edge is contaminated with even small amounts of snow or ice, and the limited experience of the flight crew in jet transport winter operations.

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Air Florida 90

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