On 13 January 1982, a Boeing 737-200 being operated by Air Florida on a scheduled domestic passenger flight from Washington National to Tampa FL failed to transition to a climb in day Visual Meteorological Conditions (VMC) after taking off from runway 36 and soon afterwards hit a bridge and vehicles on it before coming to rest in the Potomac River. The aircraft was destroyed by impact forces and 69 of the 74 occupants and 4 people on the ground were killed and 4 more on the ground were injured.
An Investigation was carried out by the National Transportation Safety Board (USA) (NTSB). The Flight Data Recorder (FDR) and Cockpit Voice Recorder (CVR) were recovered and successful download accomplished.
It was found that the Captain had just four years jet experience, all gained at Air Florida, of which two years had been in command. The First Officer had two years commercial flying experience, all at Air Florida, after USAF service. The First Officer had been designated as PF. Take off weight was 46400kg.
The flight sequence was reconstructed as follows:
- the outside air temperature had been -4°C. After a two hour period of heavy snow, this was recorded as reducing to ‘moderate snow’ at 1525 and, apart from the interval between 1540 and 1553 for which light snow was recorded, remained so. An accumulation of wet snow was observed on the aircraft prior to the commencement of de icing.
- due to a change of operative mid way through, ground de/anti icing had been carried out by two different operatives. The left side of the aircraft was done first using a single stage application of a heated mixture of glycol and water with a 30/40% glycol concentration based on the actual air temperature. The right side was then done by a different operative and, based on an incorrect assumption that the temperature was -2°C, used hot water to de ice followed by a ‘finish coat’ of a heated solution of water and 20/30% glycol. Contrary to documented Air Florida procedures, neither engine inlet blanks nor pitot / static covers were installed during de/anti icing, although other evidence did not suggest this was a significant omission in respect of the accident outcome. The de/anti icing operation had commenced at about 1450 and was complete by 1510.
- at 1525, the attached tug tried and failed to push the aircraft back off stand. Both engines were then started and operated in reverse thrust for between 30 and 90 seconds to no better effect and the engines were shut down with the reversers deployed. Another tug successfully completed the push back ten minutes later.
- the aircraft had joined the queue for departure behind a DC-9 and appeared to have been manoeuvred to expose both sides of the aircraft to the jet efflux from that aircraft as a means to removing wet snow. There was some discussion about the difference in engine EPRs which was attributed to whether the engine was taking in efflux or ambient air.
- just before 1600, take off was commenced from runway 36 with an Runway Visual Range (RVR) of 640 metres in moderate snow and vertical visibility of 200 feet having received an instruction from ATC not to delay because of landing traffic at 2.5 miles.
- during the take off roll, the First Office made a number of comments about a discrepancy between the left and right engine EPRs but the commander did not appear to have been concerned. There was no clear evidence that the abnormally slow acceleration rate of the aircraft despite the indicated achievement of the required take off EPR, was recognised by either pilot.
- eight seconds after the V1 call at 137 KIAS and two seconds after the V2 call at 144 KIAS, the stick shaker activated and about 30 seconds after becoming airborne, in a significantly nose up attitude, the aircraft stalled and struck a busy road bridge situated about 1400 metres from the upwind end of the take off runway before continuing in a more level attitude into the ice covered river below. The various impacts led to the complete destruction of the aircraft.
- the FDR readout showed that the actual EPR required for take off, although indicated, was considerably more than was indicated, but since this was never appreciated by the crew, the considerable amount of additional thrust available was never applied.
Six occupied cars and a truck on the bridge were hit and a significant part of the bridge superstructure was torn away. The probably flight path was plotted as straight ahead with a slight turn to the left once stalled.
Since no evidence that the airworthiness of the aircraft had been a factor in the accident, the investigation focused on the ways in which the prevailing adverse weather conditions and the flight crew response to them could have had an effect on what happened. Each aspect was considered separately to establish the extent of any contribution which it may have made.
This was provided as part of a line maintenance contract between Air Florida and American Airlines. It was found that there had been little communication between the two carriers about de icing and it was clear that American Airlines maintenance personnel had only limited familiarity with the 737 aircraft which was not operated by their own Airline. The solitary Air Florida maintenance representative at the airport was found to have received only limited orientation training and that his duties and responsibilities were poorly defined. It was concluded that there should have been more complete discussions between Air Florida and American so that their respective responsibilities were fully understood. It was noted that the de/anti icing treatment given to the accident aircraft was not consistent with American Airlines’ own procedures for the prevailing ambient temperature and must be regarded as having been deficient. The fact that the de icing had commenced over an hour before the aircraft took off was also noted. However, it was concluded that, in the absence of sufficient data on the effectiveness of specific de/anti-icing procedures combined with the extended exposure to continual precipitation before takeoff, it was impossible to be sure that the de/anti-icing procedures used on the aircraft were a causal factor in the accident.
- Take off commenced with frozen deposits on the wings
It was clear from the investigation that the flight crew had been aware when they began their take off roll that there were visible frozen deposits on the wings. The Investigation was concerned that, despite the flight training received and advisory material provided to pilots stressing the importance of ‘clean’ wings for take off and FAA regulations specifically prohibiting takeoff with frost, snow or ice on the wings or control surfaces, some pilots appear not to fully understand or appreciate the extent to which even a small amount of contaminant can degrade aerodynamic performance. It was considered that the accident circumstances had served to confirm wider evidence that pilots might be influenced by such factors as anticipated further delays, observations of other departing aircraft and even personal encounters with airframe icing during the cruise which might lead them to believe that some wing contamination is acceptable.
- Failure to select engine anti ice on
The Investigation found that the most significant factor in this accident was the failure of the flight crew to use engine anti-ice during ground operation and for take off. Because of this, the engine inlet pressure probe became blocked with ice, causing a significant error in the EPR reading displayed in the flight deck, as a result of which take off was attempted with an actual thrust setting which was too low. It was considered that, even if selection of engine anti ice had not occurred until just before take off rather than after start as per the checklist prompt, "had the crew turned it on at that time, the accident probably would have been averted”. It was considered that an experienced professional flight crew operating in adverse weather conditions such as prevailed for the accident take off would be expected to have routinely double checked all critical items related to safe operations, such as pitot/static heaters and engine anti-ice, regardless of their inclusion on checklists.
- Known vulnerability of aircraft type to un-commanded pitch up if wing leading edges are contaminated with frozen deposits
It was noted that operators of Boeing 737 aircraft have reported a number of aircraft pitch up or roll off incidents immediately after takeoff in weather conditions conducive to the formation of ice or frost on the wing leading edges. The investigation found that the evidence indicates that the accident aircraft did experience an un-commanded pitch up at rotation for take off which was not or could not be immediately corrected so that the aircraft became airborne in a high pitch attitude. This, combined with low thrust and high drag, was together considered to have inhibited acceleration to a safe airspeed.
ATC control of departure delays at a time when adverse weather conditions were a major constraint on runway occupancy in general and specifically on departure rates was considered. It was concluded that take off delays could have been reduced and that the system for supporting controllers in achieving this was deficient.
Although not directly related to the circumstances of the accident, it was noted that the aircraft approaching runway 36 as the accident aircraft began its take off roll, an Eastern Airlines Boeing 727, was found to have landed whilst the departing aircraft was still on the runway having received its landing clearance when ‘over the lights’ only a few seconds after 737 began to move. This was identified as a significant violation of the prevailing requirements for separation since the applicable requirements were that, under IFR, controllers must separate a departing aircraft from an aircraft on final approach by a minimum of 2 miles if separation will increase to a minimum of 3 miles within 1 minute after takeoff. This requirement is intended to ensure safe separation if the aircraft on approach executes a missed approach. However, the violation was of far more concern because both the TWR controller on duty at the time of the accident and the Head of TWR ATC stated at the Board Public Hearing after the accident that the applicable criteria are widely interpreted to allow for the "accordion effect" of landing deceleration and takeoff acceleration whereas both controller training manuals and controller tests clearly indicate that no such allowance is intended. The Investigation was additionally concerned that no operational error had been filed in respect of the violation and that the number of such errors is used as a means to monitor ATC system safety which by this criterion has shown a marked improvement which may be false.
The Investigation also documented a range of concerns at deficiencies in the Rescue and Fire Fighting Services capabilities of Washington National Airport and noted the “extensive improvements which have been made” in the six months following the accident.
The formal statement of 37 Findings of the Investigation included the following:
- The flight crew did not use engine anti-ice during ground operation or take off.
- The engine inlet pressure probe on both engines became blocked with ice before the initiation of take off.
- The flight crew was aware of the adherence of snow or ice to the wings while on the ground awaiting take off clearance.
- The crew attempted to de-ice the aircraft by intentionally positioning it near the exhaust of the aircraft ahead in line. This was contrary to flight manual guidance and may have contributed to the adherence of ice on the wing leading edges and to the blocking of the engines’ inlet pressure probes.
- Engine thrust actually produced by each engine during take off was equivalent to an EPR of 1.70 which was about 3750 pounds net thrust per engine less than that which would have been produced at the (required) takeoff EPR of 2.04.
- Although the First Officer expressed concern that something was “not right” to the Captain four times during the take off, the Captain took no action to reject the take off.
- Snow and / or ice contamination on the wing leading edges produced a nose up pitching moment as the aircraft was rotated for lift off.
- The aircraft stall warning stick shaker activated almost immediately after lift off and continued until impact.
- The aircraft encountered stall buffet and descended to impact at a high angle of attack.
- The aircraft could not sustain flight because of the combined effects of airframe snow or ice contamination which degraded lift and increased drag and the lower than normal thrust set by reference to the erroneous EPR indications. Either condition alone should not have prevented continued flight.
- Continuation of flight should have been possible immediately after stick shaker activation if appropriate pitch control had been used and maximum available thrust had been added. While the flight crew did add appropriate pitch control, they did not add thrust in line in time to prevent impact.
- Runway distance reference markers would have provided the flight crew (with) invaluable assistance in evaluating the acceleration rate of the aircraft and making a go / no go decision.
The Probable Cause of the accident was determined as:
“The flight crew'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.” It was additionally concluded that: “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 B737 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.”
The following 10 Safety Recommendations were issued to the FAA “as a result of this accident and several others involving operations in snow and ice conditions” in a letter dated 28 January 1982, just 15 days after the accident:
- Immediately notify all air carrier operators of the potential hazard associated with engine inlet pressure probe icing, and require that they provide flight crews with information on how to recognize this hazard and requiring that flight crews cross-check all engine instruments during the application of takeoff power. [A-82-6]
- Immediately review the pre-departure de icing procedures used by all air carrier operators engaged in cold weather operations and the information provided to flight crews to emphasize the inability of de icing fluid to protect against re-icing resulting from precipitation following de icing. [A-82-7]
- Immediately review the information provided by air carrier operators to flight crews engaged in cold weather operations to ensure comprehensive coverage of all aspects of such operations, including the effects of a runway contaminated by snow or slush on takeoff, and methods to be used to obtain maximum effectiveness of engine anti-ice during ground operations and takeoffs. [A-82-8]
- Immediately require flight crews to visually inspect wing surfaces before takeoff if snow or freezing precipitation is in progress and the time elapsed since either de icing or the last confirmation that surfaces were clear exceeds 20 minutes to ensure compliance with 14 CFR 121.629(b) which prohibits takeoff if frost, snow or ice is adhering to the wings or control surfaces. [A-82-9]
- Immediately issue a General Notice to all FAA tower and air carrier ground control personnel alerting them to the increased potential for aircraft icing during long delays before takeoff and when aircraft operate in proximity to each other during ground operations in inclement weather, and encouraging procedural changes where possible so that the controllers implement the gate-hold provisions of the Facilities Operations and Administration Manual 7210.3F, paragraph 1232. [A-82-10]
- Document the effect of engine inlet pressure probe blockage on engine instrument readings and require that such information be added to approved aircraft flight manuals. [A-82-11]
- Amend Advisory Circulars 91-13c, “Cold Weather Operation of Aircraft”, and 91-51, "Airplane De-ice and Anti-Ice Systems”, to discuss in detail the effects and hazards associated with engine inlet pressure probe icing. [A-82-12]
- Revise the air traffic control procedures with respect to aircraft taxiing for takeoff, holding in line for takeoff, and taking off to provide for increased ground separation between aircraft whenever freezing weather conditions and attendant aircraft icing problems exist. [A-82-13]
- Expand the training curricula for air traffic controllers and trainees to assure that instruction includes the hazards associated with structural and engine icing of aircraft. [A-82-14]
- Immediately disseminate the contents of this safety recommendation letter to foreign operators involved in cold weather operations. [A-82-15]
A further 11 Safety Recommendations were issued in a letter to the FAA dated 11 August 1982:
- Inspectors to emphasize to air carrier maintenance departments that proper maintenance of ground support equipment may be critical to flight operations and the importance of adhering to maintenance practices recommended by the manufacturers of such equipment. [A-82-79]
- Issue a Maintenance Alert Bulletin to require Principal Maintenance Inspectors to review contract agreements between an air carrier operating into a facility at which another air carrier or maintenance contractor is providing maintenance services to assure that the responsibilities of both parties and key personnel are clearly defined and that the contractor providing the maintenance is thoroughly familiar with the maintenance of the type of aircraft involved. [A-82-80]
- Issue an Operations Alert Bulletin to require Principal Operations Inspectors to require that air carrier training programs adequately cover the effects of aircraft leading edge contamination on aerodynamic performance, particularly as it affects the relationship between airspeed and angle of attack and those functions whose activation is dependent on the angle of attack, such as stall warning systems and autothrottle speed command systems. [A-82-81]
- Require revision of the B-737 Approved Flight Manual to add "anti-ice" to the normal taxi and takeoff checklist. Review the checklists for all air carrier aircraft to ensure that all action items required for a successful takeoff are included on the appropriate checklist. Special consideration should be given to items whose functions may be affected by environmental conditions subject to change during ground delay periods. [A-82-82]
- Issue an Airworthiness Directive to implement the necessary airplane modifications and/or changes in operational procedures for B-737 aircraft takeoff operations during weather or runway conditions conducive to the formation of leading edge frost, snow, or ice contamination to require either: (1) that the leading edge is free of frozen contaminant through the pre takeoff use of a ground-operable wing thermal anti-ice system, or (2) an increased stall airspeed margin at liftoff which will provide adequate pitch and roll control to counter the effects of undetected leading edge contaminants by modification of takeoff flaps configuration and/or increased takeoff airspeed schedules. [A-82-83]
- Amend Air Traffic Control coordination procedures and practices to require that terminal and en route facilities provide the Central Flow Control Facility (CFCF) with current and accurate information regarding congestion and that CFCF act on that information in a positive manner to minimize airport saturation and extensive traffic delays. Review implementation of prescribed gate-hold procedures and require their use wherever possible. [A-82-84]
- Issue a General Notice to terminal area Air Traffic Control facilities to emphasize to controllers that the separation criteria set forth in FAA Air Traffic Control Handbook 7110.65C which require a minimum of 2 miles separation do not permit deviation based upon the anticipated acceleration differences between landing and departing traffic. [A-82-85]
- Evaluate the criteria and current practices of Air Traffic Control facilities regarding the declaration and reporting of operational errors to ensure that all such errors are reported and are investigated. [A-82-86]
- Provide for essential equipment and increased personnel training to improve the water rescue capabilities at the Washington National Airport in all anticipated weather conditions, and provide necessary funding for surrounding communities and jurisdictions which will be called on to support the airport's rescue response. [A-82-87]
- Survey all certificated airports having approach and departure flight paths over water and evaluate the adequacy of their water rescue plans, facilities, and equipment according to the guidance contained in Advisory Circular 150/5210-13 and make recommendations for improvement as necessary to appropriate airport authorities. [A-82-88]
- Amend 14 CFR 139.55 to require adequate water rescue capabilities at airports having approach and departure flightpaths over water which are compatible with the range of weather conditions which can be expected. [A-82-89]
The Final Report of the Investigation was approved for release by the NTSB on 10 August 1982, just under seven months after the accident had occurred.
Note: Although the AEA ceased to exist in 2016, the most recent of their publications still contain some pertinent information. Readers are cautioned to validate the recommendations of these guidebook using more current information sources.