- SAS MD-81 in Sweden
- Accident Overview
- Accident Board Findings
- Accident Board Recommendations
- Relevant Regulations / Policy / Background
- Prevailing Cultural / Organizational Factors
- Key Safety Issue(s)
- Safety Assumptions
- Resulting Safety Initiatives
- Airworthiness Directives (ADs) Issued
- Common Themes
- Related Accidents / Incidents
- Lessons Learned
- SAS MD-81 in Sweden
On December 26, 1991, the MD-81 operated by SAS arrived from Zurich and landed at Stockholm/Arlanda airport at 10:09 pm. After landing, the aircraft was parked overnight at gate 2 with approximately 5,622 lbs (2,550 kg) of fuel remaining in each wing tank. The air temperature at arrival was +33.8° F (+1° C). During the night a ground crew member inspected the aircraft and was required to clear slush from the landing gear in order to perform the landing gear inspection. At approximately 2:00 am, December 27, this crew member noted the presence of ice on the upper surface of the wings.
The aircraft was scheduled for an 8:30 am departure from Stockholm to Copenhagen on the morning of December 27 as Flight SK 751.
On December 27, at 7:30 am, another ground crew member noted frost coverage on the underside of the wings. He checked for ice on the upper side of the left wing and the left engine air inlet. He found no ice, but detected slush on the upper side of left wing. At approximately 8:20 am the air temperature had dropped to 32.0° F (0° C).
At 8:30 am, the aircraft was fuelled with 3,086 lbs (1,400 kg) of fuel and was ready for de-icing. After consultation with the captain, a ground crew member ordered de-icing of the underside of the wings due to an earlier detection of frost. There had been no discussions regarding clear ice, since none had been detected during the morning inspection. The ground crew ordered further de-icing after the first spray to assure the removal of slush from the wings. After de-icing, the ground crew did not check for clear ice since none was detected previously.
Ice Detection Tufts
Photo showing tufts on an MD-80 wing
Photo Copyright Juan Pablo Marini - used with permission
A portion of the MD-80 fleet, including this SAS aircraft was equipped with several nylon tufts on the upper surface of the wings, intended to be used as indicators of the presence of clear ice. It was understood that clear ice was difficult to detect visually, and the tufts were intended to enhance that detection capability. Instructions associated with the tufts required that the tufts be "touched" in order to determine the presence of clear ice, and its extent. If the tufts cold be freely manipulated, the tufted section of wing was ice free.
The operator of the de-icing spray nozzle reported that during the spraying he saw the movement of one of four upper wing indication tufts. A passenger at a window seat reported no movement of the upper wing indication tufts during the spraying.
The ground crew reported the completion of the de-icing to the captain, and when asked by the captain if "They've got it good and clean under the wings?" he replied "Yes, there was a lot of ice and snow, now it's fine, it's perfect now." The conversation ended with the captain saying "That sounds fine, thanks."
While the aircraft was being de-iced, the pilots carried on with the routine checks that included the departure procedure from Stockholm/Arlanda airport.
After engine startup and pushback from the gate, the captain taxied the aircraft to runway 08. Both engine anti-icing systems were 'On' with all systems operating in the normal range. During the two and a half minutes of taxiing, the aircraft average speed was 15 knots and the captain steered the aircraft to avoid running through taxiway slush. The captain made a normal rolling takeoff. The Auto Throttle System (ATS) which automatically and simultaneously controls the engine throttle levers was engaged.
Automatic Thrust Restoration System (ATR)
As a noise abatement measure, many carriers adopted procedures to use large thrust cutbacks following takeoff. The cutbacks were, in many cases, sufficiently large that an engine failure could result in a descent if thrust were not restored on the operating engine(s). In response to this possibility, and addressing FAA and airline concerns about crew workload during a critical phase of flight, some manufacturers developed the ATR system, which would, following a thrust cutback and subsequent engine failure, automatically restore the remaining operating engine(s) to the pre-cutback thrust level. This SAS MD-80 was equipped with a version of this system developed by McDonnell-Douglas. . The system was certified in March 1983, and subsequently introduced as standard equipment, irrespective of whether the special takeoff procedure was used. In 1992, the designation of Automatic Thrust Restoration (ATR) was introduced in the FAA approved Airplane Flight Manual and in the McDonnell-Douglas Flight Crew Operating Manual.
The ATR is a Digital Flight Guidance (autopilot) System safety function that monitors engine operation during takeoff and climbout. If an engine failure is sensed by the system, due to the MD-80 autothrottle single-clutch configuration, both the failed and non-failed engine thrust levers are advanced to the corresponding in-flight takeoff (go-around) thrust level in order to attain, and maintain required engine-out climb performance. The system is automatically armed when three conditions are met:
- The Flight Director pitch axis is set for takeoff.
- The aircraft's height above the ground is more than 350 feet.
- The current Engine Pressure Ratio (EPR) of both engines is below the go-around thrust EPR.
The system is automatically activated when engine EPR's differ by 0.25 or more at the same time as engine N1 speeds differ by 7% or more in the same direction.
Simultaneous movement of all throttle levers was considered acceptable, as it was assumed that a forward throttle movement on a failed engine would not have a hazardous effect on the airplane. The single clutch system did not foresee the possibility of a multiple engine malfunction. Advancing the throttle levers on a stalled engine, as was the case in this accident, would increase the fuel flow to that engine, and result in increasing turbine temperatures that could result in turbine damage if the engine stall continued.
The captain started the rotation at 8:47 am. Three passengers saw ice coming off from the upper side of the wings as the aircraft took off. At the same time, the captain heard an abnormal noise. He could not identify the source of the noise. The sound was recorded by the Cockpit Voice Recorder (CVR) as a low hum.
Illustration of clear ice formation
At 25 seconds into the flight and climbing through 1,124 ft (343 m), bangs were heard inside the aircraft followed by vibrations and jerking motions.
An SAS pilot inspects the wing for clear ice
Photo copyright Andreas Stoeckl - used with permission
The pilots isolated the problem to the right engine and a possible compressor stall. The vibrations and the rapid changes in the digital presentation made it difficult for the captain to read the engine instruments. At 43 seconds into the flight and climbing through 2,000 ft (600m), the captain reduced power to the right engine from 1.904 Engine Pressure Ratio (EPR) to 1.870 EPR, but the problems persisted. EPR is the ratio of the outlet pressure to the inlet pressure and is a reference value for engine thrust. The autopilot was not engaged and, according to the flight recorders, throttle control simultaneously changed to an automatic mode, increasing the power settings with altitude. This was indicated discreetly on the instrument panel and went unnoticed by the pilots.
At 64 seconds into the flight, the left engine's first surge was recorded and went unnoticed by the pilots.
At an altitude of 2,616 ft (797 m) an attempt to engage the autopilot failed and activated the voice warning "Autopilot." The warning continued for the rest of the flight.
The right engine failed 51 seconds after the first surge. Two seconds later and 78 seconds into the flight, climbing through 3,206 ft (977 m) at an indicated airspeed of 196 knots, the left engine failed. Within moments, the aircraft reached its maximum flight altitude of 3,318 ft (1011m).
Immediately following the second engine failure, the two Electronic Flight Information System (EFIS) display screens went blank due to loss of both engine-driven generators. The captain made no attempts to recover the EFIS display and for the remainder of the flight relied on a backup altimeter for the altitude information.
After the engine failures, the turbine outlet temperature readings from the engine instruments were over 1,472° F (800° C). This was first noticed by the first officer. Thirteen seconds later, the left engine fire warning came on. The first officer activated the left engine fire extinguishing system. Grey smoke was visible in the forward part of the aircraft. Twenty seconds later the fire warning ceased.
A flight captain traveling in the aft passenger cabin alerted the flight attendant sitting in the cabin rear jump seat that the right engine was surging. Using the intercom, she tried unsuccessfully to contact the aircraft's flight deck. She was finally able to alert the flight deck by passing the message through the flight purser.
A uniformed SAS captain in seat 2C hurried to the cockpit and offered help. He was given the emergency checklist by the first officer and the captain directed him to start the Auxiliary Power Unit (APU). The assisting captain's voice was first recorded two minutes and two seconds into the flight, encouraging the captain to "Look straight ahead."
After the engine failures, the crew prepared for an emergency landing. The captain initiated a glide path and a shallow left turn to a north bound heading. The first officer contacted Stockholm air traffic control and declared an emergency, requesting a clearance to return to Stockholm/Arlanda airport and was cleared to a right turn for landing on Runway 01. The captain, however, continued on the north bound heading.
The flaps were gradually extended by the assisting captain at 1,378 feet (420 m) with an indicated airspeed of 165 knots. Thirty seconds later, at 984 feet, the flaps were fully extended.
Illustration of SAS flight 751 flight path
Photo of SAS flight 751 impact site
The aircraft was entirely free from clouds at 980 to 820 ft (300 to 250 m). The captain determined that a large field far to the right of the aircraft could not be reached. He therefore selected a field in the direction of the flight path for the emergency forced landing.
Seventy seconds before ground impact, the first officer is heard on the CVR asking "Shall we get the wheels down?" A reply from the assisting captain was "Yes, gear down, gear down." Seven seconds later the sound of contacts with the trees was recorded on the CVR. According to the flight recorder, at about the same time, the gear was down and locked, and the airspeed had reduced to 121 knots. Upon contact with the trees, a major portion of the right wing tore off, resulting in a bank to the right. The last data recorded one second before impact, registered the aircraft in a 19.7° right bank with an airspeed of 107 knots.
Photos of SAS flight 751 damage
Four minutes and seven seconds after rotation, the aircraft tail struck sloping ground, and came to rest at 361 ft (110 m) from the point of ground impact. The fuselage broke into three major sections. There was no fire. All 129 people on board, including 123 passengers and a crew of six survived. Seven passengers and one crew member sustained serious injuries. Another 81 passengers and three crew members sustained minor injuries. The aircraft was totally destroyed.