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This document from AeroSAfetyWorld, published in January 2007, reports on the Helios Airways Boeing 737 crash that occurred on August 14, 2005. The pilots failed to notice a misset pressurization mode selector and misidentified a warning about cabin altitude, leading to hypoxia and eventual crash. the checklists, the pressurization system, and the technical log entries related to the incident.
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missed
By Mark Lacagnina
he Helios Airways Boeing 737- 300 was climbing through 16, ft after departing from Larnaca, Cyprus, on Aug. 14, 2005, when the captain reported a takeoff configu- ration warning to operations person- nel. The warning horn that the captain heard was actually for a problem with the cabin-pressurization system, ac- cording to the Hellenic Air Accident Investigation and Aviation Safety Board in Greece. Unaware of the problem, the pilots were incapacitated by hypoxia, and the aircraft, on automatic control, continued toward Athens, entered a holding pattern and plunged to the ground after the engines flamed out. None of the 121 occupants survived. In its final report, the board said that the direct causes of the accident were:
flight management computer and the autopilot, depletion of the fuel and engine flameout, and the impact of the aircraft with the ground.” The 737-300 pressurization system was designed to maintain a cabin altitude of 8,000 ft at the aircraft’s maximum certified ceiling, 37,000 ft. The mode selector, which is in an overhead panel above the first officer’s seat, has three positions: “AUTO” (automatic), “ALTN” (alternate) and “MAN” (photo, page 21). With the system in automatic mode, which normally is selected for flight, the crew selects the planned cruise altitude and destination altitude in the appropriate windows on the mode selector, and a cabin pressure controller positions the outflow valve to maintain a programmed cabin- pressure schedule. The alternate mode is selected to change from one cabin pressure controller to the other. With the system in manual mode, the flight crew has “direct” control of pressuriza- tion, using a toggle switch to position the outflow valve. “Manual control is primarily used as a backup to automatic control,” the report said. There are four annunciator lights above the pressurization control panel. An amber “AUTO FAIL” light indicates a failure of the automatic mode. An amber “OFF SCHED DESCENT” light illuminates if the aircraft descends before reaching the planned cruise altitude set in the “FLT ALT” window.
A green “ALTN” light indicates that the system is in the alternate mode. A green “MANUAL” light indicates that the system is in the manual mode.
The mode selector had been set to manual for a pressurization-system check the morning before the accident. The unscheduled maintenance was per- formed in response to a technical log entry by the flight crew that had landed the aircraft in Larnaca at 0425 after a flight from London. The technical log entry stated that an inspection of the aft galley service door was required because the door seal “freezes, and hard bangs are heard during flight.” After conducting a visual inspec- tion of the door and the pressurization check, a ground engineer (maintenance technician) wrote in the technical log that no defects were found and that no leaks or abnormal noises occurred. The report said that although the airplane maintenance manual included no spe- cific requirement to return the mode selector to “AUTO” after the check, it would have been prudent for the ground engineer to have done so. The report also noted that a rapid decompression of the accident aircraft’s cabin had occurred during a flight from Warsaw, Poland, to Larnaca on Dec. 16, 2004. The decompres- sion occurred when the aircraft was at Flight Level (FL) 350 (approximately 35,000 ft) and near the point at which
© Kostas Iatrou/aviation-images.com
Hellenic Air Accident Investigation and Aviation Safety Board
The pressurization system mode selector was in the manual, “MAN,” position during the accident flight and was moved beyond that position by impact forces.
after checking only that the cruise altitude and landing altitude are set correctly. The aircraft departed from Larnaca at 0907. The first item on the “After Takeoff ” checklist is to check the pressurization system. “This was the second missed opportunity to note and cor- rect an earlier error,” the report said. About 0910, the flight crew was cleared to climb to FL 340 and to fly directly to the Rhodos (Rhodes) VOR (VHF omnidirectional radio). The captain’s acknowledgement of the clearance was the last recorded communication between the flight crew and air traffic control (ATC).
The aircraft was climbing through 12,040 ft, and cabin altitude was slightly below 10,000 ft, at 0912, when the warning horn sounded. Activa- tion of the warning horn in flight indicates a problem with cabin pressurization, the report said. On the ground, the warning horn sounds when the throttles are advanced and the aircraft is not in the correct takeoff configuration — that is, with trim, flaps and/or speed brakes set incorrectly. According to the quick reference handbook, among the actions that the flight crew should take in response to a cabin altitude warning or rapid depressurization are to don their oxygen masks and stop the climb. That neither action was taken is one indication that the crew re- acted to the warning horn as if it were a takeoff configuration warning. The report noted that the crew did not silence the horn, and the loud noise that it produced likely increased their stress. The captain established radio communica- tion with a dispatcher in Helios Airways’ Opera- tions Center about 0914 and reported a “takeoff configuration warning.” About one minute later, the “MASTER CAUTION” and “OVERHEAD” lights illuminated on the flight deck annunciator panel. On the overhead panel, the “PASS OXY ON” light, indicating that the passenger oxygen masks had deployed, and the equipment-cooling system “OFF” lights also had illuminated. The report noted that nine technical log entries about the equipment-cooling system had been
made in the two months preceding the acci- dent. “The crew became preoccupied with the equipment-cooling-system situation and did not detect the problem with the pressurization system,” the report said. The equipment-cooling system includes fans and ducts that direct cool air to and warm air away from electronic equipment on the flight deck and in the electrical and electronic bay. “Loss of airflow (mass flow) due to failure of an equipment cooling fan or low air density associated with excessive cabin altitude results in illumination of the related equipment cooling ‘OFF’ light,” the report said.
The captain told the dispatcher about the equipment-cooling problem, but the dispatcher did not understand what he was saying and sug- gested that he talk with the on-duty ground en- gineer — the same person who had conducted the unscheduled maintenance before departure. The dispatcher did not tell the ground engineer what the captain had reported before handing him the microphone. The ground engineer told investigators that the captain asked for the location of the cooling fan circuit breakers and that he replied that the circuit breakers were behind the captain’s seat. The ground engineer also told investigators that he did not understand the nature of the problem that the captain was experiencing. The report said that the communication difficulties likely arose because “the captain spoke with a
German accent and could not be understood by the British engineer. … Moreover, the com- munication difficulties could also have been compounded by the onset of the initial effects of hypoxia.”
Helios Airways’ training program did not specifically require that flight crewmembers and cabin crewmembers be trained to recognize the symptoms of hypoxia. The report said that the lack of this training is “a common situation in the airline industry.” The aircraft was climbing through 28, ft about 0920, when the flight data recorder re- corded the keying of the no. 2 VHF radio, which was tuned to an ATC frequency. “This marked the last known attempt of radio communication by the flight crew,” the report said. Attempts by the airline’s Operations Center and ATC to re-establish radio communication with the flight crew were unsuccessful. The aircraft continued to climb at an average rate of 3,030 fpm. The pressurization outflow valve remained about 12 percent open during the flight, and the average cabin altitude rate of climb was 2,300 fpm. Cabin altitude reached a maximum of about 24,000 ft. “The aircraft leveled off at FL 340 [about 0923] and continued on its programmed route,” the report said. The aircraft crossed the Kéa VOR about 1021 and “began what appeared to be a standard instrument approach procedure for landing at Athens International Airport, Runway 03L, but remained at FL 340,” the report said. The aircraft flew over the airport about 1029 and turned right, toward the Kéa VOR, in accordance with the missed approach procedure (Figure 1). The aircraft crossed the VOR about 1037 and entered the published holding pattern.
Two Greek air force F-16s intercepted the aircraft during its sixth circuit of the holding pattern. The F-16 pilots observed no external structural damage, smoke or fire. “One of the F-16 pilots observed the aircraft at close range and reported
at [1132] that the captain’s seat was vacant [and] the first officer’s seat was occupied by someone who was slumped over the controls,” the report said. The captain likely had vacated his seat to check the cooling fan circuit breakers. The F- pilot also saw oxygen masks dangling from pas- senger service units and three passengers sitting motionless, wearing oxygen masks. The investigation did not determine what actions the cabin crew took or whether they attempted to communicate with the flight crew after the passenger oxygen masks deployed. The cockpit voice recorder (CVR) provided data only for the last 30 minutes of the three- hour flight. The report said that the F-16 pilot’s observations indicated that few passengers had donned their masks. The passenger-oxygen system was designed to supply oxygen for 12 minutes. “In order to retain consciousness after the depletion of the oxygen from the passenger oxygen system, a per- son on board would have had to make use of one of the [four] portable oxygen bottles,” the report said. The valves in three of the bottles were found open. The report said that at least one of the bottles likely had been used by a cabin attendant. The aircraft was on its tenth circuit of the holding pattern about 1149, when the F-16 pilot saw a man who was not wearing an oxygen mask enter the flight deck, sit in the captain’s seat and don headphones. From the F-16 pilot’s description of the man’s clothing, investigators concluded that the person likely was the cabin attendant who had used one of the portable oxy- gen bottles. “The F-16 pilot may not have been able to observe an oxygen mask on the person’s face because the portable oxygen bottle mask was clear in color,” the report said. The CVR recorded sounds of an oxygen mask being removed from its stowage box and oxygen flowing through the mask. The F- pilot tried to attract the cabin attendant’s atten- tion, but he did not respond.
About 1150, the left engine flamed out due to fuel starvation. The aircraft turned steeply left
The F-16 pilot saw
a man who was not
wearing an oxygen
mask enter the
flight deck, sit in the
captain’s seat and
don headphones.
The remains of 118 occupants were recovered and examined by a forensic pathologist; the bodies of the other three occupants are believed to have been con- sumed by the post-accident fire. “Accord- ing to the pathologist’s report, the cause of death for all on board was determined to be multiple injuries due to impact, in addition to the extensive burns for 62 of them,” the report said. The pathologist’s report also said that the occupants likely were “in deep, nonreversible coma due to their prolonged exposure (over 2.5 hours) to the high hypoxic environment” when the impact occurred.
The report said that the following were latent causes of the accident:
the country. CAA audits of Helios Airways had found several deficien- cies. “Management pilots appeared to be insufficiently involved in their managerial duties,” the report said. “Training and duty records were found to be incomplete. Manuals were found to be [partially] deficient; they did not always adhere to regulations, and on some issues they were out of date. In the two months before the initiation of the first flight operations with the [accident aircraft], the airline appeared to be effectively scrambling to piece together manuals and paperwork. This suggested that an underlying pressure was prevalent to proceed with little regard for the required formalities.” Moreover, flight crew training records indicated that no follow-up action had been taken in response to the first officer’s record of insufficient checklist discipline and ineffective performance in abnormal situations. The report said that the Cyprus De- partment of Civil Aviation (DCA) ap- peared to be completely dependent on the U.K. CAA for safety oversight and that the DCA’s Safety Regulation Unit was understaffed and lacked leadership and oversight. There was no record that the DCA took action to ensure that airlines responded to deficiencies and issues considered by CAA auditors to require urgent attention.
Investigators reviewed several previ- ous occurrences worldwide involving aircraft pressurization problems. “Of interest and relevance to the [Helios Airways accident were] nine reports of pressurization problems directly attributed to the crews’ failure to set and verify the proper position of the pressurization mode selector to ‘AUTO,’” the report said. “Seven of
these concerned Boeing 737 aircraft, while the other two events concerned McDonnell Douglas aircraft. These nine reports all referred to aircraft that took off with the pressurization selector inadvertently set to ‘MAN.’” The report said that Boeing had taken or was in the process of taking several actions before the accident to reduce the likelihood of 737 pres- surization incidents. Among actions underway was a revision of the B Flight Crew Training Manual to include information on distinguishing between a cabin altitude warning and a takeoff configuration warning. “A number of remedial actions had been taken by the manufacturer since 2000, but the measures taken had been inadequate and ineffective in preventing further similar incidents and accidents,” the report said. Among actions taken in response to the findings of the accident inves- tigation was Airworthiness Directive (AD) 2006-13-13, issued in June 2006 by the U.S. Federal Aviation Admin- istration. The AD required revision of the 737 airplane flight manual (AFM) “to advise the flight crew of improved procedures for preflight setup of the cabin pressurization system, as well as improved procedures for interpreting and responding to the cabin altitude/ configuration warning horn,” the report said. The AD also required that the following message be inserted in the AFM: “For normal operations, the pres- surization mode selector should be in ‘AUTO’ prior to takeoff.” ●
This article is based on Hellenic Air Accident Investigation and Aviation Safety Board Aircraft Accident Report 11/2006, “Helios Airways Flight HCY522, Boeing 737-31S, at Grammatiko, Hellas, on 14 August 2005.” The 198-page report contains illustrations and appendixes.
