Helios Airways Flight 522

Helios Airways Flight 522 was a scheduled international passenger flight from Larnaca, Cyprus, to Prague, Czech Republic, with a stopover in Athens, Greece, operated by a Boeing 737-300. Shortly after takeoff on 14 August 2005, Nicosia air traffic control (ATC) lost contact with the pilots operating the flight; it eventually crashed near Grammatiko, Greece. All 121 passengers and crew on board were killed. It is the deadliest aviation accident in Greek history.

An investigation into the accident by Greece's Air Accident Investigation and Aviation Safety Board (AAIASB) concluded that the crew had failed to notice that the cabin pressurization system was set to "manual" during takeoff checks. A ground engineer had set it to "manual" to conduct testing before the flight, but had most likely forgotten to restore it to "auto" afterward. This configuration was subsequently missed by the crew during their pre-flight checks. This caused the plane to gradually depressurize as it climbed, and resulted in everyone on board suffering from critical hypoxia. The negligent nature of the accident led to lawsuits being filed against Helios Airways and Boeing, with the former also being shut down by the Government of Cyprus the following year.

Flight crew

In command was Captain Hans-Jürgen Merten, a 59-year-old German contract pilot hired by Helios for holiday flights, who had been flying for 35 years (previously for East German airline Interflug from 1970 to 1991) and had accrued a total of 16,900 flight hours, including 5,500 hours on the Boeing 737. The first officer was Pampos Charalambous (Πάμπος Χαραλάμπους), a 51-year-old Cypriot pilot who had flown exclusively for Helios for the previous five years, accruing 7,549 flight hours throughout his career, with 3,991 of them on the Boeing 737. Louisa Vouteri (Λουΐζα Βουτέρη), a 32-year-old Greek national living in Cyprus, had replaced a sick colleague as the chief flight attendant.

Accident

When the aircraft arrived at Larnaca from London earlier that morning, the previous flight crew had reported a frozen door seal and abnormal noises coming from the right aft service door. They requested a full inspection of the door. The inspection was carried out by a ground engineer, who then performed a pressurization leak check. To carry out this check without requiring the aircraft's engines, the pressurization system was set to "manual", but the engineer failed to reset it to "auto" upon completion of the test.

After the aircraft was returned into service, the new flight crew overlooked the pressurization system state on three occasions: during the preflight procedure, the after-start check, and the after-takeoff check. During these checks, no one on the flight deck noticed the incorrect setting. The aircraft took off at 09:07 with the pressurization system still set to "manual", and the aft outflow valve partially open.

As the aircraft climbed, the pressure inside the cabin gradually decreased. As it passed through an altitude of 12,040 ft, the cabin altitude warning horn sounded. The warning should have prompted the crew to stop climbing, but it was misidentified by the crew as a takeoff configuration warning, which signals that the aircraft is not ready for takeoff and can sound only on the ground. The alert sound is identical for both warnings.

In the next few minutes, several warning lights on the overhead panel in the cockpit illuminated. One or both of the equipment cooling warning lights came on to indicate low airflow through the cooling fans (a result of the decreased air density), accompanied by the master caution light. The passenger oxygen light illuminated when, at an altitude around 18,000 ft, the oxygen masks in the passenger cabin automatically deployed.

Shortly after the cabin altitude warning sounded, Captain Merten radioed the Helios operations centre and reported "the takeoff configuration warning on" and "cooling equipment normal and alternate off line." He then spoke to the ground engineer and repeatedly stated that the "cooling ventilation fan lights were off." The engineer (the one who had conducted the pressurization leak check) asked: "Can you confirm that the pressurization panel is set to AUTO?" However, Merten, already experiencing the onset of hypoxia's initial symptoms, disregarded the question, and instead asked in reply, "Where are my equipment cooling circuit breakers?" This was the last communication with the aircraft before the captain and first officer fell unconscious.

The aircraft continued to climb until it leveled off at FL340, about 34000 ft. Between 09:30 and 09:40, Nicosia air traffic control (ATC) repeatedly attempted to contact the aircraft, without success. At 09:37, the aircraft passed from Cyprus flight information region (FIR) into Athens FIR, without making contact with Athens ATC. The 19 attempts to contact the aircraft between 10:12 and 10:50 also met with no response, and at 10:40, the aircraft entered the holding pattern for Athens Airport, at the KEA VOR, still at FL340. It remained in the holding pattern, under control of the autopilot, for the next 70 minutes.

As the aircraft flew in a continuous loop over Athens, the Greek military decided to intervene. Sources differ on if they were contacted by Athens ATC or if they chose to intervene themselves, believing this may have been a possible terrorism incident. At 11:05, two F-16 fighter aircraft from the Hellenic Air Force (HAF) 111th Combat Wing were scrambled from Nea Anchialos Air Base to establish visual contact. They intercepted the passenger jet at 11:24 while it was undergoing the sixth loop of the holding pattern and observed that the first officer was slumped motionless at the controls, and the captain's seat was empty. They also reported that oxygen masks were dangling in the passenger cabin.

At 11:49, flight attendant Andreas Prodromou (Ανδρέας Προδρόμου) entered the cockpit and sat down in the captain's seat, having remained conscious by using a portable oxygen supply. Early media reports erroneously claimed his girlfriend and fellow flight attendant, Haris Charalambous (Χάρις Χαραλάμπους), was also seen in the cockpit helping Prodromou try to control the aircraft. According to a July 2006 television documentary, blood samples that were found in the radar controls were matched to Prodromou's DNA and also those of Haris, which led the documentary's investigators to the conclusion that the two flight attendants were trying to save the plane. However, the official investigation report published in October 2006 said the F-16 crew only saw one male in the cockpit and did not mention DNA evidence. Prodromou held a UK Commercial Pilot Licence, but was not qualified to fly the Boeing 737.

Prodromou waved at the F-16s very briefly, but almost as soon as he entered the cockpit, the left engine flamed out due to fuel exhaustion, and the plane left the holding pattern and started to descend. Crash investigators concluded that Prodromou's experience was insufficient for him to gain control of the aircraft under the circumstances. Some evidence indicates he tried to wake up the pilots and his girlfriend, who were all in a deep coma by that time. Ten minutes after the loss of power from the left engine, the right engine also flamed out and, just before 12:04, the aircraft crashed into hills in the vicinity of the village of Grammatiko, 40 km from Athens in East Attica, killing everyone on board.