A combination of basic pilot error and an electrical failure has emerged as the possible cause of the crash of a Spanair airliner at Madrid airport last month that killed 154 people.

The crew of the MD82 jet failed to extend the wing flaps that are vital for take-off and an automatic system did not alert them, according to media leaks from the Madrid investigation.

The heavily loaded aircraft, bound for the Canary Islands, smashed to the ground tail-first near Barajas airport on August 20 after struggling to climb after an unusually long take-off run. Only 18 people survived.

First accounts of Spain’s worst air disaster for 25 years suggested that one of the tail-mounted engines had caught fire. Suspicion then focused on the malfunction of an engine thrust reverser. However, pilots noted immediately that the failure of the aircraft to gain height and the subsequent stall was more consistent with wrong flap settings.

It also emerged this week that another Spanish MD82 narrowly escaped crashing after take-off at Lanzarote in June last year when it became briefly uncontrollable. The inquiry into that incident is not complete, but a wrong flap setting is the suspected cause.

Data from the “black box” recorders of the Spanair jet showed that the two engines worked normally up to the crash but that the flaps were not set, sources close to the inquiry told The Wall Street Journal and El Mundo newspapers. The flaps on the wing’s trailing edge and slats on the leading edge are essential for lift on take-off. They are one of the “killer items” that pilots triple-check before take-off.

The cockpit voice recorder, recovered from the crash, showed that the crew had confirmed “Flaps OK, Slats OK” during their reading of the check list, according to Spanish reports.

A loud audio alert should have warned the crew of the wrong flap setting the moment they applied take-off power, but it did not work. The devices were installed on airliners decades ago because the failure of pilots to set flaps caused so many accidents. Two other alarms — warning of nearby ground and imminent stall — did work in the Spanair aircraft as the pilots struggled to control it.

The alarm failure could have been caused by a technical fault or it could have been switched off. The Spanair jet had already aborted one departure from Madrid before its ill-fated take-off.

Spanish officials called the revelations incomplete but did not deny them. If the theory is confirmed, the disaster would be a carbon copy of the August 1987 crash of a Northwestern Airlines DC9 — predecessor of the MD82 — at Detroit, which also killed 154. The DC9 pilots had forgotten to set the flaps and slats and the warning system lost power and failed.

The possibility that the crew were distracted will be examined in the Madrid investigation.

Three relatives of victims of the crash filed a lawsuit against Boeing in Illinois, where the planemakers have their headquarters. McDonnell Douglas merged with Boeing in 1996.

Vital checks

— A failure to set wing flaps and slats for take-off creates a lethal condition for modern airliners

— The extended wing surfaces are vital to generate the extra lift that is needed at the relatively low speeds of take-off and landing

— Airline crews are obliged to double-check the flap and slat settings. Some airlines require crews to deploy take-off flaps before leaving the airport gates

— The alarm does not activate if the aircraft is in the air

— The performance of the Spanair MD82 that crashed conforms to an aircraft trying to take off below minimum flying speed. Witnesses said that it travelled for an extra 500 yards as the pilots coaxed into the air an aircraft teetering on the edge of a stall

— Stalls on take-off are hard to recover from

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