Aircraft has history of rudder problems
EGYPTIAN CRASH: The possibility of a jammed rudder will be one of the first things crash investigators will examine in the Flash Airlines disaster, writes Gerry Byrne.
The Boeing 737-300 is the third generation of the hugely successful 737 series. It has probably the best safety record of any jetliner of its size, yet it has also been the subject of one of the most intensive safety studies conducted on a modern airliner.
Crash investigators were stumped when United Airlines Flight 585, a 737, nose-dived from 1,000 feet into a Colorado Springs park in March 1991, killing all aboard. The pilots died before they could describe their problem and the aircraft's black boxes did not provide sufficient details to explain the crash.
Crash investigators from the National Transportation Safety Board (NTSB) suspected a rudder problem but failed to come up with conclusive proof.
When a rudder is pushed to one side and no corrective action is taken, the aircraft will turn, then rapidly corkscrew, nose-first, towards the ground. Boeing produced convincing arguments that turbulence was responsible and that, even if the rudder had malfunctioned, other flight controls on the wings - hinged flaps known as ailerons - were more than adequate to compensate.
The following year a United Airlines pilot refused to fly when his rudder pedals misbehaved during a pre-flight test. When the hydraulic rudder control of his aircraft was dismantled it was discovered it could force the rudder to go in the opposite direction than that commanded, then jam in that position, a situation known as a hardover.
The following month, at a secret meeting in Seattle, Boeing executives debated grounding the aircraft but decided the chances of a rudder malfunction causing a crash were so rare as to be negligible.
However, they did agree that the rudder hydraulic units should be checked and replaced with a modified design as soon as possible, a process which could take up to five years. Although the NTSB wanted tougher action, the US Federal Aviation Administration backed Boeing's response.
In September 1994 a USAir 737 plummeted into a hillside outside Pittsburgh, killing all 135 aboard. The plane's dive, as tracked on radar and in the flight data recorder, was chillingly the same as if the rudder had been pushed to one side and held there. While Boeing insisted pilot error was responsible, the NTSB remained convinced the rudder hydraulics were at fault. But it took almost five years before they could convincingly demonstrate that hardovers caused both the Colorado Springs and Pittsburgh accidents.
In the meantime, they also discovered that, contrary to Boeing's assertion, the ailerons were insufficient to compensate for a malfunctioning rudder at the slower speeds of landing or takeoff.
Aer Lingus and Ryanair have reported 737 rudder problems over the years. The Aer Lingus problem occurred on the ground but the more serious Ryanair incident led to a diversion and precautionary emergency landing.
At first the FAA again backed Boeing's position, that pilot retraining and minor equipment changes would be sufficient.
However, it dramatically reversed its position a year later when its own inquiry uncovered up to 40 ways in which the 737 rudder could misbehave. It ordered major overhauls of US 737 rudder hydraulics, including the installation of an additional fail-safe unit, a call which has since been echoed by other safety authorities, including the Irish Aviation Authority.
However, Parker Hanafin, the subcontractor which builds the 737 rudder hydraulic units, is unable to satisfy the immediate demand for thousands of new units, each of which must be hand-built to microscopic tolerances. Boeing has said it may be 2008 by the time the worldwide fleet is retrofitted.
Meanwhile, pilots have been trained to increase speed and perform rapid manoeuvres to overcome the force of the rudder, should a hardover occur. The extent the rudder can move at low altitudes has also been restricted. But the fear remains that less-regulated Third World airlines may not have been as vigilant as their and US and European counterparts in implementing the new procedures.
Swiss authorities last year banned Flash Airlines from its airspace, citing safety issues, but they have not been specific on the reasons. The possibility of a hardover in the disaster will be one of the first things crash investigators will examine.
The NTSB remains the leading international authority on the 737 rudder problems and, because the aircraft was US built, will be automatically invited to take part in the Egyptian government inquiry into the crash.
However, relations between the NTSB and the Egyptian aviation authorities remain soured by an NTSB finding that the 1999 plunge of an Egyptair 767 into the sea off Nantucket, Massachusetts, killing 217 people, resulted from the actions of a suicidal pilot. The Egyptians disagreed, saying technical problems caused the crash.
Gerry Byrne, a Dublin science writer, is author of Flight 427: Anatomy of an Air Disaster