Guy Norris/LOS ANGELES
Boeing's 737-700 obtained European Joint Aviation Authorities certification on 18 February after changes were made to increase stall warning.
The modifications meet the JAA's insistence that the pilot be able to identify clearly the occurrence of a stall, even after the activation of the stick shaker. The resulting changes to the speed trim system, and related wiring, mean that several early build -700s will have to be retrofitted. The JAA certification is an interim approval pending availability of retrofit kits.
JAA certification was due in September 1997 in parallel with US Federal Aviation Administration approval. The two efforts later diverged over the issue of the overwing emergency exit design.
Boeing was forced to introduce a redesigned exit to enable the -700 and -800 to be cleared by the JAA to carry their maximum passenger loads of 149 and 189, respectively. The stall warning issue emerged at the final stages of certification.
Boeing Next Generation 737 chief project engineer Pete Rumsey says, however, that "it is not unexpected that the JAA requests items at the back end of the programme. The JAA asked us to fly beyond the stick shaker. We came close to the specific requirements, but did not meet the letter of the law, which was written for older aircraft." The advanced design of the new wing means that, in a stall, "-lift degrades very gradually. The aircraft continues to be controllable", says Rumsey.
The JAA insisted on the addition of a system to raise pilot awareness. "We are adding a speed trim system that will demonstrate the stall characteristics more. It will push the nose down as the aircraft goes into a stall," he says.
Normally, the speed trim system is switched off automatically as the stick shaker is activated. The JAA ruling means that original safety systems have been redesigned to allow the trim system to activate in the event of a stall.
Source: Flight International
