Enlarge /. The Southwest has dozens of 737 in stock. The airline says it has not experienced the error described in the FAA policy.
The Federal Aviation Administration has instructed airlines to inspect the engines of their 737 aircraft after four reports of "single engine shutdowns".
Many 737 aircraft have been in hangars for weeks when the coronavirus pandemic suppressed demand for air travel. When airlines resumed operations, they found a key valve stuck after weeks of no use. The FAA estimates that around 2,000 aircraft could be affected.
"If this valve opens normally at takeoff power, it can get stuck in the open position during flight and not close if the power at the top of the descent is reduced," warns the FAA directive. This could lead to "unrecoverable compressor downtime and inability to restart the engine".
This has happened four times in the past few weeks. Alaska Airlines has confirmed that one of its aircraft suffered from the problem while flying from Seattle to Austin. Despite the unexpected shutdown of one of its engines, the aircraft was able to land safely and no one was injured, the airline says.
Fortunately, this also applied to the other three incidents: only one engine stopped and no injuries occurred. However, the FAA fears that an aircraft could experience the same malfunction in both engines at the same time, which "could result in a forced landing outside the airport".
Therefore, the FAA orders airlines to carefully inspect the engines of 737 aircraft that have been out of service for seven or more days in a row and have not flown ten times since then. If a sticky valve is discovered, it must be replaced before the aircraft can be operated again. Most airlines indicated that the mandatory inspections would not significantly affect their flight plans.
The order relates to older models of the 737 – from the 737-300 line to the 737-900 line. Boeing's newer 737-MAX line is still on the ground as the company struggles to address design and software issues.