The Federal Aviation Administration has issued a new Airworthiness Directive (AD) covering more than 10,000 Boeing 737 airplanes of all model types after several reports of a faulty pressure switch in the cockpit. There are approximately 2,500 737’s registered in the United States and more than 9,000 registered in other countries. While the FAA has no authority to order such an AD in foreign countries, most countries follow FAA directives.
After receiving several reports of faulty cabin altitude pressure switches on different models of the Boeing 737 airplane, Boeing launched an investigation. The FAA requires two switches in the cockpit for redundancy, however in several instances, both were found to be faulty. Following Boeing’s investigation, Boeing determined in November of 2020 that this was not a safety issue.
The cabin altitude press switches are designed to alert the flight crew if cabin pressure exceeds 8,000 feet. At 10,000 feet some people can begin to experience hypoxia and even loss of consciousness. A notable example of this was in 2005 when the Greek airline Helios flight 522 crashed after the crew and passengers suffered hypoxia. All 121 on board died in the crash. A subsequent investigation found that the airplane suffered a pressurization problem and everyone died from lack of oxygen.
The FAA began investigating the high rate of failure being found on both the primary and redundant secondary pressure switches and on July 15th, issued a new AD requiring operators of the 737 to conduct regular testing of the pressure switches. The design of the switches alert the crew of a pressure problem, but not if the switches themselves are not working. The FAA said the high failure rate was a substantial safety risk, but that they have not yet determined why the failures keep happening. Until a fix can be made, regular testing will be required.