FAA issues new 737 Max directive over overheating cabin risk

Tripped circuit breakers can prevent 737 Max air conditioning systems from cooling hot air pumped into the cabin.

The Federal Aviation Administration has taken additional action to address a circuit breaker issue that can cause 737 Max cabins and cockpits to become dangerously hot.

The agency on 29 June released an airworthiness directive (AD) addressing a problem, which can occur when a tripped circuit breaker causes ram air doors to close, keeping the air conditioning system from cooling hot engine bleed air.

The risk affects all in-service Max models – a total of 825 aircraft, including 737 Max 8s, Max 8-200s and Max 9s.

The FAA in February issued a prior AD to address the issue. Both that rule and the new rule require airlines to add new “non-normal checklists” to aircraft flight manuals. Those checklists provide steps pilots should take if cabins or cockpits get too hot after tripped circuits.

The orders respond to “two reports of in-flight events of excessive cabin and flight deck temperatures that could not be controlled by the flight crew”.

That February AD said the problem results from a circuit breaker in the 737 Max’s standby power control unit (SPCU) tripping. The SPCU supplies power to air conditioning and cabin pressure systems.

When tripped, the circuit sends “an unintended erroneous electrical ground signal” to “ram air door actuators”, causing the doors to close and preventing cooling air from reaching air conditioning heat exchangers. Those exchangers then cannot properly cool “hot bleed air” fed from the engines to the cabin and cockpit.

“This air conditioning system malfunction could cause an uncontrollable, excessively high temperature in the cabin and flight deck,” the FAA said.

The agency’s newly released AD addresses the same risk but involves the tripping of circuit breakers for the environmental control system (EVS), “downstream of the SPCU”.

It comes as a final rule that takes effect on 16 July.

“The FAA has found that the risk to the flying public justifies forgoing notice and comment prior to adoption of this rule,” it says.

The agency’s February AD had required airlines to add two new non-normal checklists to aircraft manuals and to revise another. The latest AD requires carriers to add another two checklists. The checklists detail steps that include making a “controlled descent” to the “lowest safe altitude” (not exceeding 10,000ft), resetting the circuit breakers and turning off the engine bleed switches.

Boeing does not respond to a request for comment, though the latest AD says the company developed the procedures for addressing the problem.

In February, Boeing had told FlightGlobal it intends to further address the problem by developing an “engineering solution to eliminate the possibility of this electrical fault”. It said it intended to introduce that fix on the 737 Max 8, Max 8-200, Max 9 and the still-not-certificated Max 7 and Max 10.

Boeing aims to have the Max 7 and Max 10 certificated this year.