Beyond the serious issues involving the Boeing 737 MAX-8, which has suffered two fatal crashes in five months, the next version of the MAX series, the MAX-10, is also at the center of safety concerns. And those concerns again involve how Boeing and the Federal Aviation Administration work together to certify the safety of new airplanes.

The crashes have renewed alarms about the extent to which the FAA has delegated certification of the new jets to Boeing.

A report in The Seattle Times said that FAA managers “pushed the agency’s safety engineers to delegate safety assessments to Boeing itself.” This comes as The Wall Street Journal also reports that the Transportation Department’s Inspector General has opened an investigation into how the new systems in the MAX-8 were analyzed and certified. The systems suspected of bringing down flights in Indonesia and last week in Ethiopia work to prevent stalling in midair.

Concerns about the MAX-10, meanwhile, focus on its ability to meet the strict standards set for evacuation in the event of an emergency: flight attendants have just 90 seconds in which a cabin must be evacuated. Every new airplane must pass tests conducted by the manufacturer and overseen by the FAA that demonstrate such a speedy evacuation.

The MAX-10, due to go into airline service next year, is the largest of the 737 series, designed to carry 230 passengers in coach class seating. Flight testing for certification is under way and test evacuations are part of that certification.

Having that number of seats in a single-aisle jet reflects the pressure from budget airlines for “densification,” a euphemism for cabins in which seat width and the space between rows of seats is reduced to new minimums.

The MAX-10 will take the art of densification to its limits. Budget airlines in Asia that have ordered the jet will have seats as narrow as 16 inches wide and a pitch (the distance between one seatback and the next) as small as 26 inches.

The passenger activist group FlyersRights filed a petition last year arguing that the squeeze on seating needed to be urgently reviewed as a safety hazard. The petition was supported in a judgment by the D.C. Circuit Court of Appeals that ruled that there was “a plausible life-and-death safety concern.”

However, Dorenda Baker, the FAA’s executive director for aviation safety, denied that there was any safety issue and said, “Nothing in your petition demonstrates that decreases in seat pitch and increases in passenger girth create an immediate safety issue with regard to passenger evacuation that necessitates rulemaking.”

This flew in the face of all logic and showed that the FAA and Boeing were on the same page in believing that although the MAX-10’s cabin would be carrying 30 more passengers than the MAX-8 there was no need for new safety rules to be applied to it.

Critics have long maintained that the evacuation tests do not come close to replicating the actual conditions of an emergency taking place on a runway and particularly at night.

Last year The Daily Beast carried out an investigation into how the MAX-10 was being certified for safe evacuation in an emergency.

At first the FAA took the same line as Boeing in asserting that the tests could not be revealed because of “the proprietary nature of the data.” Later they said that “the Boeing 737 MAX-10 is not yet approved, so it has not yet demonstrated its compliance with evacuation rules.”

Boeing claimed that it had met the rules by widening one set of exit doors by four inches.

When the MAX series of the 737 was launched and developed, Boeing was under intense pressure from airlines to match the performance of the airplane’s direct competitors, the Airbus 320 and 321 models.

The introduction of a new generation of jet engines designed for single-aisle jets—by far the largest category of airplanes used all over the world—enabled both companies to significantly improve the range, capacity and efficiency of their models.

It was simpler for Airbus to take these steps because the A320 was a more modern design, launched in the 1980s, whereas the basic design of the 737 dates from the mid-1960s.

In order to match the largest of the new Airbus jets, the A321neo (new engine option) Boeing with the MAX-10 “stretched” the 737’s cabin to twice the length of the 1960s original, that carried only around 100 passengers.

In its efforts to overcome the limitations of the 737’s original design Boeing hit several problems. One of them, placing larger and more powerful engines on the wings, is related to questions surrounding the two MAX-8 crashes. It was the weight and placing of the engines that caused a change in the way the MAX-8 handled in some situations.

And it was that change in handling that caused Boeing to introduce new software in the airplane’s flight controls, called MCAS, that is the focus of investigators as a cause of the crashes.

Another problem occurred with the increased length of the MAX-10. The standard landing gear of the 737 was not long enough to deal with the new length of the fuselage—when an airplane takes off its length rotates around the main gear wheels. The gear has to be long enough to prevent the tail hitting the tarmac at the sharp angle of takeoff.

Boeing has had to develop a completely new and ingenious way of accommodating a longer landing gear in an unchanged wheel well—the space into which the wheels retract. They have done this using a kind of double-joint. During retraction the gear’s length is shortened by being telescoped.

The MAX-10 is now undergoing flight tests, during which this new landing gear will have to be part of the safety certification process. By the time the MAX-10 reaches airlines the cause of the two MAX-8 disasters will surely have been confirmed and will have been eliminated from the airplane. The issue of safe cabin evacuation remains unresolved and neither Boeing nor the FAA will disclose details of the evacuation tests.