The Boeing 737 MAX is currently undergoing worldwide recertification in order to return the aircraft to service. Boeing and the airlines were hoping for the 737 MAX aircraft to return to service sometime this spring, however, there is a new problem that was discovered last month. Two weeks ago, I wrote “Should the Boeing 737 MAX ever return to service? Will it be safe?“. Please read this first to understand the old problem involving the MCAS system as this post will cover the new issue.
What the New Problem Involves
Assembling an airliner like the 737 requires a lot of wire. The previous version of the 737 (700, 800 and 900) has 42 miles of wire in the airframe. Aircraft wiring involves the use of wire “looms” or “bundles” to organize and install these miles of wires. The newly discovered problem was identified as having two wire bundles installed too close together in the tail of the aircraft. The problem is that these two wire bundles could cause a short circuit in the systems that they control. Like the MCAS problem, one of the systems controlled by these wire bundles is the horizontal stabilizer. The horizontal stabilizer is the control surface that allows aircraft to climb and descend. The same horizontal stabilizer control is also the object of MCAS issues.
A short circuit in any part of an aircraft electrical system could result in failure(s) of critical systems to starting a fire on board the aircraft.
Why Horizontal Stabilizer Problems are a Concern
All aircraft from a single-engine Cessna to the massive Airbus A380 fly the same way. Aircraft are built with control surfaces which control the three basic aspects of flying:
- Pitch – the ability to climb or descend and is controlled by the horizontal stabilizer. The process of controlling pitch is also known as “trimming” the aircraft.
- Yaw – the ability to turn the nose side-to-side in order to turn the aircraft. This is controlled by the rudder.
- Roll – the ability to bank the aircraft to assist in executing coordinated turns. This is controlled by the ailerons.
A loss of yaw or roll is serious but it is possible to manage turns in the failure of either the rudder or ailerons. The failure of pitch control by the horizontal stabilizer is a more serious problem. It is the out of control pitching of both the Lyon Air and Ethiopian Airlines aircraft that resulted in crashes. A runaway stabilizer is the result of the flight management computer. A complete failure of the pitch control could happen when the actual control mechanisms for the stabilizers break. Twenty years ago, Alaska Airlines flight 261 had a complete mechanical failure of the horizontal stabilizer when the jackscrew and gimble nut connection broke. The MD-80 was now flying in an out of control manner and eventually rolled completed over before crashing into the Pacific Ocean. My friend Petter of Mentour Pilot explains what happens when you lose pitch control.
When the pitch is out of control due to faulty input from the flight management computer, the pilot will pull back on the yoke to raise the nose of the aircraft if the plane is pitching down. The problem with this is that it could take a lot of strength to counteract the flight management computer inputs. This may require as much as 90 pounds of force to pull back on the yoke and a pilot not be able to apply that much effort for very long. The computer will eventually win if there is a failure of this type.
When Was This New Problem Identified
By now, everyone knows that Boeing is under intense scrutiny when it comes to the self-certification of various aircraft systems on the Boeing 737 MAX. Boeing was conducting an internal audit last month on the MAX assembly line. As part of this audit procedure, Boeing notified the FAA, which is the aircraft certifying body in the U.S. of the problem. The key factor here is how Boeing will design and implement a fix. Boeing engineers need to be extremely careful in solving this problem without creating new electrical wiring problems or wire damage during the fix.
Are Other Models of the 737 Involved?
What we know is that this problem has been identified with the latest 737 series, the MAX. Boeing will have to take a look at the previous generation of 737s, the “next generation” (NG) for the possibility of the same problem existing there too. The NG series includes the 700, 800 and 900 models of the 737. There are thousands of Next Generation Boeing 737s in service.
What Would the Fix Look Like?
At this time, Boeing thinks that they can fix the wire bundle problems by installing wire clamps. This process could take about two hours per aircraft. Boeing will need to supply the necessary parts to airlines that have the 737 MAX already in their fleets. The undelivered aircraft will have to be fixed before they can be delivered to their customers. What happens if this problem affects the 737 NG? The FAA could issue an Airworthiness Directive (AD) requiring airlines to correct the problem.
Possible 737 MAX Engine Issues
Another safety issue with the 737 MAX could be with the LEAP-1 engines manufactured by CFM International. The FAA is already aware of issues with this engine which involve one of the engines’ rotors which could shatter and fail during flight. The FAA is not requiring an immediate fix at this time and they are looking into requiring an inspection process of this engine before the 737 MAX returns to service.
My Take on the Lastest Problems
Many people have asked me why the 737 MAX is grounded when the Boeing 737 has been in commercial airline service since 1967. This is an excellent question because the 737 MAX is not a new aircraft but a follow-on to an existing aircraft program. The problem with the MAX series is the size and mounting location of the new LEAP-1 engines which change the airflow characteristics over the wings that generate lift. This is why it was necessary to equip the 737 MAX with the MCAS system. Normally, a follow-on (or modification) rollout of an existing aircraft is fairly simple.
The wiring issue is a completely different problem, it has been positively identified as a problem so a fix can be easily engineered. Compared to the MCAS issues, testing and computer code changes are more difficult and time-consuming to successfully achieve.
The last thing that the Boeing 737 MAX program needs are more problems. I have said before that ultimately, the Boeing 737 MAX will return to service and it will be the most scrutinized airliner ever made.
