Leeham News and Analysis
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Leeham News and Analysis
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Bjorn’ s Corner: New aircraft technologies. Part 45. Continued Airworthiness
By Bjorn Fehrm
January 12, 2024, ©. Leeham News: We are discussing the different phases of a new airliner program. After covering the Design and Production, we now look at the Operational phase of a new airliner family.
For the operational phase, the airplane must pass scrutiny for Continued Airworthiness. Today, we discuss the different means available to the Regulator, such as Airworthiness Directives ( ADs) and System Bulletins (SBs) to the OEM to make sure any detected issues get noticed and corrected.
Figure 1. The Boeing MAX 9 Door Plug Emergency AD issued last week. Source: FAA.
Airworthiness Directives
During its operational life (which is around 25 years compared to the two years in production), there is regulatory oversight and certification of all aspects of the operation of the aircraft. We have a fresh example from last week with the Airworthiness Directive that the FAA issued for the Boeing 737 MAX 9.
A possible manufacturing lapse to ensure four stop bolts were fitted had the rear plug door on the lefthand side of an Alaska 737 come loose and leave the aircraft during climb-out from Portland International Airport.
The FAA issued an Emergency AD number 2024-2-51 on the 6th of January (Figure 1) to ground all US MAX 9s until inspections have verified they are safe to fly.
The Emergency AD is the highest form of escalation in the Safety Monitoring and Reporting (14 CFR 21.3, 14 CFR Part 39) part of the FAA regulations. The different levels of actions demanded of the operator by the OEM and the Regulator are summarized in Figure 2.
Figure 2. Safety Monitoring and Reporting (14 CFR Part 21.3, 39). Source; Boeing.
The Airline, OEM, and the Regulator are obliged to continuosly monitor the delivered product in its operation. Depending on the safety risk, different actions are initiated if an issue is suspected or detected:
- OEM Service Letter: The OEM sends the operator formal information about something to observe or know about the airplane.
- OEM Service Bulletin: The OEM sends the operator a formal action to be performed on the aircraft within a specified time. It can be an inspection or modification that shall be performed with a prescribed modification kit.
- FAA SAIB: The Regulator issues a Special Airworthiness Information Bulletin about the aircraft.
- FAA NPRM: The Regulator issues information on a changed or new regulation that the operator will have to comply with. The Regulator accepts comments to the NPRM and then issues a regulation where input from comments has been considered.
- FAA IAR: The Regulator issues a changed or new regulation that comes into force without a previous consultation via an NPRM.
- FAA AD: The Regulator issues an Airworthiness Directive, AD, which stipulates an action for the operator to be performed before a time limit, or the aircraft loses its airworthiness. The Emergency AD, which was issued in the MAX 9 case, was an AD that required immediate action.
As can be seen, the certification of an aircraft is the approval of the design. The production certificate is an approval that if the production is done according to the design certificate and with methods and procedures as stipulated in the production certificate, it’s considered safe for use when delivered.
But from the instance of delivery, the airplane is subject to the above monitoring and can at any time receive an SB and AD that require further action to keep the aircraft certified as safe to fly.
Part of the actions to keep the aircraft safe to fly is acted upon during maintenance. Many SBs and ADs are structured so these can be executed at the next hangar maintenance of the aircraft. We look deeper into the maintenance part of Continued Airworthiness in coming Corners.
The SB’s are normally categorized in importance and timing of incorporation. Still sometimes they issue several SB’s on an existing problem were most of them does not solve the problem and finally they get it right. Sometimes I have seen SB’s issued with no improvement just to sell new parts. So the owner/operator has to have the engineering skill to judge the quality of the SB issued. Some packages of SB’s really solve old problems and you want them into your fleet fast. The planned time of operation is also a factor as a narrowbody engine you often want to sell off around 30 000 cycles and a widebody engine at below 10 000 cycles if you own them.
Per above, I have seen SBs that were considered non critical that basically used the multi layers of safety as an out not to conduct the SB until a major inspe3cion period (5 years I believe)
The biggest nightmare of anyone that works on equipment that can kill people (in my case Boilers and their explosive properties) is that a safety is used as an operating control.
I saw one example that while not intentional, due to the design of the system, the High Temp safety was the operating control (the boiler fired until the safety was tripped.)
Rather than two controls in series, there was one. That had to be re-wired to make it two temp cutoff limits in series.
We never ever bypassed an operating control that put us in the situation where the safety was the only control.
The lady who wrote the definitive report on the first Space Shuttle disaster came up with the phrase, Normalizing Deviation. They definition is they push the limits , you get away with it and that becomes the new normal. They then use the push point as normal or safe, and then exceed that until there is a failure.
Modern Aviation in a nutshell.