January 19, 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 biggest item in a regulator’s Instructions for Continued Airworthiness: the required Maintenance to keep an airliner airworthy.
For an aircraft to keep its operational certification, it must be subject to a maintenance program approved by the local regulator. A country regulator, in turn, relies on a proposed maintenance program from the OEM, which was developed in a prescribed procedure called MSG-3 (from Maintenance Steering Group’s 3rd process version).
In addition to the OEM’s proposal for a maintenance program, the local regulator can prescribe extra conditions and actions to cater to local weather conditions or operational stresses (like a dusty environment). In the end, it’s the local regulator that decides what must be in an aircraft’s maintenance program that the operator presents to him for approval.
Figure 1 describes the main blocks in a modern maintenance program as suggested by an OEM. The chart is from an ATR maintenance brochure from 2020, which has a good high-level description of the maintenance program for the aircraft.
But before we go into a modern maintenance program, let’s look at the history of airliner maintenance. It went from a non-regulated and individual process for each airliner to a structured US regulator (the FAA)-demanded maintenance program as the big jet airliners (Boeing 707, Douglas DC-8) were introduced. The focus on a safe flight of passengers resulted in the first version of a Hard-Timed maintenance process.
Components in an aircraft were set a use limit, after which it should be removed (for instance, a hydraulic pump), sent to a repair shop, and “Zero-timed” by exchanging worn components. The maintenance levels and times were grouped into groups denoted by letters. We now give an example of a typical early Hard Timed system. It uses Flight Hours (FH) as the limit, but real systems used a combination of Flight Hours, Flight Cycles (FC), and Calender time limits:
The above hard-timed program did maintenance on a number of parts irrespective of how the aircraft was used. There is a major difference in wear on the aircraft if the climate is benign or harsh, like sandy or dusty. And many short hops are tougher on the aircraft than longer sectors. The hard time process also did not allow for systems where system self-checks can determine the time for overhaul, so-called On-Condition Maintenance.
Gradually, the airframe OEMs, the regulators, and the maintenance industry worked to change the hard-time maintenance systems to a more flexible and On Condition-based system. This is the system we have today, called MSG-3. We go through what it means in the next Corner.
Bjorn:
Are the engines not on their own schedule?
Yes they are, I should have mentioned it.
Lots of components have their own life limits for replacement or heavy maintenance like engine parts, props, landing gears. Now smart diagnosing software are giving information to the operator which LRU or engine should be replaced for a cheaper shop visit than flying to component failure. One can see it as dynamic hard times. Then it is up to the component shop to take smart action and not just do an incoming test and conclude it passes CMM test limits and send it back with a new 8130-3 tag.
Yes, this is the new MSG-3 maintenance program we discuss Friday.