By Bjorn Fehrm
April 20, 2017, ©. Leeham Co: We will start the second article in series on keeping airliners operational by discussing how the structure is kept fit.
There are three areas that are more key to flight safety of an airliner than others. The aircraft’s structure, the engines (already discussed) and the flight control system. We will start with the structure.
The structure of airliners has been of the stressed skin type since the 1930s, Figure 1. Stressed skin means the skin of the aircraft, including its stiffening members (frames, ribs, longerons or stringers), carries the loads. The operational integrity of stressed skin structures was no big problem until pressurized cabins were introduced in the 1950s.
As airliners needed to fly at higher altitude than 10,000ft, it was necessary to pressurize the cabin. This was to get the oxygen partial pressure to be high enough, so the passengers would not suffer hypoxia (lack of oxygen).
Pressurization of the fuselage is through the air conditioning systems pumping compressed air into the cabin pressure vessel. The vessel is formed by the aircraft’s skin and the front bulkhead (behind the nose radome) and the rear bulkhead (behind the rear galley/lavatories). The pressure regulates by outlet valves on the rear pressure bulkhead letting out more or less air.
When the aircraft climbs, the outlet valves let the pressure differential between outside air and air in the cabin rise to keep a pressure altitude in the cabin of below 8,000ft (modern aircraft decrease this to around 6,000ft).
The pressure differential will subject the skin and bullheads of the aircraft to stresses. The problem is not the absolute level of the stresses, but that it’s repeated up to 5-6 times a day for an airliner.
Gradually, the metallic materials in the structure are running into metal fatigue. The material becomes more brittle and minor cracks that were not propagating further before start to grow. When the cracks have grown to a length were the remaining material around the crack can no longer retain the stress, the material ruptures.
Figure 2 shows the ruptured cabin roof of Southwest flight 812, caused by metal fatigue due to cabin pressure cycling.
The picture of the Southwest Boeing 737-300 shows how modern design principles have avoided a catastrophic failure for the flight. The structure in this part of the aircraft is designed to fail safe standards.
Fail safe means that a total failure of the structure shall not happen even though a part of the structure fails. In this case, the skin ruptured around a lap joint, which had manufacturing faults in it. The skin was ruptured and it no longer held the aircraft together at the ruptured area. But the fuselage did not break; it was kept together by the skin’s stiffening supports, the longerons and stringers (the longitudinal beams in the picture).
For areas that cannot be designed fail safe, for example the landing gear or aircraft engine pylons (called struts in the US), the designer will design to a safe life standard. This requires that he lowers the stress levels so the metal cannot fatigue for, e.g., 30,000 airline missions and the structure is easily inspected for any anomalies that can occur. The safe life of the component is then set at a fraction of the calculated life, for a landing gear, e.g., 10,000 cycles.
During the whole operational life of both fail safe and safe life structures, the maintenance plan prescribes inspections. These are designed so the inspection method (visual or with inspection tools) shall observe any abnormalities (cracks, deformations, leaks…) well before the given hard limit is reached for safe life parts and before any part failures can occur for fail safe designed parts. The latter have hard limits as well but these are often the total design life for the aircraft.
Short range airliners were initially designed for ~50,000 cycles, after which they should be scrapped. As an example, the Airbus A320 had a design life of 48,000 cycles and 60,000 flight hours when it entered service (it was calculated with a flight hour to flight cycle ratio of 1.35h per flight). With modern flights lasting longer (close to two hours) and aircraft use per day increasing, Airbus has been busy increasing the life limits in steps, with limits now at 90,000 cycles and 180,000 flight hours.
It will not be possible to keep all parts of the aircraft fatigue free for such a long time. The maintenance plan for the aircraft has therefore specific limits for the different parts of the aircraft for:
In the next Corner, we will dig deeper in the safe life and fail safe maintenance procedures.