Bjorn’s Corner: Keeping airliners operational. Part 2

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.

Figure 1. The first modern maintenance program was formed around the Boeing 747. Source: Wikipedia.

Airliner 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.

Figure 1. Aircraft stressed skin fuselage structure, also called a semi-monocoque structure. Source: FAA handbook.

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).

Fail safe or safe life structure

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.

Figure 2. Ruptured cabin skin of Southwest Airlines flight 812. Source: Wikipedia.

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:

  • Inspections with gradually more and more sophisticated methods as the time passes. At a certain limit, it will be required to remove interiors and inspect the naked metal behind panels, hat-racks, cargo room liners and floors. These occasions are part of the expensive heavy checks for the aircraft.
  • Replacement of parts. Most replaced parts, like landing gears, goes to a maintenance shop for renovation. Other critical structural parts like bolts or straps are replaced and the old parts go to scrap.

In the next Corner, we will dig deeper in the safe life and fail safe maintenance procedures.

8 Comments on “Bjorn’s Corner: Keeping airliners operational. Part 2

  1. Almost all heavy checks on passenger aircraft airframes have moved to areas with lower labor rates over the last 20 years. While engine and component maintenance stayed where it was, labor rates are a limitted part of total costs.

  2. I dont see why its in Airbus ( or any other manufacturers) interest to extend the number of cycles and hours its planes fly.
    I would have thought the only worthwhile effort was to reduce the maintenance requirements during the 48k cycles, which i understand they do as well.
    48K is 22 years at 6 cycles per day and every day of the year, so maybe 25 years with some down time.

    • Probably does not matter one way or the other. The airplane is economically obsolete at 48K. Steps to reduce the maintenance life cycle cost also probably extend the life.

    • Because future business may depend on a product that gives greater value to purchaser.

      In recent times fuel efficiency and high labour costs have driven earlier retirement. (I am curious what happened with 757s.)

      Manufacture support for ongoing maintenance is crucial, Lockheed Georgia charges for structural analysis to define inspections so L382/C130 wings don’t break, I don’t know if that is just for subsequent owners (I’d check other manufacturers contracts as well).

      Boeing had to be taught by customers to make more reliable airplanes, as Airbus and Embraer later did, and teach suppliers as well. That was one reason for engineering departments at operators like PA, UA, and AC.

      • Intended usage is a factor. Using a 747 on short-haul routes will wear it out, though the “SR” modification package for JAL helped. OTOH, the 737 was designed for many cycles.

  3. The aircraft is not obsolete after 25 years, when fuel is cheap.

    Obsolesce comes in two forms: technical and economic.

    Technical obsolescence would be the life limit of the aircraft.

    Economic obsolescence is when it no longer is cheaper to fly an old (paid for) airplane than to make payments on a new, and more efficient one.

    In today’s low-fuel-cost environment, there is real value in flying aircraft for much longer, and avoiding shelling out for a new airplane. Both Allegiant and Delta have done very well, indeed, pursuing this approach.

  4. Good topic,

    Operational reliability is essential, takes preparation – as WestJet found out the hard way with B767s.

    Best to introduce a type on local runs first, international is especially risky due curfews and high away-from-base costs. America West found that out the hard way much earlier, helped drive the company under.

Leave a Reply

Your email address will not be published.