Aviation Safety Aging Aircraft Safety Issues
Tuesday, 15 November 2011 20:47
By Alan Price
1. A number of years ago, in recognition of the corrosion/fatigue issue with “Aging Aircraft”, an industry study committee convened and has been very active since then in identifying areas of concern, and being proactive in recommendations to deal with these issues. Two things occur as aircraft age: Corrosion becomes an issue, and second, the number of cycles (times the cabin has been pressurized) becomes a factor. The SWA 737-300 has 39,000 cycles, which is a lot of wear and tear, but with a properly maintained aircraft, usually not a problem.
2. The location and size of this breach in the fuselage is of concern, since it was unexpected. Three more SWA aircraft have shown to have similar cracks. There are proven and simple fixes for these issues, but we have to identify them and stay ahead of the problem. The NTSB investigation working with SWA and Boeing will identify the problem, craft a solution, and the FAA will issue an “AD – Airworthiness Directive” to fix the problem. These vary, depending on the nature of the problem; from recommendations to mandatory, with timelines which may be many months to comply, to immediate no flights until fixed and/or inspected.
3. This story has a happy ending – a testament to two things: Commercial aircraft are greatly “over-designed”, built to more than withstand forces they will encounter in a normal lifetime. For example, a test set of wings of any new type aircraft is placed in a stress jig to flex the wings thousands of times, long before the actual aircraft will encounter such cycles, to insure there are no failures. Additionally, a set of wings is stressed to prove it can withstand 150% of normal expected stress and taken to its failure point to show its durability.
Even with a 5′ rip in the upper fuselage, the aircraft was structurally sound and did its job.
The SWA pilots and cabin crew did an exceptional job. From the interviews I’ve seen with passengers, they were said to be calm and professional, and kept the passengers informed and calm. In an emergency descent from altitude, due to any abnormal and in this case due to a rapid decompression, the pilots have to answer one fundamental question as they begin their descent: Is the aircraft structurally sound? If so, they can make a rapid descent; if not, care must be taken to not exacerbate the condition which necessitated the emergency descent – in this case, a tear in the cabin ceiling which led to a rapid decompression – and the descent is made at a slower airspeed. Pilots and flight attendants are trained yearly on these procedures.
As a side note, rapid decompressions are not normally harmful although they are frightful. Depending on the altitude at which they occur, we have tables called “Time of Useful Consciousness” (TUC), which predict how long one will remain conscious without supplemental oxygen. At 35,000′, after a rapid decompression, TUC would only be 60 seconds or so, maybe less. Thus, the oxygen masks drop and the pilots begin a rapid descent to a lower altitude not requiring oxygen (10-13,000′).
In the cockpit, we have quick don oxygen masks, which can be put in place within a second or two. In the cabin, flight attendants have portable oxygen bottles to allow them to walk the cabin and assist passengers. In the cockpit, there are numerous warning systems to alert the crew should cabin pressure fail – these are both aural and visual, and are UNMISTAKABLE. The crew knows when a decompression occurs, both from these warning systems and from their perceptive senses (fog in the cabin, noise, cold, etc.)
Finally, rapid decompressions are not very common, but we train for them regularly. You may recall that golfer Payne Stewart’s Lear jet crashed some years ago, after flying for several hours, with all onboard already dead from oxygen starvation, hypoxia. This obviously occurred due to an oxygen leak/decompression. A rapid loss of cabin pressure is good in most cases in that there is no doubt about what has happened.
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