FAA Mandates B737 Inspections
Following Incident Aboard SWA 812, Agency Acts In Public Interest
This, in my opinion, is a rational reaction to the metal fatigue fracture incident experienced by Southwest Airlines Flight 812. Southwest had already voluntarily taken over 80 737s out of service for immediate inspections of their fuselage causing delays and cancellations on Sunday/Monday. Boeing then recommended that all operators inspect their earliest production models 737s. Yesterday the FAA issued an Airworthiness Directive mandating that all 737-300/400/500 series aircraft undergo structural inspections before they can be returned to service. This will no doubt cause delays and cancellations during the week of April 5-12, but it really is the prudent thing to do.
WHAT HAPPENS WHEN A CABIN DECOMPRESSES AT ALTITUDE
First it is important to note that pressurized aircraft are designed in such a way that if there is a significant structural failure, only the local area will be compromised while the rest of the structure remains solid and effective. SW 812 was an example of this, a 3-foot hole opened up, but despite flying along at cruising speed and altitude the rupture did not get any larger or affect the pilots’ ability to control the aircraft.
There’s a misconception that oxygen is pumped into aircraft cabins because there isn’t enough oxygen for us to breathe at high altitude. In reality, the same percentage of oxygen exists at high altitude as at sea level. But as altitude is increased, there is less pressure, which in effect leads to less oxygen available in a given breath. Because of this, aircraft are pressurized to simulate a lower altitude inside the cabin. As an aircraft climbs to a high altitude, the cabin is pressurized, and as it descends for landing, it is gradually de-pressurized. This is similar to inflating a balloon, followed by slowly letting the air out.
If a sudden loss of cabin pressure does occur, the balloon deflates rapidly (or all at once) expelling the breathable air very quickly. Sensors in the cabin detect the pressure loss, releasing oxygen masks above each passenger. An explosive loss of pressurization is an emergency that pilots simulate and engrain into their minds many times in training.
Some emergencies allow for a slow and methodical response, but this particular situation requires an instinctive reaction from memory. Priority No. 1 is restoring the ability of the pilots to breathe properly. In the cockpit, pilots have quick access to emergency oxygen masks, and are trained to put them on in mere seconds. Priority No. 2 is quickly and safely descending the aircraft to a lower altitude. At the lower altitude (below 10-12000’) the air pressure is sufficient to sustain consciousness.