Analysis: Rapid descent

I got my hands on the official Flight Crew Training (FCTM) manual for the big Boeing 737-700 for this story.

The 737-700 is a later version of the 737 that Air New Zealand flew for many years and the type that had the catastrophic engine failure sadly killing a passenger on the Southwest Airlines flight this week.

I do so to get the exact procedure for an emergency descent when cabin pressure is lost, a-la a window blows open. The FCTM actually doesn’t call it an emergency descent – only media do – but a Rapid Descent.

First though let me explain how it is you manage to exist in an aeroplane at 32,000 feet. Actually, how you exist at any altitude above where humans can’t really get enough air to exist – around 14,000 feet. (I use feet as that’s what most of the worlds pilots/planes use for telling their altitude – Russia uses metres.It's a bit funny, we’re metric but use feet when flying.)

The how is cabin pressurisation. From just after take-off air is drawn from the engines, cooled, conditioned and pumped into the cabin. As you get higher the more air is pumped in. It remains that way until just before landing when it all gets dumped over-board.

At say 40,000 feet the cabin environment in most jets is around 8,000 feet, in the high tech Boeing 787 it’s 6000 feet. It’s certainly much warmer in a plane than if you were actually standing atop an 8000 foot hill.

Audio of the entire Southwest Airlines event

Now should that pressurised air inside the cabin decrease for whatever reason – and there are a few - and so the ‘cabin altitude’ increases, the plane must be descended as quite simply, the fare payers can’t exist up there where the plane burns less fuel and costs less to run.

If the cabin exceeds 14,000 feet the passenger masks automatically deploy – the crew can also manually deploy them before then. Pilots put on their own full face masks as soon as an issue arises as they really need to be on oxygen and quickly. Then, according the FCTM, they select a lower altitude on the autopilot, select a mode called Flight Level Change - which begins a descent at a set speed to that altitude, they pull the thrust levers back to idle and extend the speed brakes (they are the panels that pop out of the top of the wing of a plane causing large amounts of drag. They are often used in normal operation descents and also deploy on landing).

And down she comes. Fast!

Southwest flight 1380 passenger Marty Martinez (above left) was cool and calm to buy $8 wi-fi to transmit photos to his Facebook account. But like other passengers, he failed to follow the mouth-and-nose instruction for his oxygen mask.

As a result of doing this the vibration and buffeting increases a lot as the plane is heading toward earth so quickly, but it is a fully controlled maneuver designed to get low as possible as quickly as possible.

The 737’s Flight Crew Training Manual states this about a ‘Rapid Descent’;

This maneuver is designed to bring the airplane down smoothly to a safe altitude, in the minimum time, with the least possible passenger discomfort.

The reason for my research was because I keep reading certain media prattling on about emergency descents - ‘unscheduled nose-dives’, ‘horror flights’ where ‘prayers are said’ and ‘goodbye texts are sent to loved ones’.

Plus of course the story has the almost mandatory Twitter pic of a passenger wearing a yellow oxygen mask with a look of horror. On pics of the Southwest Airlines flight pictures it showed almost all the passenger’s not wearing masks right at all! I saw one passenger on board the Southwest flight quotes as saying didn’t know what to do when the mask dropped from the roof!

Southwest 1380's exploded engine, a piece of shrapnel from which shattered a window. The Southwest plane had engines manufactured by CFM, a General Electric subsidiary, not the Rolls Royce engine that has sparked the recall that has embroiled Air New Zealand.

Now I’m not saying it wouldn’t have been scary – anything out of the ordinary on a flight can be scary for some, and certainly shrapnel smashing a window and a passenger being blown out – I’ll explain, because the aforementioned pressurisation inside the plane is much greater than the air outside, a person gets blown out the window not sucked! - and certainly a yellow mask dropping down is definitely out of the ordinary, but reports on airliner incidents makes for laughable reading.

The blown-out window.

It does have me a bit split though. The longtime media man in me realises why stories are told that way, however the even longer time aviation nut knows those quotes are from the ‘special passengers’ on board who media always seem to interview.

You know the ones, they read, text, play on their tablets, sleep or simply ignore the, certainly on Air New Zealand flights, very entertaining and very important safety information video. The one that shows you things like what to do when a yellow mask falls out of the roof!

Mouth and nose!

Grayson Ottoway is an aviation reporter and airshow commentator.

This is supplied content and not commissioned or paid for by NBR.