DJ FLIGHT FORCED TO LAND IN BRISBANE

[Maca44]

Cracked window brings jet down
03-12-2005
From: AAP
NINE people on a Virgin Blue flight into Brisbane were taken to hospital after the pilot was forced to make an emergency landing.

The pilot of Boeing 737 travelling from Townsville to Brisbane made a precautionary descent from 34,000 to 10,000 feet at 10.30pm (AEST) yesterday when an outer windscreen cracked.
Ambulance crews treated 11 of the passengers on the aircraft as soon as it landed at Brisbane Airport.

An ambulance spokesman said nine people suffering from nausea, headaches, nose bleeds and ear aches were then taken to the Princess Alexandra and Royal Brisbane Hospitals.

He said the symptoms were caused by decompression and the injuries were not life threatening.

 

[NM]

This will be interesting to see the official report. If only the outer layer was cracked, why was there a rapid change in cabin pressure? Was it a pressurisation system failure at the same time, human error in changing the cabin pressure or selecting the wrong cabin pressure control system setting during the rapid decent?

I believe the 737 windscreen is a 3-layer laminate, and it is designed not to allow a crack in one layer to propagate to the other layers. The only thing that would normally cause a crack through all three layers would be an external strike of some sort and that is most unlikely at 35,000 feet - not too many migrating geese up that high over north Queensland.

So a crack due to age or material weakness that was perhaps triggered by temperature change should be contained to only one laminate layer. Such a crack is not going to cause rapid depressurisation. So the depressurisation must have been through the pressure relief valves used to control the cabin pressure. This is controlled by the Cabin Pressure Control System (CPCS) that can be setup to operate in several modes, including constant pressure differential (inside to outside) or climb/decent mode. The 737NG has a variable pressure differential system permitting different maximum differentials depending on cruise altitude. For cruise at FL370 (37,000 feet) the max differential will be 7.8 PSI to maintain a cabin altitude of 8000 feet.

A rapid decent should not cause a rapid change in the cabin altitude (ie pressure) under normal conditions. I suspect that is why they want to look at the FDR information. Perhaps the wrong mode was used during the decent. Under normal conditions, the CPCS would be set to a maximum rate of change for the cabin pressure. 8000 feet cabin altitude would be maintained until decent below FL280 and then the change would be in line with the rate of change selected.

A rapid decent is an appropriate action to take when a windscreen crack is identified. Getting down below 13,000 feet would be the primary objective just in case it broke and a rapid decompression resulted. I think the 737 max altitude without cabin pressurisation is about 13,600 feet and a cracked windscreen would result in a target IAS of around 250 KIAS (to reduce potential for impact damage at the lower altitude). But with the cabin pressurisation system in the correct mode of operation, there should be little change to the internal pressure during a precautionary decent from FL350 to FL130.

At FL350 the cabin altitude will have been at 8000 feet anyway, so the cabin would still be pressurised at FL130. Its just that if the pressurisation failed at FL130, the chances of anyone experiencing hypoxia is greatly reduced.

We will have to keep an eye open for the official report.

 

[Alan in CBR]

This will be interesting to see the official report. If only the outer layer was cracked, why was there a rapid change in cabin pressure? Was it a pressurisation system failure at the same time, human error in changing the cabin pressure or selecting the wrong cabin pressure control system setting during the rapid decent?

As you say, the report will be interesting. Perhaps the depressurisation was deliberate - equalise cabin pressure with the outside to minimise stress on the remaining layers of the windscreen (and minimise the effects if they were breached)? Dunno.

 

[QF WP]

Great report, NM. Ever thought of being an Air Crash Investigator????

 

[NM]

Great report, NM. Ever thought of being an Air Crash Investigator????

That would be a ver sad job indeed. But probably very interesting at the same time. I will leave that to Spotty.

 

[QF WP]

Yes, he's the only one that I know that goes to the strangest places I've never heard before, then back to civilisation...he must do some serious miles.