Design Errors For Cargo Door on Boeing 747
The first rule in designing and building a pressurized hull is: Don't cut
holes in it. If one must cut holes in the hull for essential reasons make
sure the holes are small well sealed. And then if the hole fails to seal,
make sure the hull does not come apart.
Those principles were violated in aircraft and submarines to the dismay
of the families of the victims of inadvertent door openings in pressurized
hulls.
The nuclear submarine Thresher was sunk because a small hole in the hull
had a valve installed backwards so the water kept on coming in and sunk
the boat and killed everyone on board.
UAL Flight 811 had a cargo door open inflight and nine passengers were
sucked out to their deaths from their seats above the cargo door.
The design errors on doors for Boeing 747 are as follows.
1. The hole is too large. It is large to accommodate passengers' spare clothes
and other non-essential items. A small hole will depressurize slowly and
not allow huge amounts of fast moving air into the hull.
2. The hole has a complicated door system for locking and unlocking which
is prone to misuse. In fact two Airworthiness Directives have been issued
against that specific door, the forward lower lobe cargo door.
3. a. The door is hinged on top and opens upwards. The tradition of passengers
entering by a front hinged door and baggage loaded into a top hinged door
goes back to stagecoaches, buggies, cars, trains, and buses. And when those
cargo doors opened the penalty was bashed baggage and damaged doors. At
speeds above 200 knots such an error has catastrophic results. The door
opens and the airstreams pushes the door up and tears it off it upper hinges
taking away as large part of fuselage skin, exposing the nose of the aircraft
to huge amounts of powerful fast moving air. Top hinged doors on fast moving
pressurized hulls is a fatal design error. They also exist to ease the loading
of the non-essential items by the baggage handlers.
b. Doors hinged on the front are slightly better but the door will still
flap around and eventually break off leaving a large hole. The air pressure
will attempt to keep the door closed. A front hinged door, when it opens
in flight, will at least give the crew time to slow the airplane down to
reduce flutter and possibly allow safe return to land.
Cargo doors, and passenger doors, and engine doors, and access doors
will all be left open or open by themselves sooner or later. The effect
should be mild and easily corrected. The consequences for that small oversight
should not be total destruction and death.
Why do the doors inadvertently open? Good question, as all why questions
are. Let's make a list:
1. It is never closed fully.
2. It was backdriven manually.
3. It was back driven electrically.
4. Fuselage flexing allowed door to spring open.
5. Electrical short to door opening system.
6. Cargo shift into door.
7. Internal explosive force against door.
8. Locking pins shearing and releasing door to open.
9. Intentionally/unintentionally opened by crew inflight.
10. I'm sure there's more.
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