FBW, ditching in water

[JF Mezei]

You may have undoubtely heard that a US Airways aircraft lost both
engines and managed to ditch into the hudson river and stay intact with
all occupants safely escaping.

It is unfortunate that it will probably take over a year for NTSB to
release its reports, but this has raised a couple of questions which the
generally clueless media have not handled:


The A320 cockpit has a "ditch" switch which closes all air valves to
help keep the cabin afloat. (it is also used during de-icing to prevent
de-icing glycol fumes from being distributed into cabin). Was it
activated, and is it powered by the RAT ? (For the BA 777 crash, they
found that the fire fighting fuction was not fully functional when an
engine was turned off for instance).



Secondly, it will be interesting to hear the NTSB's analysis of
aircraft's behaviour. One pilot interviewed mentioned that at low speed,
it is very hard for a pilot to maintain the right attitude to ditch in
water. Did the FBW computers make this job much easier by multiplying
control surface movements to compensate for low airpeed withouth going
into stall ? Or did the pilot have to fight the computers to get the
plane to do what it needed to do ?

Does anyone know why there was no apparent effort to close the doors to
prevent the aircraft from further flooding ? (especially since the
aircraft was moving towards salt water which would destroy the cockpit
and reduce amounts of information usable by NTSB).

It is possible to secure the overwing exits once they have been opened ?
Since the pressure would be in reverse direction, the plug doors would
truly have to be locked in place. (same for the main doors).


.
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[matt weber]

On Fri, 16 Jan 2009 07:49:48 -0500, JF Mezei

Post by JF Mezei
The A320 cockpit has a "ditch" switch which closes all air valves to
help keep the cabin afloat. (it is also used during de-icing to prevent
de-icing glycol fumes from being distributed into cabin). Was it
activated, and is it powered by the RAT ? (For the BA 777 crash, they
found that the fire fighting fuction was not fully functional when an
engine was turned off for instance).

Fortunately the aircraft was a low altitude, and I suspect there was
sufficient windmilling of the engines to provide hydraulic and
electrical power. CMF56 engines will in fact turnover even a slight
breeze on the gound!

Post by JF Mezei
Secondly, it will be interesting to hear the NTSB's analysis of
aircraft's behaviour. One pilot interviewed mentioned that at low speed,
it is very hard for a pilot to maintain the right attitude to ditch in
water. Did the FBW computers make this job much easier by multiplying
control surface movements to compensate for low airpeed withouth going
into stall ? Or did the pilot have to fight the computers to get the
plane to do what it needed to do ?

When you get outside the nominal flight envelope (and gliding is
outside the envelope), the flight control systems are designed go
into 'direct mode'.They do exactly what they are told, period. Even
non-FBW aircraft have features to adjust the feel and behavior of
control surfaces based upon airspeed. The aircraft equivalent of
variable ratio power steering.

Post by JF Mezei
Does anyone know why there was no apparent effort to close the doors to
prevent the aircraft from further flooding ? (especially since the
aircraft was moving towards salt water which would destroy the cockpit
and reduce amounts of information usable by NTSB).

The aircraft was a writeoff as soon as it went into the water and the
avionics bay went underwater, with or without salt (and that close
the ocean, it is nearly ocean salinity anyway).
The money these days isn't in the airframe, it is in the avionics and
engines. The engines were going to need overhaul even before they got
wet. Now both the engines and the airframe are probably toast,so the
cost to repair is probably several times the market value of the
aircraft, so the aircraft is a writeoff. My guess is the aircraft was
probably worth about ~15 million.

Post by JF Mezei
It is possible to secure the overwing exits once they have been opened ?

Don't think so. It doesn't look like the escape slides were detached
initially, and that would prevent the doors from closing.

Post by JF Mezei
Since the pressure would be in reverse direction, the plug doors would
truly have to be locked in place. (same for the main doors).

That assumes you don't get water coming in through the cabin
pressurization system, or through the vents in the floor to the cargo
compartments. The cabin floors have had to be vented in the aftermath
of the D10 accidents where the cabin floor collapsed when the cargo
compartment de-pressurized, cutting the links to the control surfaces.
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[Scott M. Kozel]

Post by JF Mezei
The A320 cockpit has a "ditch" switch which closes all air valves to
help keep the cabin afloat. (it is also used during de-icing to prevent
de-icing glycol fumes from being distributed into cabin). Was it
activated, and is it powered by the RAT ? (For the BA 777 crash, they
found that the fire fighting fuction was not fully functional when an
engine was turned off for instance).

The "A320 ditching switch" has been reported on in the news media, but
it seems to me to be an urban legend.

After all, ditching is an incredibly rare event, and is extremely
dangerous in a jet airliner, as ditching would result in (at the
minimum) cracks in the fuselage, which would let in a lot more water
than open valves. More likely, the wings and fuselage would break up on
impact.

What these pilots did was incredible airmanship ...
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[JF Mezei]

Post by Scott M. Kozel
The "A320 ditching switch" has been reported on in the news media, but
it seems to me to be an urban legend.
After all, ditching is an incredibly rare event, and is extremely
dangerous in a jet airliner, as ditching would result in (at the

It is on the overhead panel, towards the bottom of the centre. Just to
the right of the cabin pressurisation controls.

Ditching Switch ( Guarded )

* Normal - Selects normal system operation.
* ON - Commands operating system to close the outflow valve,
emergency ram air inlet, avionics ventilation inlet and extract valves
and pack flow control valve.
o Outflow valve will not automatically close if under manual
control.

for a good look at various cockpit layouts:

http://meriweather.com/flightdeck.html

It is on the overhead panel section labeled "Keep that ice off the
wings and windows" when you mouse over it.

A previous version of that site had more serious titles. This was setup
by someone who used for work for UA. It had been taken off line after
9-11, but eventually came back.

Post by Scott M. Kozel
What these pilots did was incredible airmanship ...

Pilots train for all engines-out situations now. Not sure if they train
for ditching per say though. But flying the aircraft as a glider is part
of their training.
.
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