GA Aircraft ditching - Irish Sea - 16th Dec

[Islander2]

KeepItStraight said:

Induction icing is unlikely to cause a power lose due to the location of the intake plus the Comanche is fitted with automatically opening alternate air doors which can be also opened manually.

You may well be right, and this could well have no bearing at all on the accident in question.

However, it is certainly interesting to note that the alternate air door design in the the PA30 is poor and has been responsible for engine failures (and, I understand, double engine failures) through air inlet blockage in the past. From an air accident report:

The original design of the alternate air door on the normally aspirated PA30's and PA39's puts the door downstream of the air filter and in an area of high vibration. A failure of the door hinge or shaft will lead to an immediate engine failure. A modification exists to move this door to a lower vibration area on the lid of the air filter. If not modified, this door should be inspected thoroughly and frequently (we have had an Australian fatality due to this).

And from a separate Twin Comanche incident arising from alternate air door failure:

I was flying my PA-30 over water together with my friend Michael in his PA-39. My position was some 200 ft behind and to his right side (we flew many thousands of miles that way, with the leading aircraft calling for both as "Comanche Formation"). All of a sudden it looked like he applied brakes: His aircraft slowed down and in a second I was at his level. I throttled back and we exchanged ideas about what went wrong, as all gauges were normal. All of a sudden he started to accelerate and soon operation was normal. This repeated itself several times to our amazement. However, he was able to make it safely to destination. After landing we started to look for a possible cause and it did not take long to find that one of these doors got loose and partially blocked the air passage. When it moved back and forth the size of the free passage was changed. Fortunately, there was never a full block.

Thus the lack of prop RPM stability through a wildly fluctuating manifold pressure could be explained! Of course, as I said, this could well have no bearing at all on the accident in question.

[mm_flynn]

Islander2 and I threw 'induction blockage' out as an idea to explain low MP on one (or depending on how you read MLB's post both) engine(s). There have been several answers of how common forms of intake blockage shouldn't effect this aircraft. Moving away from this particular case - are there any system failures that can result in less than atmospheric MP other than an induction blockage? (ignoring the case of a faulty gauge)? Are there any sources of inflight induction problems other than, icing/snow, broken alt air door, others??

[mark147]

Moving away from this particular case - are there any system failures that can result in less than atmospheric MP other than an induction blockage?

To see the MP reduce, I believe you must either have an increased intake restriction (e.g. throttle closure, intake blockage/icing etc.) or increased engine RPM (effectively causing increased suction from the engine and hence increased pressure difference across any existing intake restriction, intentional or otherwise).

So I can see how a failure of a CSU would show up as reduced MP as well as increased RPM. But a failure that just results in decreasing MP (and no RPM change) can't, to my mind, be caused by anything other than an intake restriction of some kind.

Mark

[IO540]

You would need a helluva RPM increase to drop MP really significantly.

On my TB20 (IO-540-C4 engine) I see about 1/3 inch MP change from 2300rpm to 2500rpm.

However, we don't know anything about ManxLadybird's flight conditions.

[Islander2]

You would need a helluva RPM increase to drop MP really significantly.

True, but it's the converse that is more interesting. If it were a failed alternate air door, the MP could have been fluctuating between 17" and, say, 22" ... which, presumably, would put the prop on and off its fine pitch stops and present a fluctuating or 'unstable' RPM!

[IO540]

Yes, very much so. At the low MP end the governor would drop out of regulation, and at the top MP end it would overshoot a bit.

So a messy/intermittent intake blockage could manifest itself as wildly varying RPM.

Metars for 16/12/09 around 1200:

METAR EGNH 161250Z 32009KT 9999 -RA FEW023 SCT038 06/04 Q1011
METAR EGNH 161220Z 32008KT 9999 -RA FEW022 SCT033 06/04 Q1011
METAR EGNH 161150Z 32009KT 9999 -RA FEW022 SCT033 06/04 Q1012

For some reason can't get the skew-t from Wyoming Univ. Some from here for the general area suggest solid IMC from a low level (which the metars don't support) to c. 15000ft with 0C around 3000ft. Meteox.com shows strong scattered returns for the area.

Could it have been snowing?

[Flyingmac]

Can anyone tell me what depth of water the aircraft sank in?

[icantfindagoodname]

MLB,

Congratulations on a fantastic outcome and thank you for posting on this thread.

A lot of different emergency agencies where alerted and on standby for you - and I would just like to thank them all.

Have a great Christmas and I hope your back in the air very soon.

[Oldpilot55]

The Irish sea is not particularly deep.

I would think around 20 metres or so, where the ditching occurred.

[Flyingmac]

If it's only in 20mtrs it would be fairly easy to bring it to the surface, but doubt it would be worth the effort. Easy dive though.

[Captain Stable]

Easy dive though.

I think not. Poor visibility, strong current, BLOODY FREEZING even in a drysuit, and a relatively very small thing to find as well as quite light so it could have drifted for quite a distance. The expense of mounting a dive to locate it would far exceed the replacement value of the airframe.

[englishal]

Perhaps there is an ROV vessel in the area already due to the oil and gas ops which can salvage it? Not cheap of course but it depends if it has to be salvaged for any reason....

[rocco16]

Quote:
Easy dive though.
I think not. Poor visibility, strong current, BLOODY FREEZING even in a drysuit, and a relatively very small thing to find as well as quite light so it could have drifted for quite a distance. The expense of mounting a dive to locate it would far exceed the replacement value of the airframe.

Not difficult to dive on the slack although vis may be an issue. Site would need to surveyed by decent side scan sonar to locate and/or a form of magnetic detector. Not sure it would be worthwhile going to the effort.

[mm_flynn]

Neither insurance nor the AAIB appears to have pulled out the previous Twin Comanche that went down in the area. The salvage value of the airframe will be pretty much nil (how much for a light twin with no engines or avionics ??)

The AAIB report will probably be a factual description of the events of the flight, ending with

The pilot was unharmed but was later flown to a mainland hospital as a precaution.
Further investigation as to the cause of the total power loss could not be carried out as the wreckage was not recovered from the sea.

(From the AAIB report of 8 years ago - same type, same place).

[Flyingmac]

Had vis of 20mtrs plus in UK waters at 60mtrs depth. Cold not an issue if properly equipped. Current is rarely a problem at slack on a neap tide. Sidescan sonar much cheaper these days and very good when used with GPS. Our RHIB can run a search pattern at 40 knots. Still not worth it though. Price of Aluminium too low. Fuel too high. Won't bother.

[BEagle]

ManxLadybird, my warmest congratulations on an excellent piece of flying. Whilst the cause of the double engine failure is a mystery still, the description you provided of your descent and pre-ditching drills was quite outstanding. Were you taught all those things you described, or was it instinct? Either way, that was a 'copybook' ditching.

I'm sending you a PM as I think that your account needs a wider audience - many of us have already learned a lot from your post and I thank you for that!

Congratulations again; enjoy a relaxing Christmas!

[herman the crab]

40 kts? How do you stop the 'fish' from coming out of the water at that speed? And I would think the data is unusable too - we struggle much above 4 or 5 kts to get quality usuable data.

HTC

[Flyingmac]

Fairly flat sea bed. Find target at decent speed with a good sounder and mark with M.O.B button on GPS. Positively ID target with slow speed sidescan pass. Job's a good'un.

[Tropical John]

Perhaps there is an ROV vessel in the area already due to the oil and gas ops which can salvage it? Not cheap of course but it depends if it has to be salvaged for any reason....

There is! It is the Northern Canyon, on contract to Centrica. Operator Hallin Marine ( www.hallinmarine.com ). Vessel has both ROVs and diving spread onboard. Operating in Morecombe Bay area now until approx beginning of February 2010.

Operating cost per day of the Northern Canyon > Hull value of of a PA30 so unlikely to be worthwhile. I need to declare personal interest - but happy to help if someone wants it back

[herman the crab]

Well your RIB must have some pretty fancy equipment on board as I've not come across an echo sounder that gives usuable data at that speed in a RIB either!

HTC