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What QNH would you use if ditching?

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What QNH would you use if ditching?

Old 10th Jun 2012, 20:52
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What QNH would you use if ditching?

Was having a discussion with one of our Captains today and he asked me this question.
Night, engines failed, no radio altimeter obviously, what QNH would you set?
After thinking for a second, I figured of course it would be the Current QNH.
He was very convinced that he should set standard.
I don't see how standard would be useful considering you almost never have standard pressure or temperature.

If Current QNH will show you elevation at airdrome when youre on the runway, shouldn't it read 0 above sea?

I'd appreciate input on this.

Also while on this topic, another irrelevant ques, I am trying to remember the little concept from flight school.
When youre at std 1013, and change to QNH which is 1030.
Changing from low to high gives you a lower alt.
LOW - HIGH - LOW
Just making sure this is correct.
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Old 10th Jun 2012, 21:38
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gcc_
Was having a discussion with one of our Captains today and he asked me this
question.
Night, engines failed, no radio altimeter obviously, what QNH would you set?
Depends where you are when the engines fail, but why not ask ATC if they can give you the QNH, if thats not posible due to a remote location then use the last known QNH.

If over the sea; then when you think you are close to sea level, lights on and use the mark one eye ball, if over land when you switch on the lights and you dont like what you see then turn them off and hope for the best.

Last edited by Above The Clouds; 10th Jun 2012 at 21:41.
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Old 10th Jun 2012, 22:24
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Also while on this topic, another irrelevant ques, I am trying to remember the little concept from flight school.
When youre at std 1013, and change to QNH which is 1030.
Changing from low to high gives you a lower alt.
LOW - HIGH - LOW
Just making sure this is correct.
Not the way you've written it. If you change the altimeter setting from 1013 to 1020 the altimeter reading will be higher. What the LOW-HIGH-LOW thing is referring to is that if you fly from a low pressure area to a high pressure area and DON'T change your altimeter setting, it will read low.

As for ditching I'd definitely set the best QNH I had access to. Either the local QNH of a near by airfield or the area QNH. It won't be perfect but it'll be a lot better than setting 1013 which would be useless. What was your captain's reasoning for setting 1013?

Last edited by AerocatS2A; 10th Jun 2012 at 22:25.
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Old 10th Jun 2012, 22:34
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If it is daytime VFR it doesn't matter. At night it might be nice to have an accurate altimiter setting but with wing landing lights on it would be the same. Ask Sully if he ever looked at the altimeter and his reply would be WHY? Any competent pilot in visual conditions should be able to ditch by eye ball navigation. The altimiter is not required. This may not work for an autopilot monitoring pilot, only for a real pilot.
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Old 10th Jun 2012, 22:37
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Bubbers, Sully was using the energy circle... That is how he knew that he would not make those airports.
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Old 10th Jun 2012, 23:38
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I didn't say Sully used his eye balls to decide where to land but how to ditch. What I said was the eye ball in visual conditions is the only way to touch down in the proper attitude and speed. He did it well and we all hope we could have done the same. The altimiter is unnecesary once you have chosen your touch down ditching spot.

Hopefully we all could have done the same thing. We aren't really trained for it but we all should have the skill to do it.

The new autopilot monitor guys with low time won't be able to do it unfortunately. The old guys can.
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Old 11th Jun 2012, 00:38
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energy circle

everything I read says he didn't use the ''energy circle"... also, FWIW 4 pilots repeated the scenario in the simulator and turned right away to LGA and all made it.

so I really don't think the energy circle was working...I understand he was in a situation that didn't display this option.

and for the question, use the best one available by radio, or by knowledge of the general pressure situation over the ocean. standard is one way to go without additional knowledge...but if you know you are flying over an area of high pressure...you might want to adjust things


but you would be advised of lowest useable flight levels if pressure was way off.
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Old 11th Jun 2012, 05:59
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Heavens to murgatroyd,

You are landing an airliner with no engine power into the water and you are worried about what QNH to set? I would have thought there would be other things higher on the "to-do" list in this situation.
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Old 11th Jun 2012, 06:41
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I would have thought there would be other things higher on the "to-do" list in this situation.
Yes! Like getting out the tide tables to correct the altitude of the water surface. According to PANS-OPS 26.4.9.3 (vii) para. 6a, night ditchings may only be carried out within 1hr either side of high water (increasing to 1.5hrs in spring tides). This only applies to latitudes above 43N, obviously.

Stabilised approach criteria come into their own in these situations as an overrun of the designated landing area could be critical. Also, after touchdown, all usages of left and right must be replaced with "port" and "starboard", the F/O should be addressed as "first mate" and the in-charge cabin crew as "bosun".
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Old 11th Jun 2012, 07:05
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Is qnh available? Sully was VFR with QNH. Simply if you can see the water all the rest of the BS is not in question. BE THE AVIATOR!
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Old 11th Jun 2012, 08:52
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If Current QNH will show you elevation at airdrome when youre on the runway, shouldn't it read 0 above sea?
Yes, I am very sure of that if temperature is 15c

When youre at std 1013, and change to QNH which is 1030.
Changing from low to high gives you a lower alt.
LOW - HIGH - LOW
Just making sure this is correct.
Somehow correct, if you are looking at your altimeter reaction while you are changing the setting,your indicated altitude will go up,hence you will be lower.If you dont change your setting while you enter a low pressure ,you will be lower than indicated.

Remember of Altimeter errors though

Ditching is a visual maneuver.

Last edited by de facto; 11th Jun 2012 at 09:20.
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Old 11th Jun 2012, 09:19
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comply with the ditching checklist and set landing elevation at 0 and QNH 1013 that's the sea level pressure
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Old 11th Jun 2012, 09:22
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I highly doubt checklist states to set qnh 1013.....
Elevation 0 in the pressurization selector ok but qnh 1013... Nahhhhh
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Old 11th Jun 2012, 11:16
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All those wittering on about using lights and setting pressurisation need to re-read the OP post#1

As to the QNH - the best I had.
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Old 11th Jun 2012, 12:01
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Why not just wait until your belly touches the water? When it does, zero your altimeter and you'll be able to read a reasonably accurate QNH (within instrument limitations) from your kollsman window. Especially handy should you have to go missed off your initial ditching attempt for any reason...

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Old 11th Jun 2012, 12:08
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Snoop

Tell the passengers to look out of the windows. When they all scream together, flare!
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Old 11th Jun 2012, 13:16
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At 200' cross check with your radar altimeter. If they don't agree go-around and reset your altimeters. Do the after takeoff checklist, climb checklist, descent checklist and before landing checklist, and then, and only then, attempt the ditching.

The investigators will be all over you if you continue the approach with the wrong altimeter settings!

Repeat after me - procedure, SOP, procedure, SOP, procedure, SOP... it will save you.
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Old 11th Jun 2012, 13:40
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Who cares? Haven't you got better things to be doing? If you really need height info use the rad alt!

Dear me!
I think the poster has the right to a discussion.
Now since we are in the tech forum,are you sure the RAs would work ?
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Old 11th Jun 2012, 17:59
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I think honestly for a night ditching it would be best to set up a very shallow glide similar to a 'glassy water' landing. however that maybe difficult to do with no power....but I don't think an accurate altimeter would be of that much help in the scenario you describe
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Old 11th Jun 2012, 18:10
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Was having a discussion with one of our Captains today and he asked me this question.
Night, engines failed, no radio altimeter obviously, what QNH would you set?
After thinking for a second, I figured of course it would be the Current QNH.
So that was you guys that over-ran MSP by an hour?


I sense a need for RNP (DITCH) procedure plates!

Last edited by FlightPathOBN; 12th Jun 2012 at 01:24.
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