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Old 18th Jun 2017, 17:31   #1 (permalink)
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Setting altimeter in cruise

On the bus (can't speak for other types), when in the cruise and upon obtaining the destination airfield QNH, many pilots temporarily change the altimeter over to QNH and preset the value they will be needing later on, and then go back to STD.

At a previous airline, a trainer suggested not to do this, as it can inadvertently result in a TCAS RA. I.e, if cruising at FL350, flying a thousand above someone in the opposite direction, we enter destination QNH of 983 that's 600ft below STD so the transponder will churn out 34,400 momentarily and that can cause the TCAS to give an RA.

Thoughts about the above and this procedure in general?
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Old 18th Jun 2017, 17:41   #2 (permalink)
 
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I was under the impression the transponder always transmitted altitude with reference to 1013.25?
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Old 18th Jun 2017, 18:39   #3 (permalink)
 
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Thats correct and besides modern airliners allow you to set the regional QNH in a standby setting without having to change the actual altimeter setting from standard.

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Old 18th Jun 2017, 19:51   #4 (permalink)
 
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I remember being in the hold one time in an old A320 when the aircraft below was indicating -07 on TCAS. I enquired with ATC as to his altitude. ATC asked him to check his altimeter to which the response was "now corrected". He then indicated -10 for the remainder of his hold.

Last edited by Superpilot; 18th Jun 2017 at 20:55.
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Old 19th Jun 2017, 07:28   #5 (permalink)
 
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Line pilots are supposed to follow SOPs and not devise their own procedures. Creativity in cockpit is fraught with danger. It's not airbus SOP. Besides it is very myopic action. What if QNH changes? If you get in the habit of pre setting you are likely to forget to change that to the new setting.
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Old 19th Jun 2017, 08:55   #6 (permalink)
 
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Thinking ahead shouldn't stop at only the next step. It should also consider it's complete impact.
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if cruising at FL350, flying a thousand above someone in the opposite direction, we enter destination QNH of 983 that's 600ft below STD so the transponder will churn out 34,400 momentarily and that can cause the TCAS to give an RA.
If you think this is smart then good luck to you. I have experience of training crews of 12 airlines in the east at different times and none of them have this in their SOPs. Jetstar Australia and two other airlines changed FMA callout priority for go around and during ILS they landed up in IMC 40ft and in worst case 14ft above ground. They all changed the procedure. One doesn't have to be over smart. Surely you can check with your operations before doing that.
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Old 19th Jun 2017, 09:07   #7 (permalink)
 
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You pre-set the landing minima on the PFD before TOD don't you? what if the approach changes?
Surely everyone does that. Setting minima is one step of the approach procedure. It doesn't change by itself like QNH. Not a good example.
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Old 19th Jun 2017, 09:10   #8 (permalink)
 
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Preset QNH all you like, I can't find any fault with this practice. And of course it has no impact whatsoever on TCAS or your transponder.

Just be sure you put it back to standard! This got my sim partner (and arguably myself) a reprimand during a sim check. We were dealing with a complex failure with no autopilot. My sim partner, the FO, was flying and out of habit he preset the QNH in preparation for the approach but forgot to revert back to standard, or he didn't pull the knob hard enough, I don't know. So throughout the descent he was on QNH whilst I remained on standard. It wasn't a big enough difference in QNH for the ECAM to register and I only noticed this error when he started to level of way too early!
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Old 19th Jun 2017, 09:13   #9 (permalink)
 
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Quote:
Originally Posted by Superpilot View Post
I remember being in the hold one time in an old A320 when the aircraft below was indicating -07 on TCAS. I enquired with ATC as to his altitude. ATC asked him to check his altimeter to which the response was "now corrected". He then indicated -10 for the remainder of his hold.
I'd be inclined to suggest the aircraft was holding at the wrong level due to a mis-set altimiter.
The transponder therefore the TCAS references 1013.25 otherwise just one aircraft being on a different pressure setting talking to a different service would compromise the TCAS systems effectiveness.
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Old 19th Jun 2017, 09:19   #10 (permalink)
 
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Exactly correct, TAd.
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Old 19th Jun 2017, 09:31   #11 (permalink)
 
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Vilas:- I should add that I was referring to pre-setting the QNH on the ISIS, in my haste to reply I inadvertently thought he was referring to the ISIS.

I don't advocate setting it on the main Altimeter and then re-setting STD.

Sorry for any confusion.

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Last edited by ACMS; 19th Jun 2017 at 09:45.
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Old 19th Jun 2017, 13:30   #12 (permalink)
 
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If it's such a bad idea then why does airbus allow to preset the QNH without leaving the STD mode on their new jets?
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Old 19th Jun 2017, 13:41   #13 (permalink)
 
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As does Boeing...
My point exactly.
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Old 19th Jun 2017, 16:42   #14 (permalink)
 
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In the F70/F100 ("the better Airbus") you can change the altimeter setting without switching from STD to QNH...
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Old 19th Jun 2017, 19:46   #15 (permalink)
 
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Good point Tango!
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Old 19th Jun 2017, 20:06   #16 (permalink)
 
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We're professional pilots. What's so difficult about following SOPs?
As an FO, one of the worst things I'd hear on the FD was "I usually do it this way."
My unvoiced thought was 'Well aren't you the ace?'
Any captain who fails to follow SOPs (without good reason) is making life difficult for his FO and reducing safety.
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Old 19th Jun 2017, 20:28   #17 (permalink)
 
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Which SOP are you referring to?
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Old 19th Jun 2017, 20:45   #18 (permalink)
 
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There are a LOT of people in this forum who have no idea what they are talking about, all too keen to blast the "pilots MUST follow their airlines SOP lines" without actually knowing the facts, probably having never flown an airliner themselves. I despair.
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Old 19th Jun 2017, 20:49   #19 (permalink)
 
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If you change the Altimeter setting whilst in the cruise, won't the AP pitch up or down momentarily to recapture the Altitude?
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Old 19th Jun 2017, 21:06   #20 (permalink)
 
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Nope. The AP will hold the memorized ALT. Besides, we wouldn't have this topic if the AP would chase the new indicated altitude.
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