Question for airliner pilots

[Peter Dowson]

I'm relaying this question from Ray Proudfoot and I here because I believe we
have some airliner pilots, do we not?

Ray and I had a lengthy discussion about when to change from STD to QNH
pressure during the descent. Ray thought it should be when or close to the TL,
disregarding current clearances, but I said I thought it should be when we
get our first clearance to an altitude (rather than a Flight Level), and are
setting this altitude on the MCP.

My logic here is that the autopilot is targetting on whatever the altimeter
is set to. In particular, the descent rate to make a specific clearance will
be calculated and the autopilot will proceed on the basis that the
altimeter will read the target altitude when it gets there -- but it could be up
to 1000 feet difference depending on the QNH/STD difference. If the target
altitude is the first valid one below the TL then that could be quite a big
change.

To me, it seems much more logical to allow the automatic mechanisms to do
their job correctly and smoothly by providing them the correct inputs they
need.

Of course the same logic applies to the climb to a Flight Level, but of
course in this case the check would be that you were on STD pressure.

Comments from any real-world pilots would be appreciated. Which is "right"?
Are there rules? References? If the "right" way is the one I feel is
illogical, why is it right?

Bear in mind that I am using an EFIC control panel and PM's lovely PFD/ND
which allows be to swap between QNH and STD with ease, a single button
press.

Regards,

Pete

[Matt Olieman]

18,000' MSL and above..... always at set at 29.92 and 17,999' and below,
local ALT settings. As far as I know there are no variances to that rule.

Matt O.

"Peter Dowson" wrote in message
news:411493.82130@wb.onvix.com...
> I'm relaying this question from Ray Proudfoot and I here because I believe

> we
> have some airliner pilots, do we not?
>
> Ray and I had a lengthy discussion about when to change from STD to QNH
> pressure during the descent. Ray thought it should be when or close to the

> TL,
> disregarding current clearances, but I said I thought it should be when
we
> get our first clearance to an altitude (rather than a Flight Level), and
> are
> setting this altitude on the MCP.
>
> My logic here is that the autopilot is targetting on whatever the
> altimeter
> is set to. In particular, the descent rate to make a specific clearance
> will
> be calculated and the autopilot will proceed on the basis that the
> altimeter will read the target altitude when it gets there -- but it could

> be up
> to 1000 feet difference depending on the QNH/STD difference. If the
target
> altitude is the first valid one below the TL then that could be quite a
> big
> change.
>
> To me, it seems much more logical to allow the automatic mechanisms to do
> their job correctly and smoothly by providing them the correct inputs
they
> need.
>
> Of course the same logic applies to the climb to a Flight Level, but of
> course in this case the check would be that you were on STD pressure.
>
> Comments from any real-world pilots would be appreciated. Which is
> "right"?
> Are there rules? References? If the "right" way is the one I feel is
> illogical, why is it right?
>
> Bear in mind that I am using an EFIC control panel and PM's lovely PFD/ND
> which allows be to swap between QNH and STD with ease, a single button
> press.
>
> Regards,
>
> Pete
>

[Peter Dowson]

On 4/21/2006 6:26:48 PM, Matt Olieman wrote:
>18,000' MSL and above.....
>always at set at 29.92 and
>17,999' and below,
>local ALT settings. As far as
>I know there are no variances
>to that rule.

Ah, only in America. Sorry, I don't fly there. Most parts of the world have
things called Transition Altitudes and Transition Levels, which are
distinct, and vary quite considerably (and which are mostly much lower than the US
18,000).

Anyway, thank you, but I have my answer now, from another source, quoted from
the UK AIP:

=====================================
The extracts from the UK AIP below explain in more detail (I've highlighted
the most important bits). In simple terms, though, when outbound you set QNH
until ATC clears you to climb to a FL at which point you set 1013.2/29.92
unless a vacating report is required at an intermediate altitude. When
inbound and cleared from a flight level to an altitude you set the QNH as soon
as you have vacated the FL unless you are asked for a vacating report at an
intermediate FL in which case, you keep 1013.2 until you've passed that
level.

Here are a few examples.

1. You're flying at 4000ft on the QNH 1023 when you receive clearance to
FL80. As soon as you leave 4000ft, you re-set your altimeter to 1013.2.

2. You're flying at 4000ft on the QNH1023 when you receive clearance to FL80
to report passing 5000ft. In this case, you leave 1023 set until you've
passed 5000ft and then you re-set your altimeter sub-scale to 1013.2.

3. You're flying at FL120 on 1013.2 when you receive clearance to descend to
6000ft, QNH 1021. As soon as you vacate FL120, you re-set your altimeter
sub-scale to 1021.

4. You're flying at FL120 on 1013.2 when you receive clearance to descend to
6000ft, QNH 1021 to report passing FL80. In this case, you leave 1013.2 set
until you have passed FL80 in the descent and then you re-set your
altimeter sub-scale to 1021.

==============================

Handily, all this is exactly what I've been doing for years, simply because
it seemed so logical! :-)

Regards,

Pete

[David Rabiner]

Pete,

I thought Mode C transponders send altitude based upon what the altimeter is

reading. If that's the case, then a prematurely changed altimeter setting
would send erroneous altitudes to ATC, which would, of course, impact ATC's

ability to separate traffic and accurately inform pilots of the location of

nearby aircraft.

Warm regards,
David

[David Rabiner]

Well, let me correct myself. Mode C transponders apparently send pressure
altitude. ATC's equipment then adjusts the pressure altitude transmission
to the current altimeter setting. As a private pilot, I guess I should have

known this.

Again, this is also all USA stuff, with the steering wheel on the left. ;-)

David

[Peter Nielsen]

When an aircraft has to climb until or above the Transition Altitude,
Flightlevels are used, and the altimeter is set to the standard pressure 1013
(QNE). If the aircraft stays below the Transition Altitude it is called
Altitude.
When an aircraft has to descend below the Transition Level, Altitudes are
used, and the altimeter is set to the local QNH. If the aircraft has to level
off above the Transition Level, you should use Flight Levels.

For the United States, the Transition Altitude is 18000ft. Above this
Altitude, Flight Levels are used. For Belgium, the Transition Altitude is fixed at
4500 ft. For Germany Transition Altitude is 5000 ft, and for the
Netherlands Transition Altitude is 3000 ft. The transition Layer is always minimum
1000 ft thick. So if the local pressure is 1013 Hpa, the Transition Level is
FL040 (Netherlands). One (1) Hpa is about 30 ft. So if the local pressure
is 1000 Hpa, the Transition Layer will get smaller, so the Transition Level
will be higher. How much? Well at least 13 x 30 = 390 ft. The nearest
Flightlevel is then FL045 as Transition Level

[Peter Dowson]

On 4/22/2006 3:37:04 PM, Peter Nielsen wrote:
>When an aircraft

Your tutorial is very nice, and thank you for it, but my question was way
beyond such elementary matters, which I do really understand thoroughly I
assure you. As it is I did receive answers from another source which I included
earlier in this thread.

Thnk you anyway for your contribution.

Regards,

Pete

[Don McKellow]

Real life info:

On climb, nver set STD untill actually at Transition Altitude.
Except in USA it is accepted to set QNH on descent when cleared below
Transition Level, however for accurate TCAS operation, on descent the QNH should
be set at exactly Transition Level.

Regards
Don

[Peter Dowson]

On 4/23/2006 7:19:30 AM, Don McKellow wrote:
>On climb, nver set STD untill
>actually at Transition
>Altitude.

Actually the UK AIP seems to disagree, and I've now found references in CAP
85, Aviation Law for PPL applicants, Appendix 11, which states "after
clearance to climb above TA has been given and climb commenced, vertical position
will, unless spcially requested by ATC, be expressed as a flight level
provided that the aircraft is no more than 2000 feet below TA".

Interesting that the 2000' part doesn't seem to appear in the AIP reference.
The CAP 85 I have is dated Feb 1997 so things may have been changed since
then. I don't have any of the new European joint authority books I'm
afraid.

Regards

Pete

[Peter Dowson]

On 4/23/2006 7:19:30 AM, Don McKellow wrote:
>however for accurate
>TCAS operation, on descent the
>QNH should be set at exactly
>Transition Level.

Are you sure about TCAS operation being dependent upon the pilot setting the
altimeter correctly? I really hope that is not the case. Surely TCAS works
on pressure differences, as for Transponder -- in fact doesn't it rely on
the transponder settings, which use pressure altitude, not some possible
misreading off the altimeter dial?

Anyway, I have found a website from which I can get the actual rules. It has
to be subscribed to, but that is free, so I will try to join it and come
back with the actual rules.

Regards,

Pete

[Peter Dowson]

On 4/23/2006 8:22:32 AM, Peter Dowson wrote:
>Anyway, I have found a website from
>which I can get the actual rules.

Okay, I found all the details. That 2000' part has been removed. The current
UK AIP explicitly says, for take-off and climb, that "when cleared for
climb to a flight Level, vertical position will be expressed in terms of Flight
Level, unless intermediate altitude reports have been specifically
requested by ATC".

For descent it states "on vacating the Flight Level, the pilot will change to
the aerodrome QNH unless further Flight Level vacating reports have been
requested by ATC, in which case the aerodrome QNH will be set following the
final Flight Level vacating report".

I've now got this clarified in two separate definitive ways, so I'm happy!
;-)

In case anyone else is interested I attach the relevant section of the AIP.

Regards,

Pete

[nrasch0402]

I have asked this question a few times of my pilot buddies and the answer varies with the different companies. More ofthen than not, following the "keep it simple stupid" rule, when cleared to climb up to a FL, they go ahead and set 29.92, when cleared on the way down below 18k, set local pressure. Often times, with two more more altimeters to set, the PF will go ahead and set his altimeter, but the PNF will set the altimeter which the FMC references ony when passing 18K. Of course the text book answer is the latter. Reason being is you may be give a descent (or climb) clearance that takes you through transition, only to be told during the descent (or climb) to hold at an intermediate altitude. This is not such a big deal when there is a small difference between the local altimeter setting and 29.92, but there are those days...

(I know, its all heresay, hope it helps anyway)

[dodiano]

Ok this is my humble opinion as a humble Airline Pilot! You change STD to QNH at Transition Altitude in the U.S is 18,000 feet, In central America is 19,500 feet MSL in Cuba transition altitude is 5000 feet MSL so in the case of Cuba which is really weird you fly to Flight Leve 060 that is 6000 feet passing 5000 you change to QNH... that is Why sometimes the center while on Descent from for example and this is in the U.S. ¨DESCEND FROM 230 TO 100 THOUSAND ALTIMETER 30.06¨since you´ll be crossing the transition they give you the altimeter setting from taht sector for example when approaching LAX they give you te altimeter from Palomar cause you´ll be almost 100 miles from the airport crossing 18,000 feet then when you get to LAX approach you´ll set the LAX altimeter since you have copied on the ATIS if not you can always ask for it... Normally is a small difference!!

Hope it helps

Roberto

[3202b]

Thomas Cook in the Uk change over straight away when cleared below the TL.

[dodiano]

Basically what he says is true Crossing or about to cross the Transition ALtitude! When it blinks in the case of the bus is when we do it...

Regards,

Roberto