Isn't this risky? I thought once they pass transition level they should change to local pressure... But is this not risky to put local pressure qnh before... risky in terms of separation wheter vertically or horizontally.
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1$\begingroup$ How would it affect anything horizontally? If they are cleared down to an altitude below the flight levels then they are allowed to pass through all altitudes between. $\endgroup$– JimJun 18, 2022 at 5:30
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$\begingroup$ Standard pressure is set above transition level. If you used local pressureway above some airplanes would be flying at different altitudes thus potential for risk of trafic or obstacles @Jim $\endgroup$– Yosef JabbourJun 18, 2022 at 5:42
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$\begingroup$ Good. Carry that thinking even further… $\endgroup$– JimJun 18, 2022 at 5:45
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$\begingroup$ We are talking about descnding here, and the possible risks involved in using local pressure qnh above the tranition layer .(since when airplanes are above transtion layer standard pressure 1013.25hp reference system is used....) having different references provides problems. An airplane 3000 ft might collide with a 4000ft ...I mentioned not only vertically but also horizontally due to the fact that airplane can converge you from any direction ...hope this clarrified my point @Jim $\endgroup$– Yosef JabbourJun 18, 2022 at 5:51
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$\begingroup$ So you are descending and have been cleared to do so. You’ve set and are holding a vertical speed. Does fiddling with the knob on the altimeter affect anything about the flight path? $\endgroup$– JimJun 18, 2022 at 5:57
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2 Answers
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There is no risk in changing the altimeter at any time during the descent, as long as it is correct by the time you level off at your proper assigned altitude.
During the descent your displayed altitude may either be behind or ahead of whatever it would display if you were to level off above or below the transition. But that doesn't matter because you are not leveling off, you are in a descent and your altitude is constantly changing.
If you were to receive modified instructions to level off before the transition and you have switch settings early you would need to switch back. Otherwise, the vertical and lateral airspace around your descent would have already been verified clear by ATC before issuing the descent clearance.
The only practical effect is instrument lag: Your altimeter may indicate a different altitude than another aircraft, (that already has adequate lateral IFR separation!) as you fleetingly transition through that same altitude. It is a temporary condition, of extremely short duration, and is not a safety concern.
answered Jun 18, 2022 at 5:34
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$\begingroup$ ... and the lag issue is not really an issue. ATC is required to ensure separation from all other IFR traffic at ALL altitudes between the altitude you are departing and the altitude you are cleared to. They do not watch your altitude and clear other traffic through your flight path at altitudes that you have already passed through! Also, ATC does not see the altitude displayed on your altimeter, Mode C transmits an unencoded altitude which does not vary no matter what altimeter setting you dial in. $\endgroup$ Jun 18, 2022 at 20:45
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$\begingroup$ When you fly on autopilot (i.e. most of the time) and ATC tells you to "climb FL230", you dial 23000ft and 1013hPa into the computer and wait until it levels off at the correct altitude. Why would you postpone pressure setting until midway through the climb? That gains you nothing and creates the risk that you forget about it. Same for descent. $\endgroup$ Jun 20, 2022 at 20:49
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$\begingroup$ @RainerP. is your question directed at me, or the OP? $\endgroup$ Jun 21, 2022 at 3:05
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Yosef you are correct in the sense that setting local QNH before transition altitude during descent may be risky in the rare occasion the clearance to descent is abrubtly changed to an altitude above the transition level. In this case the plane's altimeter would read "wrong" unless the crew remembers to correct the altimeter setting.
Some may think this kind of situation is unlikely, but nevertheless it is possible. A plane may make a navigation error prompting ATC to halt your descent. Unforseen traffic may emerge (emergency, high priority military etc.) making it necessary to halt your descent.
Whatever the procedure pilot chooses to use for descents, it should contain memory item to check for correct altimeter setting when levelling off. If the changing of altimeter setting (well) above or below transition layer is based on SOP, usually a checlist but could also be a requiered memory item, it's ok, as the SOP's would likely contain altimeter setting checks for all actions relating to vertical movements. If, however, the QNH adjustment is done against SOP out of habit or for convinience, it is a potential safety issue.
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1$\begingroup$ Actually, this scenario is possible, but the consequences described are not possible. If an approved descent was halted by ATC, and acknowledged by the Pilot, the pilot would reset the altimeter setting as appropriate (depending on the new altitude requested by ATC ). If there was no response from the pilot (radio failure or whatever), ATC would immediately be responsible for ensuring separation from all other IFR traffic until communications was re-established. $\endgroup$ Jun 18, 2022 at 20:41
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$\begingroup$ I can only speculate as to how this is dealt with in SOP's, but just as you described, the premature setting of QNH brings an extra step to this kind of situations, and is therefore not KISS. Let's say you are cleared from FL200 to 6000ft, you set QNH. Something happens and ATC commands to stop descent at FL180. There is a chance you forget to reset altimeter, and end up at not FL180, unless the altimeter check is on checklist or a memory item for you. $\endgroup$– Jpe61Jun 18, 2022 at 21:10
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1$\begingroup$ Well, If the individual responsible for doing something he/she is required to do by standard operating procedure does not do it, and that causes a problem, yes that would be bad. But, for this very reason, as I mentioned in another comment, the altitude reading on the altimeter is Not what is transmitted by Mode C. If you are flying along at 18,000 on your local altimeter setting, The ATC controller will see that on his screen, and you will likely get a call from him/her to fix it. $\endgroup$ Jun 18, 2022 at 23:47
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$\begingroup$ Let me reiterate my point: setting QNH at, say, top of descent, adds an unnecessary hole in the slice of cheese. I've seen my fair share of cluttered checklists that had unnecessarily out of place items. Luckily most of them got cleaned out over time. I do, however see your point and I've slightly edited my answer @CharlesBretana $\endgroup$– Jpe61Jun 19, 2022 at 5:58
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2$\begingroup$ and, to be explicit, I can unequivocally say your point is also a valid one. But it also applies in the much more common scenario where the descent is not canceled. If you wait til you level off at your lower altitude, and get busy doing other things during descent, and then forget... Changing QNH when starting the descent "gets it done" at altitude, when you are likely less busy. $\endgroup$ Jun 19, 2022 at 12:55