Operation with transition level below FL180
 

Hi all,

This question not depends on a specific aircraft type, but it might be different based on your company SOPs or personal criteria, reason I would like to know more about others' opinion.

The scenario is this, in several places your transion level is below FL180, for example Europe or some South American countries. Lets set for example FL040. If you get cleared for the approach above the transition level, would you wait to cross below it and swith to Baro reference, or as soon as you are cleared for the approach make the switch?

I have found the next text regarding this as well

The reference for the
altimeter setting should be done during the APPROACH CHECKLIST
after passing the respective transition level.

Your comments will be appreciated

SOP will vary with each airline and may depend on how low transition is. Airlines which traditionally operate in areas with low transition will usually have the 'altimeter' check on the checklist fairly late in the approach sequence.
One system which seeks to prevent a last minute rush when transition is low, is:
At top of descent set STBY altimeter to landing QNH.
When ATC clear the aircraft to an ALTITUDE, the PF altimeter is set to landing QNH.
At or approaching TRANSITION the PNF altimeter is set, then all three crosschecked.

The idea of this is:
1. If the worst happens and later the crew forget to set QNH on all altimeters, at least PF will have it, and so, hopefully, will the autopilot (installation dependant).
2. If there is a significant split between QNE and QNH, it should become obvious when the altimeter cross check is done on approach that one has been forgotten, whereas if both have inadvertently been left on QNE the error may be missed with potentially serious consequences.
3. Until transition, PNF is able to report any levels required by ATC.

However, where there are crossing restrictions in the STAR, careful monitoring of the vertical path is required, and of course if any intermediate level off at a FLIGHT LEVEL is required, PF can NOT set QNH until ATC assign an altitude referenced to QNH (although there is no reason not to set the STBY at TOD).
In some installations, using such a SOP would require the PNF transponder to be selected for altitude reporting, not the PF transponder.

Not saying this is the best system, but have seen it work well as a SOP.

Works in the climb, too. Take for instance China, with a transition altitude at the metric equivalent of 9,850 ft. Always have been taught to WAIT to set the altimeter until passing transition level / altitude. Main reason being, what if you are told to "stop climb or descent" prior to passing the transition? For instance you are cleared in the USA to descent from FL 360 to 8000 feet. On the way down Center says "maintain FL180 for traffic". If you set your altimeter already (esp if it is a low QNH) you may cause a loss of separation. This EXACT clearance happened to me once. Skipper had "pre-transitioned" his altimeter and I had not. He does not do that any more, as the altitude alert going off during level-off scared the bejeezes out of both of us!

Another gotcha (back to China now) is a STAR with altitude restrictions. For instance you come out of Beijing on the TONIL and have a crossing restriction at 8860 feet (or 2700 meters or whatever the heck off-kilter number it is). You are cleared to climb to a high altitude "on standard". I see a lot of people switch their altimeters AT THE POINT OF RECEIVING THE CLEARANCE. As long as you get a subsequent "unrestricted" climb clearance prior to leveling at the 8860 ft crossing restriction, NBD. However if the controller does NOT clear you to climb unrestricted, you are expected to level at 8860 feet, ON QNH AS YOU HAVE NOT CROSSED TRANSITION ALTITUDE YET!

Yes I know they don't give a hoot usually. But that's the way it's supposed to be. You transition, AT TRANSITION. That's the whole point.

Its rare in EU land to get cleared from FL36OHHH to a low altitude. Usually when you get to around FL100 thats when they (ATC) start to reference QNH. But for argument sake, I would preselect the QNH (B737NG) until we get lower before selecting it. BUT, I would take into consideration of High QNH settings because you are in level bust territory.

It's a while since I've checked but the UK AIP used to say set QNH as soon as cleared to an altitude unless further flight level vacating reports are required.

So for example if at FL360 and ATC say "Descend now 2,500 feet QNH XXX" and nothing else you would immediately set QNH.

The ICAO PAN OPS last time I checked says in this case set QNH unless further level flight above the TL is anticipated.

As has been previously stated airlines have SOPs around this and also have a third altimeter where QNH can be set at top of descent.

Thinking Threat and Error Management one issue amongst others is busting MSA on descent if QNH not set and risk of CFIT when pressure is significantly below 1013.

Getting cleared for an ILS approach from a Flight Level is common where I fly. We keep STD (or QNE) until intercepting the glideslope beam, then we set QNH on all three altimeters and complete the Approach Checklist from there. Yes, it becomes a bit backwards since Approach aids is part of that checklist :rolleyes:

To me the most logical is:

Cleared to an altitude - All altimeters set to QNH
Cleared to a Flight Level - All altimeters set to STD

That said, there's traps involved like a stop climb or.. hikoushi had some examples from China.