When to set STD on altimeter
Join Date: Oct 2004
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It's a strange one this because in quiet a few caa and aip's the pilot and ATC internal dept are completely at odds with each other. I have worked with sops written as per UK method and approved by foi. But aip and ATC expect Change at transition. I did ask and was told not to worry about it ATC were run by the military and were clueless.
Even in Europe there are 4 different ways of doing it in aip's. Pilots do not read aip's as a rule.
Personal experience looking at a fleet of 20 aircraft and 60 pilots says there are more alt busts changing at transition. We had a group that were adamant to change at transition and a group that changed when cleared The first group accounted for 95% of the busts. The second did have a couple of busts but they were going to happen what ever the setting was being told to level off with less than 500 ft to go and climbing at 2500ft per minute.
Even in Europe there are 4 different ways of doing it in aip's. Pilots do not read aip's as a rule.
Personal experience looking at a fleet of 20 aircraft and 60 pilots says there are more alt busts changing at transition. We had a group that were adamant to change at transition and a group that changed when cleared The first group accounted for 95% of the busts. The second did have a couple of busts but they were going to happen what ever the setting was being told to level off with less than 500 ft to go and climbing at 2500ft per minute.
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To clarify
ATC systems correct reported level for QNH below the transition layer. So if two aircraft are operating below the transition layer at the same vertical distance above ground, one has 1013.2 set and the other QNH, then they will display different altitudes to ATC. So for example if the transition layer is A100/FL110, then FL070 will display as something other than 7000 to ATC, the size of the difference depending on the pressure difference between QNH and 1013.2.
For this reason other AIPs at ENR 7.1 (not just in Argentina) require QNH to be set when operating below the transition layer.
ATC systems correct reported level for QNH below the transition layer. So if two aircraft are operating below the transition layer at the same vertical distance above ground, one has 1013.2 set and the other QNH, then they will display different altitudes to ATC. So for example if the transition layer is A100/FL110, then FL070 will display as something other than 7000 to ATC, the size of the difference depending on the pressure difference between QNH and 1013.2.
For this reason other AIPs at ENR 7.1 (not just in Argentina) require QNH to be set when operating below the transition layer.
Why would you design the system to potentially transmit a bogus altitude because the pilots set the altimeter wrong?
Wikipedia can be a bad source so google "mode c pressure altitude" because that is what is transmitted, the pressure altitude.
And the pressure altitude is independent of the local pressure. Then the ATC system corrects for local pressure and voila you get a correct readout.
Now the number on the planes altitude indicator may be different from the one displayed on the ATC system because the pilot already set standard pressure. But that's not a problem is it?
This is incorrect. The altitude is always broadcast independent of aircraft altimeter setting.
Why would you design the system to potentially transmit a bogus altitude because the pilots set the altimeter wrong?
Wikipedia can be a bad source so google "mode c pressure altitude" because that is what is transmitted, the pressure altitude.
And the pressure altitude is independent of the local pressure. Then the ATC system corrects for local pressure and voila you get a correct readout.
Now the number on the planes altitude indicator may be different from the one displayed on the ATC system because the pilot already set standard pressure. But that's not a problem is it?
Why would you design the system to potentially transmit a bogus altitude because the pilots set the altimeter wrong?
Wikipedia can be a bad source so google "mode c pressure altitude" because that is what is transmitted, the pressure altitude.
And the pressure altitude is independent of the local pressure. Then the ATC system corrects for local pressure and voila you get a correct readout.
Now the number on the planes altitude indicator may be different from the one displayed on the ATC system because the pilot already set standard pressure. But that's not a problem is it?
So coming back on to topic if there is other traffic at 8000 or 6000 on QNH above or below the aircraft operating on 1013.2 at 7000 and the pressure difference between QNH and 1013.2 is > 6HPa the wrong way then the aircraft on 1013.2 will be deemed to be in vertical conflict (assuming a 1000FT vert separation standard is applicable). Similar applies to a climbing flight that sets 1013.2 while still below the transition layer. That is the reason why climbing flights should not set 1013.2 while still below the transition layer.
Oh, and let's not get hung up about Mode C (or even Mode S, for that matter). Radar is a dying technology, rapidly being replaced by ADS-B and with accelerating pace of technology change who knows what next.
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The situation above would only be possible when the aircraft is fitted with extremely old first generation encoding altimeter with a first generation mode C transponder. Your talking about a 40 year old setup.
Transponders have had their own pressure cell since then and it will only squirt the pressure on 1013. What the pilots altimeter settings are makes not the slightest bit of difference.
The transponder is calibrated every 3 years for IFR machines.
The only aircraft that are even remotely likely to have an encoding altimeter transponder setup will be privately owned antiques with old none digital transponder selectors, Russian or Military.
If its a digital display it will have its own pressure cell for the altitude data.
Those are standalone transponders.
For air data computer machines the transponder can get the data from the data bus in its raw state ie 1013.3 and uses that and the EFIS system converts the same data for QNH for display to the pilots. But again what the pilots set won't change what the transponder transmits.
Most digital transponders be it S or C allow you to see what it is transmitting. Just fly manual and select it and fly level on the backup altimeter and wind the QNH up and down on the main altimeter, the value shown on the transponder won't change.
There is also a theory out there which I think does come from Australian expats is that you have to be very careful and slow changing between standard and QNH and vice versa and not to use the press for standard button (which you must use your finger not your thumb to press) because the sudden change in value might trigger a TCAS event. Complete ignorance of altimeter systems, but they are adamant that you have to do it that way and change at transition.
Transponders have had their own pressure cell since then and it will only squirt the pressure on 1013. What the pilots altimeter settings are makes not the slightest bit of difference.
The transponder is calibrated every 3 years for IFR machines.
The only aircraft that are even remotely likely to have an encoding altimeter transponder setup will be privately owned antiques with old none digital transponder selectors, Russian or Military.
If its a digital display it will have its own pressure cell for the altitude data.
Those are standalone transponders.
For air data computer machines the transponder can get the data from the data bus in its raw state ie 1013.3 and uses that and the EFIS system converts the same data for QNH for display to the pilots. But again what the pilots set won't change what the transponder transmits.
Most digital transponders be it S or C allow you to see what it is transmitting. Just fly manual and select it and fly level on the backup altimeter and wind the QNH up and down on the main altimeter, the value shown on the transponder won't change.
There is also a theory out there which I think does come from Australian expats is that you have to be very careful and slow changing between standard and QNH and vice versa and not to use the press for standard button (which you must use your finger not your thumb to press) because the sudden change in value might trigger a TCAS event. Complete ignorance of altimeter systems, but they are adamant that you have to do it that way and change at transition.
Join Date: Apr 2008
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I would set standard when I have been cleared to a FL and once above the MSA most of the time, however, Some SID's have not below altitudes for terrain clearance even though the final level is a FL. So for navigation purposes I would want QNH set for longer. If the final level on a SID is a low FL in an area where the transition altitude is low, you may want to set Standard soon after take off especially when there is very low pressure as setting it late could result in an alt bust. I don't believe there is a single rule that will cover all situations, we need to use common sense and airmanship as each departure is different. In the UK when cleared to an altitude in the descent we tend to set QNH straight away, this never seems to be a problem. When in Europe the controllers seem to expect you to set QNH when passing transition level and not before. Out of interest what do you call standard once you have set standard? "Standard" or "one zero one three" or "ten thirteen"
Three aircraft below the transition altitude, A is 7000ft above 1013 pressure level, B is 7000ft above 1000 pressure level - the current QNH.
A's transponder will be sending 7000 to radar, B's will be sending 7390. These figures are then corrected on ATC display to show A as being at 6600 (his actual altitude above the QNH pressure level) and B at 7000.
Aircraft C is opposite direction at 6000 on QNH. Good chance for breakdown of separation with A, as A has the incorrect pressure set on altimeter.
A's transponder will be sending 7000 to radar, B's will be sending 7390. These figures are then corrected on ATC display to show A as being at 6600 (his actual altitude above the QNH pressure level) and B at 7000.
Aircraft C is opposite direction at 6000 on QNH. Good chance for breakdown of separation with A, as A has the incorrect pressure set on altimeter.
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@topdrop, in your scenario, if it was in CAS then ATC would have everybody on QNH.
If A had been cleared above TL and had set 1013 but subsequently had been "stopped off" then ATC would ensure QNH is passed again.
Any incorrect altimeter setting has potential for a loss of separation, full stop.
Personally, since the introduction of RVSM, I very rarely noticed any Mode C with an error of 100 feet let alone the accepted tolerance of 200 in any commercial aircraft,
If A had been cleared above TL and had set 1013 but subsequently had been "stopped off" then ATC would ensure QNH is passed again.
Any incorrect altimeter setting has potential for a loss of separation, full stop.
Personally, since the introduction of RVSM, I very rarely noticed any Mode C with an error of 100 feet let alone the accepted tolerance of 200 in any commercial aircraft,
