Pilots Lounge

(This article was published in Ireland's vACC site (www.sateir.org). We publish this fine article after both the author and the stuff of ireland vACC gave us their kind permission to do so. We thank them for their generosity.F.D.)

This guide will give you a bit of background about altimeter setting procedures, especially regarding the selection of a Transition Level.

Altimeter setting procedures were developed to provide pilots with adequate vertical separation between aircraft and adequate terrain clearance during all phases of flight. Let's start with a couple of definitions to build the puzzle:

So then, with the mumbo-jumbo out of the way, what exactly is this all about? This is how it works:

A pilot will be given a QNH by ATC on startup and will fly an altitude based on that altimeter setting until he reaches the published Transition Altitude. At this point, he sets his altimeter sub-scale to 1013,25 hPa and continues to his cruising level. Above the Transition Altitude, he will report his vertical position to ATC as a Flight Level. When he commences his descent, he will be given the local QNH and Transition Level by ATC. He will continue to fly Flight Levels until he passes the Transition Level where he sets his altimeter to the QNH given earlier. His vertical position will be given to ATC as an Altitude from this point until he lands.

Using QNH around an aerodrome ensures that the pilot has an accurate indication of his height above mean sea level thus helping him to maintain adequate terrain clearance. Using the standard setting of 1013,25 hPa (29.92 in Hg for those with a non-metric subscale) above the transition altitude ensures vertical separation between aircraft en-route using flight levels.

Now what about an aircraft at transition altitude and another at the transition level at the same time. Let's say you're working Dublin Approach and you have one aircraft at 5000ft (TA) and another at F60 (TL). The QNH is 1020. We know the aircraft at 5000ft is at 5000 feet above mean sea level. But how high is the aircraft at flight level 60? 6000 feet? No, definitely not. So let's work out how high he really is.

We know that an altimeter works on the principle of changing atmospheric pressure. As the aircraft goes higher, the atmospheric pressure decreases, a little drum in the instrument inflates and turns the needles to indicate a higher level. For each hectopascal drop in pressure, the needles indicate approximately 30 feet higher. (The value is actually a little less and changes once you're higher up, but we'll keep a round number to work with.) When an aircraft is parked overnight and the QNH drops a hectopascal or two, the altimeter will read a higher altitude the next day. Thus the need for an updated QNH from ATC, or if you know your altitude, you'll turn the knob to set your altimeter to that altitude and read the QNH off the subscale.

Back to the aircraft at F60. We know his subscale is set to 1013 hPa and it is reading 6000 feet Pressure Altitude (a fancy name for the flight level with two zero's added to the end). To get his altitude above mean sea level we'd have to set it to the QNH, 1020 hPa in this case. Remember now, his vertical position is exactly the same. Turn the knob to set the subscale to 1020. We've set a difference of 7 hPa which is equal to 210 feet (30ft per hPa remember?) Now a little ditty, find yourself a tune to remember it:

"Wind on hectopascals, wind on altitude. Wind down hectopascals, wind down altitude."

What this means is that we've wound on 7 extra hectopascals which will translate to 210 feet of altitude we've wound on. The altimeter will be reading 6210 feet and this is the actual altitude of the aircraft, even though it is still at F60. What separation do we have? We have 1210 feet, well above the legal 1000 feet. But what about the guy at F70 you may say. Ah, he'll be at 7210 feet AMSL. And there we are, simple as that.

Or so you thought. Another day breaks and the QNH is 1005. You have the same aircraft at the same levels. How high will the aircraft at F60 be now? We mull about it a little, wind down 8 hPa, that's 240 feet. We're winding down hPa so we wind down altitude…..hang on! He's at 5760 feet AMSL! Only 760 feet separate him from the bloke at 5000 feet. That won't do will it? No, and that's what this is all about.

Someone came up with the idea of changing the transition level with the QNH, which in turn means that the transition layer will always be greater than 1000 feet but less than 1500 feet. Using the same kind of calculations we've been doing above, they came up with the following table, which you can use when you work to avoid those uncomfortable reductions in separation. Unlike some countries, Ireland has a standard transition altitude for all its aerodromes, viz. 5000 feet. If you were to work somewhere else, just adjust the table up or down accordingly.

TRANSITION ALTITUDE	QNH	TRANSITION LEVEL
5000 feet	959 to 977	75
	978 to 995	70
	996 - 1013	65
	1014 - 1032	60
	1033 - 1050	55
	1051 and above	50

As an example, let's take a day with a rather low pressure in Dublin. The QNH is 980. Vertical separation between an aircraft at 5000 feet and another at F70, the transition level, will be 1010 feet. This in spite of what looks like a 2000 feet separation. So could you descend someone to 6000 feet? Yes, as soon as he passes F70, he sets the QNH and there he is. Still separated from the traffic at 5000 feet. But now you cannot have another at F70. This is quite important. If you need to use both the transition altitude and level, you just don't have space to pop someone in between.

And that's it as far as this guide goes. Remember to put the applicable transition level into your ATIS and to pass the latest QNH when you clear someone below it.