Transition Altitude and Level
Transition Altitude and Level
(This is a glossary page, what the TA and TL is, and how it is handled in the VACC Austria FIR.)
(CAUTION: This information is not yet verified completely, please handle with care until further notice. If you are a controller and know better, please correct it!)
What's that about?
Aircraft measure altitude with a barometer - ambient air pressure. This ambient air pressure varies with weather. If aircraft did not correct this air pressure, they would fly wrong altitudes. Why? Let's assume the aircraft always uses an average pressure of QNH1013:
- When ambient pressure is low, then the aircraft would fly too close to the ground, where the outside pressure matches the average.
- When ambient pressure is high, aircraft would fly too high, where outside pressure matches the average.
So, in order to keep safe altitudes, aircraft would normally have to adapt their pressure settings constantly. If the did not do that, their altitude could vary as much as 1.400ft, depending on the local QNH. bearing in mind, that vertical separation between aircraft must be 1000ft or more (300ft on approach), then this could be lethal. This is, why airports publish their ambient pressure with ATIS.
But what to do in high altitudes? Nobody constantly knows ambient pressure at 36000ft, and enroute aircraft would constantly need to change their settings - errors are inevitable.
So ICAO has adopted a "layer" concept:
- In high altitude, all aircraft fly according to average ambient pressure (QNH 1013). This might be "wrong" according to local QNH, but if all enrout aircraft are "equally wrong", they are safe.
- In low altitude, where aircraft take off and land, they need safe altitude from ground - local QNH is inevitable. When aircraft approach an airport, they have to change their settings to local QNH.
- What about in-between, so they won't conflict? There is a safety layer, depending on the QNH value, to keep safe distance between enroute and lower areas. This safety layer ensures that in real, the lowest Flight level is more than 1000ft apart from the highest altitude an aircraft is aiming for under local QNH.
It would be easy to set this layer thick enough to accomodate every possible QNH - but inefficient: The layer would be up to 3000ft, and that in congested airspace around airports - they need the space for stacking aircraft. Therefore, the safety layer varies according to QNH:
QNH -977: +3000ft QNH 978-1013: +2000ft QNH 1014-1050: +1000ft QNH 1051+: + 0ft
(Caution: This does not mean, that the layer is actually' 3000ft thick. It just means: At QNH below 977, an altitude plus 3000ft is high enough to keep 1000ft distance from the next flight level, so that 10000+3000ft=13000ft are still well apart from FL130. On the other hand: At high air pressure, an aircraft at 10000ft (+0ft) is still more than 1000ft apart from an aircraft flying on FL110.)
The result is two thresholds, where QNH settings change:
-------------------------------------------------------------------------------- Enroute altitudes: in FL (100ft) with standard QNH 1013 -- Transition Level (TL) : From which all altitudes are to standard QNH 1013 --- Safety layer: Thick enough to keep enroute altitudes and local altitudes apart -- Transition Altitude (TA): Up to which local QNH is used --------------------- Local altitude (for everything near the ground) --------------------------------------------------------------------------------
Switching QNH settings
When an IFR aircraft takes off, pilots change their barometric settings after TA and before they reach TL (as they could be too high at TL with local barometric settings) - in other words: They have to cross the TL with QNH set to 1013.
When an IFR aircraft descends towards an airport, it's the other way around: They have to cross TL with QNH1013 and reach TA with local QNH (bearing in mind, that TA is at the altitude of local QNH, so they have to change before to avoid passing it inadvertently).
Where is this Layer?
Starting with spring 2016, transition altitude in Austria has been raised to A10.000ft. Bearing in mind different QNH, the next available flight level (Transition Level) is:
QNH up to QNH TL 943 977 130 978 1013 120 1014 1050 110 1051 1088 100
This is not a problem for most major airports in Austria - TL is far above any terrain. It is a problem in LOWI/Innsbruck.
In LOWI, transition altitude is A10.000ft too, but with the exception ...
- that some arrivals need higher (local QNH) altitudes before switching to levels: ELMEM has 13.000ft for example.
- that ATC is free to (and will) issue higher (local QNH) altitudes when he wishes (they don't do it for fun, but for safety).
Caution: There are two more rules:
- Charts go first: If charts cite barometric altitudes, they have to be applied, regardless the transition altitude. If you approach LOC DME West for LOWI, you have to level at ELMEM with 13000ft (local QNH, whichever TA there is).
- ATC goes "even more" first: If ATC orders a barometric altitude, then pilots have to fly it, regardless of TA. Example: In high traffic at LOWI, the RTT holding must be stacked, and the stack starts at 10.000ft. With 6 aircraft in the holding, the highest is not at FL150, but at 15.000ft. ATC must take care to keep them (laterally) apart from transit aircraft, like those flying from Munich to Bolzano.
So the hierarchy is:
- ATC orders always go first
- With no ATC orders, follow the charts
- With no orders from the charts, follow your calculation of TA and TL according to the TA published in the charts and the QNH published in ATIS.
What to make sure when clearing flight levels or altitudes
If controllers clear altitudes, they have to be consistent (as ATC is always first to follow). First clearance comes from delivery, so DEL must take care to clear the right thing.
- in LOWW, it is easy: Initial climb altitude is TA, so it is always "climb initially 5000ft altitude".
- In LOWI, things are more difficult: Clearing via RTT has a TA of 11000ft. Add the safety layer, then you end up with a FL much higher than that. This is, why many controllers choose to clear departures from LOWI initially to FL160: This is always a flight level.
But what, if the aircraft has a lower requested flight level (RFL), because the destination is near, like EDMM? Then you have to calculate: Take TA, add the safety layer, and you have the minimum FL. If the RFL is below, you have to raise the RFL in the flight plan. The formula is: TA (11000ft) + safety layer (example: QNH 1019: 1000ft) = lowest FL (FL120). But it must be correctly even/odd, so for an eastbound flight, it would be FL130.
What makes it even more difficult, is local deviations from TA in the charts: The ADILO departures have MSA 13.000ft. Add the safety layer (QNH 1019: 1000ft) = lowest FL140, which is already (west) even. The KPT departures have MSA 11.500ft, + safety layer (QNH 1019: 1000ft) = 12.500ft, so lowest is FL130, but west is even -> FL140.
Approach controllers may set their TL, but they have to be consistent: If they order barometric altitudes, they must order it to everyone, and they have to maintain safe distance to all FL-flying aircraft.