SPOTLIGHT - Altitudes, Flight Levels, and Heights

[Darren Hill]

Introduction

When should you report at a flight level and when should you report at an altitude? At a first glance, these two terms appear to refer to the same thing: altitude – something that often confuses newer pilots (even in the real world!).

Why not impress your fellow London controller during your next flight and avoid the embarrassment of “passing flight level 3000ft”!

 

The concept of “altitude”

Altitude refers to the aircraft’s vertical displacement above a set reference datum: mean sea level. For altitude to be read off the altimeter, QNH must be set in the subscale.

QNH is the atmospheric pressure at an aerodrome adjusted to mean sea level using ISA. On the runway, an altimeter set to the QNH will read the airfield elevation.

Mean sea level is not the only reference datum we can use to determine vertical displacement. It may be optimal or even required to use the ground itself as a reference datum for the flight operation being conducted safely.

Height refers to the aircraft’s vertical displacement above an airport or ground level reference datum. For height to be read off the altimeter, QFE must be set in the subscale.

QFE is the atmospheric pressure at an aerodrome. On the runway, an altimeter set to the QFE will read zero.

The limitation of using QNH and QFE lies in the fact that they are simply measurements of current atmospheric pressure. Since the atmosphere varies both laterally and, more significantly, vertically, it may not always be suitable or possible to utilize a current reading of atmospheric pressure. In these instances, a standard reference datum is used as defined by ISA (International Standard Atmosphere).

Pressure altitude refers to the aircraft’s vertical displacement above the 1013.25hPa/29.92” Hg reference datum, known as standard pressure. For pressure altitude to be read off the altimeter, QNE (1013/29.92) must be set in the subscale.

 

 “Flight Level” or “Altitude”?

To aid in conveying which reference datum we are flying on, the convention is to prefix the vertical displacement with the term corresponding to the reference datum:

Altitude 5000ft = 5000ft above mean sea level on QNH.

Height 5000ft = 5000ft above aerodrome level on QFE.

Flight Level 50 = 5000ft above the standard reference datum on 1013hPa/29.92”.

Note: Flight levels less than FL100 are spoken without the leading zero, eg: FL90 and not FL090.

Great, we have defined our terminology and yet the question remains: when do we fly on QNH, when do we fly on QFE, and when do we fly on standard pressure?

 

Transition Altitudes and Levels

Ensuring pilots are on the correct pressure setting for their altitude in imperative to maintaining ATC separation. For every 1hPa difference in pressure setting, the altimeter will read an average 27ft (often rounded to 30ft) deviation.

As an example, let’s imagine a Ryanair 737 at FL270 and an EasyJet A321 at FL260 cross over in opposite directions along the same airway – meaning there only exists the minimum 1000ft of vertical separation. The Ryanair pilots have inadvertently left their altimeters on the Stansted QNH of 1031 hPa, while the EasyJet crew have correctly set standard pressure. Since the Ryanair’s altimeter setting is (1031-1013) 18hPa too high, the altimeter overreads by around 490ft. Thus, there only exists 510ft of separation between the 2 airliners!

To prevent situations such as these, the concepts of transition altitude and transition level have been created to ensure all aircraft are properly separated based off a predefined procedure.

Transition altitude is the highest altitude to be flown on QNH, which is fixed for an aerodrome. The transition altitude can be found on the charts and AIP entry for the airport. Where a transition altitude is not defined, such as for a small airfield or outside controlled airspace, the transition altitude is fixed at 3000ft.

Transition level is the first available flight level which is separated from the transition altitude by at least 1000ft. The transition level is determined by ATC based off the atmospheric pressure at the aerodrome.

In general, the following descriptions detail how pilots are expected to operate with respect to the transition altitude and the transition level. For a detailed description that covered specific instances, refer to the UK AIP ENR 1.7 Section 5.

 

Departures/Climbs (think TA)

On departure, and for any operation below the transition altitude, the pilot should set QNH and refer to their vertical displacement by altitude.
“Scottish Control, SHT2R is passing 2400ft for 5000ft, SANBA1Y departure”.

Upon climbing through the transition altitude or when cleared to a flight level by ATC, the pilot should set 1013hPa/29.92” and refer to their vertical displacement by flight level.
“Climb FL190, SHT2R”

Should the flight leave controlled airspace above altitude 3000ft, the pilot should set 1013hPa/29.92” and refer to their vertical displacement by flight level.

 

Arrivals/Descents (think TL)

On descent or when operating above the transition level, the pilot should have 1013hPa/29.92” set and refer to their vertical displacement by flight level.
“Descend FL80 level by DAYNE, SHT2R”

When passing through the transition level or when instructed to descend to an altitude by ATC, the pilot should set QNH and refer to their vertical displacement by altitude.
“Descend to altitude 3500ft, SHT2R”

Should the flight reenter controller airspace above altitude 3000ft and below the transition altitude for that airspace, the pilot should set QNH and refer to their vertical displacement by altitude.

 

Height

Reference to height by setting QFE is only practically used when flying within a circuit, to aid the pilot in their height above the aerodrome.

ATC will explicitly state if a circuit is to be conducted based off QFE.
“… not above height 1500ft …”

 

Disclaimer

This post was authored as a generalized guide for assisting members to wrap their heads around the concepts associated with “altitude”. Should any information contradict a local procedure or a clause in the AIP, then the official source is to be used.

For a comprehensive detailing of the UK’s altimetry procedures, refer to the AIP ENR 1.7 (https://nats-uk.ead-it.com/cms-nats/opencms/en/Publications/AIP/).

Edited August 27, 2022 by Darren Hill