Altimeter Setting Procedure

Altimeter Setting Procedures

Basic Concepts

11.1Introduction. In order to ensure separation and to make sure that when flying an instrument procedure the aircraft is actually at or above the procedure design minimum altitude, it is essential that the aircraft altimeter subscale is correctly set to the appropriate reference pressure. History is littered with accidents caused by incorrect altimeter setting and despite the best intentions of the Air Traffic Controllers, the basic responsibility remains with the pilot to ensure that whatever he/she does with an aircraft, it must be safe.

11.2Terrain Avoidance. During instrument departure or arrival procedures the aircraft must be flown according to the published flight profile. Until the aircraft is at or above a ‘safe in all cases’ altitude, the altimeter must be referenced to mean sea level so that the pilot knows exactly how high the aircraft is. All obstacles shown on approach and departure plates are referenced to sea level and likewise, all altitudes required by the procedures are

also referenced to sea level. As sea level pressure (QNH) varies geographically and the terrain avoidance problem is geographic in nature, the reference setting must be a local QNH.

11.3Lowest Useable Flight Level. This is the flight level that corresponds to or is immediately above the established minimum flight altitude.

11.4ATC Separation. Once above the ‘safe in all cases’ altitude, the problem ceases to be terrain avoidance and becomes avoidance of other air traffic. In this case it essential that all aircraft have at least one altimeter referenced to the same subscale setting so that a standard separation can be applied regardless of the sea level pressure. According to ICAO ISA the average barometric pressure is 1013.25 hPa and this (when rounded down to 1013 hPa) is defined as the Standard Pressure Setting (SPS).

Altimeter Setting Objectives

11.5Objectives. The two main objectives of altimeter setting procedures are to:

•Provide adequate terrain clearance during all phases of flight especially departure and arrival.

•Provide adequate vertical separation between aircraft

11.6Altimeter Subscale Settings. There are three altimeter subscale settings that can be applied at any aerodrome. These are:

11.7QNH. This is the observed barometric pressure at an aerodrome adjusted in accordance with the ISA pressure lapse rate to indicate the pressure that would be observed if the observation was carried out at sea level. If QNH is set on the altimeter subscale, the altimeter would read aerodrome elevation at touchdown.

11.8QFE. This is the observed barometric pressure at an aerodrome which, if set on the altimeter subscale, would result in the altimeter reading zero at touchdown.

Altimeter Setting Procedure 11

Procedure Setting Altimeter 11

Figure 11.1

11.9 QNE. A situation can occur where the QNH is below the lowest altimeter subscale setting. For instance, if the altimeter subscale will not read below 940 hPa and the QNH is 935 hPa it would appear that the altimeter is useless. If, however, the altimeter subscale is set to a standard setting (e.g. 1013h Pa) then it would be possible to calculate what the altimeter would read at touchdown where the QNH is 930 hPa and the altimeter subscale is set to 1013 hPa using the ICAO ISA. Assume that the aerodrome elevation is 100 ft AMSL. On touchdown the altimeter will read:

1013 - 930 = 83 (amount of pressure wound on) 83 × 27 ft (ISA interval) = 2241 ft + 100 ft = 2341 ft.

In this case, 2341 ft is the QNE. A pilot would be instructed by the ATCO:

“G-CD set 1013 land with QNE 2340”

There is a popular misconception amongst pilots that QNE is 1013 hPa. When used as a reference as opposed to a QNH, 1013 hPa is defined as the standard pressure setting (SPS). Therefore QNE is what the altimeter will read at touchdown with SPS set.