General: Altimeter Setting & Vertical Reference

Question 1

What units of pressure are used?

Answer 1

• Hectopascals are the notified units for the measurements of pressure for flying within UK airspace.
• Pilots are normally expected to carry out their own conversion from hectopascals to inches of mercury.
• ATCOs will only provide pressure settings in inches of mercury when specifically requested by an aircraft (Appendix A)

Question 2

What pressure setting is used for flight levels?

Answer 2

• 1013.25 hPa
• Consecutive IFR cruising levels above the transition altitude are separated by pressure intervals corresponding to 1,000ft in the ISA up to & including FL410 (then 2,000ft)

Question 3

What are QNH & QFE?

Answer 3

• QNH indicates altitude
• QFE indicates height above the reference datum

Both of these are rounded down to the nearest whole hectopascal.

Question 4

What is the Regional Pressure Setting?

Answer 4

• The Regional Pressure Setting is a forecast of the lowest QNH value within an Altimeter Setting Region (ASR)
• Value of which is made available hourly for the period H+1 to H+2 (in whole hPa)
• Airspace below TMAs & CTAs does not form part of the ASR Regional Pressure Setting system. QNH of an adjacent aerodrome should be used for aircraft at / below the Transition Altitude.

Question 5

What is the Transition Altitude?

Answer 5

• The altitude at / below which the vertical position of an aircraft is controlled by reference to altitude (or height when QFE used)
• Unless otherwise notified the TA for civil aerodromes is 3,000ft

Question 6

What is Transition Level?

Answer 6

The lowest Flight Level available for use above the TA & shall be located at least a nominal 1,000ft above the TA to permit the TA & the TL to be used concurrently in cruising flight with vertical separation ensured.

(see appendix A)

Question 7

What is Transition Layer?

Answer 7

• The airspace between the transition altitude & transition level.
• The first available flight level above the TA is separated from the TA by a minimum pressure interval corresponding to a nominal 1,000ft

Question 8

What must ATCOs do when transmitting pressure on the RT?

Answer 8

• Always issue pilots with correct pressure setting & check read-back
• When hPa lower than 1000 ATCOs shall clearly specify the unit of measurement * pay attention to the read-back

Question 9

What is Aerodrome Elevation?

Answer 9

The elevation of the highest point on the landing area.

Question 10

When is QFE threshold for an instrument runway used?

Answer 10

QFE threshold is used when the threshold elevation is 7ft or more below aerodrome elevation.

Question 11

What pressure info shall be given to arriving aircraft?

Answer 11

• Aircraft at / below TL given QNH.
• When aircraft cleared to descend from a Flight Level to an altitude, the appropriate QNH shall be included in the same transmission.
• Transition Level must be passed to the pilot in due time (RT / ATIS)

Question 12

Use of Level by ATCOs in airspace adjoining Class G vertically

Answer 12

• ATCOs should not allovate a level to an aircraft which provides less than 500ft vertical separation above the lower limit of a CTA or below the upper limit of a CTR/CTA to provide some vertical sep from aircraft operating at the lower/upper limit in Class G