Transition altitude and transition level

Free lesson · Meteorology · DGCA CPL and ATPL

Why every aircraft sets 1013 at the same point, and the sums that go with it.

Why does everyone switch to 1013 at the same height?

Transition altitude

Four cards. Tap Next.

  1. Same height, different readings Two aircraft are both at 10,000 ft. The real QNH here is 1013, but one pilot still has 1000 hPa set from one airfield and the other 1020 hPa from another. Their datums differ by 20 hPa, about 600 ft, so their altimeters disagree by 600 ft at the same height. On different settings, ATC cannot keep them apart safely.
  2. Everyone on 1013 So above a set height everyone sets the same standard pressure, 1013.25 hPa. Now the altimeters all read pressure altitude, expressed as flight levels (FL50 = 5,000 ft, FL300 = 30,000 ft), and separation works.
  3. TA, TL and the layer Transition altitude (TA): the highest altitude at or below which you fly on QNH. Transition level (TL): the lowest flight level at or above which you fly on 1013.25. Transition layer: the airspace between them.
  4. Only on 1013 A flight level exists only with 1013.25 set. Every other reading is a height or an altitude, never a flight level. Levels are spaced by the pressure for 500 ft, in hundreds of feet: 3,500 ft pressure altitude is FL35.

Memory hook: Climbing: at the TA, set 1013. Descending: at the TL, set QNH.

Takeaway: Below the transition altitude fly on QNH (altitudes, terrain clearance). Above it everyone sets 1013.25 and flies flight levels, so all aircraft share one datum. The transition layer lies between the TA and the TL.

Can you do the altimeter sums?

Worked sums

Worked sums, one per card. Use 1 hPa = 30 ft unless told otherwise.

  1. QNH from QFE QFE is 1000 hPa at a field 90 ft above sea level. What is the QNH?
  2. The 600 ft field Field elevation 600 ft, QFE 990 hPa. What is the QNH, and what does the altimeter read on the ground with 1013 set?
  3. A 27 ft example QFE 1000 hPa, field elevation 270 ft, and use 1 hPa = 27 ft. QNH?
  4. Metres QNH 1005 hPa at an aerodrome 160 m above MSL. Use 1 hPa = 8 m. QFE?
  5. Without ATIS You are on the ground at Delhi (778 ft) with no ATIS. How do you find the QNH?

Takeaway: Height ÷ 30 = hPa. QNH = QFE + elevation ÷ 30. QFE = QNH - elevation ÷ 30. On 1013 the field reads (1013 - QFE) × 30.