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A commercial air transport flight is to be conducted in a single-engined piston aircraft. At the most critical point during the flight, the distance to be travelled to a safe landing place is 10 NM. Given the following information, what is the lowest altitude the aircraft should fly over this critical point?

Terrain elevation: 0 ft
Gross glide gradient: 10.2%
TAS: 110 kt
Expected tailwind component: 10 kt
1 NM = 6080 ft

  • A

    5966 ft

  • B

    6232 ft

  • C

    5648 ft

  • D

    6475 ft

EASA Part CAT.POL.A.320 En-route – single-engined aeroplanes

(a) In the meteorological conditions expected for the flight, and in the event of engine failure, the aeroplane shall be capable of reaching a place at which a safe forced landing can be made, unless the operator is approved by the competent authority in accordance with Annex V (Part-SPA), Subpart L — SINGLE-ENGINED TURBINE AEROPLANE OPERATIONS AT NIGHT OR IN IMC (SET-IMC) and makes use of a risk period.

(b) For the purposes of point (a), it shall be assumed that, at the point of engine failure:

(1) the aeroplane is not flying at an altitude exceeding that at which the rate of climb equals 300 ft per minute, with the engine operating within the maximum continuous power conditions specified; and
(2) the en-route gradient is the gross gradient of descent increased by a gradient of 0.5%.


Net glide gradient = Gross glide gradient + 0.5% = 10.7%

The glide gradient is a still air gradient. We must therefore work out the still air distance with the expected wind conditions.

Ground Speed = 110 kt + 10 kt = 120 kt.

Ground distance = 10 NM
Ground distance = Still Air Distance x (GS / TAS)
Still Air Distance = Ground Distance / (GS / TAS) = 10 / (120 / 110) = 9.17 NM
(from NM to feet) 9.17 NM x 6080 = 55 753.6 ft (still air distance)

Still air distance = (height difference / net gradient) x 100
(Still air distance / 100) x net gradient = height difference
Height difference = (55 753.6 / 100) x 10.7 = 5966 ft

The terrain is 0 ft, so the minimum altitude for the aircraft to glide clear at the critical point is 5966 ft.

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