FC5394 - Cleared to flight level below transition level

Initial Report

Published transition level by ATIS FL85. We were visual on approach to the airport. We were cleared to maintain FL50 by ATC. Once level, we were then re-cleared to an altitude of 4000’ on QNH 971. 1mb is about 30 ft so setting the local pressure setting resulted in the aircraft’s altitude being lower than the cleared altitude by a couple 100ft. The aircraft then climbed back up to the cleared altitude, exceeded it slightly then levelled off with no further issues. If the pressure setting was even lower this would cause further issues regarding clearance busts and potentially terrain clearance and MSAs. If the crew were in IMC this could become a dangerous situation with terrain. My other concern following this event is potentially having a wider issue with interpretation/understanding/SOPs of transition altitudes and levels. Having flown in other countries under other aviation authorities, this was always followed: When climbing, cleared to a flight level, set standard at the transition altitude. When descending, cleared to an altitude, set QNH at the transition level. The issue in this being non-standard is having aircraft potentially flying around on different altimeter settings below/above TA/TL.  I frequently observe crew when cleared to an altitude immediately setting QNH when above the TL and vice versa, setting standard when cleared to a level but below the transition altitude.

CHIRP Comment

QNH as low as reported here is sometimes referred to as a ‘double low’. CHIRP believes it would be unusual for a controller to issue a flight level below the transition level owing to losing another level in the stack, but since extreme lows of pressure were relatively rare in UK airspace, such a procedure would not usually cause a problem, as was described in the report. There is an acknowledged discrepancy between the ICAO standard procedure and the one used in UK airspace relating to pressure settings and transition levels, which can be confusing for foreign crews. Not least because the controller cannot be certain if QNH has been set by flight crews. Future work is being considered to change the SOP to align the UK with ICAO for UK airspace. However, in a situation of uncertainty, as described in the report, the crew still had the opportunity to question ATC whether it was a FL or altitude instruction if they were unsure – it is always better to check than to assume in such situations. Finally, the last sentence of the report, although describing an apparent anomaly, is in fact the correct procedure in UK airspace.

This report usefully highlights that not all procedures are not the same in UK as for ICAO, which can clearly cause confusion for non-UK trained/based crews.