New Strumble HMU Procedures

(Effective 23rd March 2023)

The Strumble Height Monitoring Unit (HMU) is located approximately 14NM to the east of Strumble (STU) VOR.  The horizontal coverage area for the HMU is a 13.8 NM radius circle centred on position N51 56 00’ W004 40’ 00.  In the vertical dimension, the coverage is from FL 290 to FL 410 inclusive.  The site is unmanned and designed to operate continuously.

Whilst straight and level flight through the area of coverage should normally result in successful monitoring, operators of aircraft making a flight with the specific intention of getting monitored should, where possible, plan to meet the requirements outlined below:

Pre-Flight Procedures.

Operators proposing to divert from an optimum route in order to fly over the Strumble HMU are strongly advised to check the NATCMA website for serviceability information.  Every effort will be made to ensure that the promulgated information is accurate; however, operators should note that the equipment may become unserviceable without warning or prior notice.

Within the area of coverage of the STU HMU, the airspace is defined as Free Route Airspace (FRA). The flight should be planned to route within the area of coverage of the STU HMU via appropriate FRA intermediate points. Example routeings in FRA that transit the area of coverage include:


Dispatchers should ensure that Item 18 of the flight plan includes both aircraft registration (if it is not already included in Item 7) and “RMK/HMU FLT STU” .

If the aircraft does not possess RVSM approval, an initial level outside the band FL290 to FL410 should be filed and an additional remark in Item 18 should be added e.g. “RMK/HMU FLT STU RVSM LEVEL FOR OVERFLIGHT”

In Flight Procedures.

Crews should aim to fly straight and level while within the coverage area of the respective HMU.  Failure to do so may invalidate the result.  Prior to an overflight of the Strumble HMU, crews should transmit “… for HMU Flight” to ATC on initial contact and, if not RVSM approved, a request for a level between FL290 and FL410 (inclusive) should be made.  Operational requirements permitting, the controller will endeavour to allow the aircraft to route through the HMU coverage area in straight and level flight.

The HMU is capable of tracking many aircraft simultaneously.  To that end, provided the above procedures are followed, aircraft height will be monitored and there will, therefore, be no necessity to carry out a second consecutive overflight.