Strumble HMU Removal from Service


Strumble HMU will no longer be available for use from 1st November 2023.


Strumble HMU was commissioned to monitor the height-keeping performance of aircraft in support of RVSM implementation in North Atlantic airspace during the 1990’s. The system has reached the end of life phase and ICAO have decided to take advantage of innovations in Height Monitoring technology by introducing ADS-B based height monitoring within the North Atlantic. The ADS-B Height Monitoring System utilises existing data within the North Atlantic and allows for far greater value to customers than embark on a costly refurbishment programme of Strumble HMU.


We have worked closely with the FAA and NATS to introduce the AHMS within Shanwick Airspace in the North Atlantic.  After a period of testing and validation, the new system went live in July 2023.  It is currently available to all ADS-B equipped aircraft on Mondays within Shanwick airspace and availability will gradually increase to include more North Atlantic airspace and greater time coverage in line with resource. Further details of monitoring procedures are available on the NATCMA website.


We recognise the removal of Strumble HMU will have an impact on some airspace users – particularly those without ADS-B or those that routinely operate only in UK domestic airspace and the NAT CMA is available to support those users where possible.


Alternative Height Monitoring Systems are as follows;



USA, Thailand, Australia and China

Multiple ICAO regions currently developing implementation plans for AHMS including South America, Africa, Europe and Russia.



Europe – Linz, Geneva, Nattenheim

USA – Atlantic City, Ottawa (Results provided by NAARMO, FAA)

Japan – Setouchi, Niigata, Sendai


Availability of these systems can be checked through the relevant Regional Monitoring Agencies websites. Links to each RMA can be found on our NAT CMA website.


The NAT CMA would like to take this opportunity to thank all of our customers who have used Strumble HMU over the last 25+ years.  We remain available to answer any questions and provide advice where we can. Please feel free to contact us for more information.