HMS Rev. 9c
Questions & Answers
This Q&A section provides detailed insights into the latest updates to the Helideck Monitoring System (HMS) standard, Revision 9C, as developed by the UK Civil Aviation Authority. Whether you're an offshore operator, vessel crew member, or aviation safety professional, you'll find answers to common questions about the rationale behind the changes, operational implications, and guidance for a smooth transition from Rev. 9B. Explore the key differences, safety enhancements, and technical clarifications that support safer and more consistent helicopter operations offshore.
The Rev. 9C update is essentially a ‘sweep-up’ exercise comprising a collection of relatively minor changes aimed at addressing issues that emerged during the approval and roll-out of Rev 9B. The Rev.9C ‘compliance’ date presently stated in CAP 437 9th Edition (see para. 6.11) of 01 October 2024 has been extended to 01 October 2025 following consultation with the HMS providers.
A single report was received of the rotor downwash from a landing helicopter causing an MSI/WSI exceedance. Such an exceedance is of no consequence to the stability of the helicopter on the helideck. However, the change in landing status during the landing manoeuvre could present a significant distraction to the pilot at a critical point in the flight and compromise safety. The 30 second delay will normally ensure that any MSI/WSI exceedances generated by the helicopter will not be triggered until after the helicopter has landed.
The possibility of down wash from the helicopter affecting anemometer measurements is noted in section 4.1 of the HMS standard. It is for the vessel to ensure that the anemometer(s) is located where this will not occur or else provide additional anemometers at unaffected locations for the HMS to switch to during helicopter operations.
Although relatively unusual in practice, helicopters are allowed to operate to vessels under way and the HMS needs to be able to support these operations.
Under the Rev.9B standard, switching between red and blue status for significant heave rate takes place when the rounded value of the SHR is greater than or equal to the limit (blue to red) and is less than the limit (red to blue). This has the effect of reducing the SHR limit from 1.3 ms-1 to 1.25 ms-1, resulting in a modest reduction in the availability of the helideck. In Rev. 9C this is corrected by switching when the ‘ceilinged’ value of the SHR is greater than the limit (blue to red) and less than or equal to the limit (red to blue).
In aviation, the legal definition of night is from 30 minutes after sunset until 30 minutes before sunrise measured at surface level. This varies by location (latitude) and season and can easily be determined by the HMS using the GPS position of the vessel and an almanac. Lower helideck motion limits apply at night and it is not difficult to imagine situations where it would be convenient for the vessel to delay switching to night limits, potentially compromising safety. Commercial pressure features as a contributory factor in many aviation accidents and removing the opportunity for taking risks should enhance safety.
The HMS repeater light test facility allows the functioning of the lights to be regularly tested to maximise serviceability of the system and, hence, availability of the helideck.
For the most part, the upgrade from Rev.9B to Rev.9C entails a minor HMS software update. However, on some vessels it may be necessary or advantageous to relocate the motion reference unit (MRU). Vessels should contact their HMS provider for advice. In addition, vessel operators should ensure that the meteorological sensors (anemometers in particular) are compliant with CAP 437 9th Edition in terms of location, accuracy and sample rate.
A small number of vessels may need to relocate their motion reference units (MRUs), otherwise no significant challenges are expected.
No further revisions of the HMS standard are presently envisioned. Helicopter manufacturers may wish to develop bespoke MSI/WSI limits for individual helicopter types but this is already catered for in the present standard. Laser anemometers may provide a solution to avoiding the effects of rotor down wash on smaller vessels where options for location are limited; note that laser anemometers may currently only be used for the HMS functionality and not for general Met reporting.