UK CAA HOFO ACAS Rulemaking 2025 & a 2004 Tornado / AS332L Airprox
UK CAA Rulemaking 2025
The UK Civil Aviation Authority (UK CAA) has recently consulted on a general update to the Air Operating Regulations for offshore helicopters (Subpart K: Helicopter Offshore Operations). This included introduction of Aircraft Collision Avoidance System (ACAS) requirements.
The UK CAA wrote that:
An internal CAA study produced in 2005 cited flight in uncontrolled airspace and mixing of civilian and military air traffic as two of the single largest factors associated with risk bearing Airproxes (during the 2001-2004 study period).
The majority of offshore helicopter operations take place off the north-east coast of the UK where both of these factors prevail.
It is therefore proposed to add a requirement for ACAS II to the Air Operating Regulations in order to reduce the risk of MAC…
Wording similar to the CAT.IDE.A.155 requirement for aeroplanes will be [sic] proposed for addition to SPA.HOFO.160 (new para. (d)) in order to ‘future proof’ the requirement.
A two-year period is proposed to allow time for all aircraft to be upgraded and to allow the costs to be spread.
In there consultation, UK CAA note:
Following a high profile very near miss between a military aircraft and an offshore helicopter in February 2004, all helicopters currently used for UK offshore operations are voluntarily equipped with ACAS, mostly ACAS II. Some aircraft are equipped with ACAS I, about half of which are already scheduled to be upgraded to ACAS II.
We will examine that 2004 incident next.
RAF Tornado F3 / Bristow AS332L Airprox 5 February 2004
The UK Airprox Board (UKAB) published its analysis of this incident:
The RAF Panavia Tornado F3 from RAF Leuchars was tasked as the target aircraft in an exercise and “required to simulate an aircraft flying a 20nm square search pattern between 250-1000ft asl” for two other Tornadoes. The fighter’s…
…AI [Airborne Interception] radar [GEC AI.24 Foxhunter] was serviceable and they were squawking with Mode C selected on.
They were receiving an Air Defence Information Service (ADIS) from the Control and Reporting Centre (CRC) at RAF Neatishead, Norfolk.
The surface wind was westerly at about 35kt, with the 2000ft wind registering at 50kt; the weather was slightly hazy at low level with an in-flight visibility of about 8km and no cloud to affect the flight.
The Bristow AS332L was returning to Aberdeen in VMC along the Helicopter Main Route (HMR) 117 from the Auk A installation (then operated by Shell) in the Central North Sea at 09:07. They were receiving a NATS offshore Flight Information Service (FIS) and were squawking the assigned code with Mode C. The helicopter was en route at 1000 ft rather than the more usual 2000 ft because of a 55 knot headwind, as they were entitled to do in Class G airspace.
Neither aircraft was equipped with any form of ACAS. Both were operating in uncontrolled Class G airspace, and beyond radar cover at that time (multilateration was not introduced until 2010). Neither NATS nor Neatishead were aware of the other traffic, undermining their ability to provide a useful information service.
After about 20 nm on the HMR and approaching 119 nm range from the ADN [Aberdeen North] VOR at 125kt with the autopilot engaged, when the helicopter crew…
…suddenly became aware of a “roaring” noise coupled, “milliseconds later” by the sudden onset of harsh and severe turbulence which started with a roll and yaw to the right. The P1 grabbed the controls and looked left, across the cockpit toward the aircraft’s 10-10:30 position and saw the P2’s windscreen and quarter light filled with what they thought was the underneath of the rear section of a Tornado in a steeply banked turn away from the helicopter at a range of about 50ft. The Tornado then reversed the turn and passed ahead before climbing away toward the NW.
The suddenness meant no avoiding action was taken by the helicopter. The AS332L crew called Aberdeen ATC, reporting “…we’ve just had an Airmiss [sic] with a Tornado he came extremely close…the turbulence was such it kicked the autopilot out”. The controller advised that he had no reports of military traffic in the area whereupon at 0941:30, the AS332 pilot added, “…we lost about 500ft in the event and he’s now climbed to about 3000 feet I guess”. Subsequently:
He assessed the risk as “high” and added that the jet had closed unseen from their 8 o’clock.
Meanwhile the Tornado pilot recounted that:
About 119nm SE of Aberdeen at 5630N 00103E, after turning onto the heading of 305° at 320kt whilst flying in ‘25° wing’ he suddenly spotted a blue/white & red helicopter at extremely close range in the 12:30 position about ¼nm away and co-altitude.
To avoid the AS332 he immediately pulled up, rolled left, then reversed the roll right to re-acquire the helicopter. During this manoeuvre he exceeded the ‘never exceed’ angle of attack limits of the Tornado, but full power was not applied to prevent any downwash affecting the AS332.
We understand that this manoeuvre overstressed the Tornado’s wing box. The Tornado pilot….
…assessed that he probably passed about 100ft above, slightly to port, of the other aircraft at the closest point…
Their RAF station described the event as a “very close call”.
UKAB note that:
In the interests of improving flight safety a meeting was held between HQ STC FS [Headquarters RAF Strike Command Flight Safety] and the AS332 crew’s company that resulted in some useful changes to SOPs. Helicopter crews operating on the HMRs, but not in receipt of a radar service, will now select A0040 on their transponders. Tornado crews will set their interrogators to that code when operating in the vicinity of the HMRs in an effort to improve ‘electronic’ conspicuity and detection.
Liaison between the RAF & NATS on low level military traffic was improved. The RAF “agreed to use the ADN VOR/DME as a common reference point for traffic information”.
It is evident in the UKAB report that there was a complacent military expectation of no helicopters traffic below the 1500 ft HMR floor let alone other traffic in Class G airspace, outside of any exercise area.
Only in 2014 did the UK CAA first reveal, in CAP1145, that:
In a study performed in 2005, it was found that, between 2000 and 2004, there were 21 Airprox occurrences involving helicopters flying to/from offshore platforms.
Of these one third (7) were Cat B (safety not assured) and two thirds (14) Cat C (no risk of collision).
Just over half (11) involved military aircraft, three involved other helicopters, three involved fixed-wing commercial air transport, three light aircraft and one a model aircraft. Three quarters (16) occurred in Class G airspace, four in Class D and one in Class F [a now defunct airspace category in the UK].
…a high level review of risk bearing (Cat A and Cat B) Airprox reports has identified the two most significant underlying factors to be flight in Class G airspace and military traffic. Both of these factors coincide for North Sea helicopter operations.
Of note is that UK CAA committed to no actions and made no recommendations in 2014 in CAP1145 on this matter.
Multilateration was already being investigated by the time the UKAB reviewed the 2004 incident. It was subsequently introduced in 2010 with the assistance of Oil and Gas UK (now Offshore Energies UK) whose members both hosted equipment offshore and funded the implementation and continued operation of the service.
In their report on the 2004 incident the UKAB expressed surprise that commercial air transport helicopters had not been fitted ACAS, which they called “an invaluable piece of equipment which had undoubtedly proved its worth and could potentially have given the AS332 crew a vital early warning of the presence of the jet and any other traffic around”. Their surprise is odd because the extant operating rules, JAR-OPS 3 (specifically ACJ OPS 3.398) wrote against the use of ACAS II for, what ultimately were proven to be, spurious reasons (such as the ability of helicopters to meet the climb or descent rates needed). ACJ OPS 3.398 the elaborated on the limitations of ACAS I being:
…not capable of resolving the bearing, heading or vertical rates of intruders accurately. For this reason, pilots should not attempt to manoeuvre solely on the basis of TA [Traffic Advisory] information (for example in IMC).
An indication given to the flight crew that a certain intruder is a potential threat.
The Tornado/AS332L case further illustrates the weakness of ACAS I when a helicopter is being approached from behind.
Following the 2004 incident Bristow initiated a trial to demonstrate the integration of Rockwell Collins ACAS II using AS332L G-TIGE (coincidentally the first helicopter equipped with IHUMS, and now preserved at The Helicopter Museum after flying 42,000 flying hours). The included operating against an ‘intruder’ aircraft, a BAe 146, to demonstrate the Resolution Advisories (RAs) of ACAS II could be successfully performed. An RA is more useful than a mere TA as it is:
An indication given to the flight crew recommending:
a) a manoeuvre intended to provide separation from all threats; or
b) a manoeuvre restriction intended to maintain existing separation.
Bristow then started retrofitting four different fleets with ACAS II, which has since became available on new production aircraft.
UKAB did acknowledge that:
This equipment argument applied equally to the Tornado but here the Board was briefed that progress had been made and while funding had been allocated much development work was still needed to create a ‘military CWS [Collision Warning System]’.
While ACAS II was fitted to the UKMFTS BAE Systems Hawk T2 fleet, the retrofit of military CWS to the rest of the RAF frontline fleet proved controversially more difficult. In July 2012 two Tornado GR4s from RAF Lossiemouth, operating independently and without a CWS, collided of the coast of Scotland with three fatalities.
Safety Resources
The European Safety Promotion Network Rotorcraft (ESPN-R) has a helicopter safety discussion group on LinkedIn. You may also find these Aerossurance articles of interest:
- Limitations of See and Avoid: Four Die in HEMS Helicopter / PA-28 Mid Air Collision
- Alpine MAC ANSV Report: Ascending AS350B3 and Descending Jodel D.140E Collided Over Glacier
- ATC Assisted Sikorsky H-60 Collision
- Mid-Air Collision of Guimbal Cabri G2 9M-HCA & 9M-HCB: Malaysian AAIB Preliminary Report
- AAIB Highlight Electronic Conspicuity and the Limitations of See and Avoid after Mid Air Collision
- Military Mid Air Collisions
- Military Airprox in Sweden
- North Sea S-92A Helicopter Airprox Feb 2017
- Mid Air Collision Typhoon & Learjet 35
- USMC CH-53E Readiness Crisis and Mid Air Collision Catastrophe
- Avoiding Mid Air Collisions: 5 Seconds to Impact
- Fatal Biplane/Helicopter Mid Air Collision in Spain, 30 December 2017
- A319 / Cougar Airprox at MRS: ATC Busy, Failed Transponder and Helicopter Filtered From Radar
- Merlin Night Airprox: Systemic Issues
- Alaskan Mid Air Collision at Non-Tower Controlled Airfield
- Mind the Gap: Hover Taxi Collision
- Taxiing AW139 Blade Strike on Maintenance Stand
- S-92A Collision with Obstacle while Taxying
- RLC B407 Reverses into Sister Ship at GOM Heliport
- Pilatus PC-12 Collided with Pick-Up Truck During Dusk Take Off
- Investigation into Collision of Truck with Police Helicopter
- South Korean Fire-Fighting Helicopter Tail Rotor Strike on Fuel Bowser
- Second Time Unlucky: Fatal Greek Wirestrike High-Wire Illusion
- Ground Collision Under Pressure: Challenger vs ATV: 1-0
- Fatal ATC Handover: A Business Jet Collides with an Airport Vehicle on Landing
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