GA1384

Sleap and Wickenby, separated by 106nm, both operate on the same frequency, which under certain conditions (notably during high pressure systems) leads to interference and confusion. Although this might seem surprising at such a distance, the issue likely stems from atmospheric ducting, a recognised phenomenon where signals travel well beyond their intended range.

Edited excerpt from Skybrary:

“Radio Interference” can occur when two airports share the same aeronautical frequency but are several hundred nautical miles apart. Atmospheric conditions result in aircraft inbound to one airport having interference from the ATC and aircraft using the same frequency at another airport. The controller on the ground may have no knowledge that the aircraft they are controlling is suffering radio interference because they may not be able to hear the transmissions relating to the other airport. Specifically, in unusual propagation conditions, especially high-pressure situations, transmissions from authorised aeronautical transmitters using the same frequency may interfere with transmissions from stations which are well beyond the protected range.

While both airfields have a standard Designated Operational Coverage (DOC) of 10nm and 3000ft – though not published – it appears this protection is being exceeded in certain conditions. DOCs define the volume of airspace within which transmissions are expected to be reliable and free from interference. If transmissions from another airfield’s aircraft are being heard within this protected zone, there are clear safety concerns.

It was not clear whether both airfields were aware of the problem, whether other pilots had experienced it, or if it had previously been reported. CHIRP followed up with the reporter and received the following clarifications:

• Does the issue affect both airfields? Yes, it sounds virtually identical at each end. The issue is apparent only when airborne above 2000ft; only aircraft are heard, not ground stations. The issue is most apparent when descending for the overhead join procedure.
• How often does it happen? Fairly regularly above 2000ft, and more so during high-pressure (ducting) conditions.
• Are other pilots affected? Likely, based on anecdotal responses from pilots at both airfields.
• Are the airfields aware? Unclear.
• Has it been reported before? Not to the reporter’s knowledge.
• Any MORs submitted? No.

CHIRP has now raised the issue with the CAA, who are working with both airfields to explore a resolution – likely involving a frequency change at one or both. It’s a strong example of how reports to CHIRP can drive real safety improvements; as an independent and trusted voice, we’re able to bring concerns to the right people and help shape practical outcomes. But it all depends on your input – meaningful change starts with someone choosing to speak up and report.

Meanwhile, whilst the frequency overlap still exists and we await a resolution, local mitigations could reduce the risk when the atmospheric conditions dictate. For instance, pilots could consider including the airfield name or runway orientation in all joining and circuit calls, eg “Downwind 23 at Sleap”; this would provide better situational awareness than standard calls. While this goes beyond CAP413 standard phraseology, good airmanship and judgement should always prevail where confusion may arise. Clarity is more important than strict adherence to brevity when safety is at stake.

As to why the issue has persisted, this may be a case of normalisation of deviance, where an abnormal situation becomes accepted simply because “it’s always been that way”. The airfields and/or regulator can’t address a risk they don’t know about and which hasn’t been reported. Some GA pilots are understandably hesitant about using the Mandatory Occurrence Report (MOR) MOR system, but reporting helps build a clearer picture of risk leading to safety improvements. Without reports, patterns go unnoticed and hazards persist.

A short guide to occurrence reporting

Under UK regulations, pilots must submit an MOR for any event that could affect flight safety. Just as valuable are voluntary reports about less serious incidents or potential hazards. Near-misses, navigation errors, airspace infringements, technical issues, or even uncertain situations that “could have gone wrong” all provide valuable learning opportunities. Whether mandatory or voluntary, reporting near-misses, miscommunications, technical issues or situational confusion helps improve safety for all in the wider GA community by building awareness of risks.

Submitting an MOR is straightforward using the CAA’s ECCAIRS2 online portal, which works on most devices. Here are two useful links:

• Submit a Report via ECCAIRS2
• CAA Overarching Guidance & Portal

If something didn’t feel right, it’s worth sharing. You might help someone else avoid the same problem.