Not quite an airprox but...

[Genghis the Engineer]

(Okay, maybe it was, but not filed as such.) Anyhow...

I was teaching circuits at an airfield at 360ft amsl. 1000ft circuit height, QFE 1005, QNH 1018. Conventional left hand circuits.

Whilst we were in the middle of downwind, a light aircraft appeared in my 10 O'clock on what at first appeared a converging course. I left it for my student to spot once I'd satisfied myself that he was slightly higher than us and not descending.

He flew slightly ahead and above us - probably 100ft above, least separation around 500ft. Closer than wise, but not a problem - hadn't been called to me by the tower.

I called it up to the tower, who told me that the aircraft was transiting the overhead at 2000ft then asked for an altitude check from both of us.

Me: 1000ft, QFE 1005
Him: claimed 2000ft, QNH 1017.

In theory, that put him 550 ft above me, if he was actually at the claimed altitude.


Now, here's a few thoughts:-

(1) 550ft vertical separation, within an ATZ, between aircraft not notified to each other, is perhaps a little minimal.

(2) I don't think that we were that far apart. I know my student was at the right height, because I was monitoring his altimeter. So, possibly the other aircraft was a little lower than declared. Thinking about it, if they were cleared through the ATZ *at* 2000ft, that technically would be an IFR clearance - but VFR (we all were), rather than "not below", which would be a more normal clearance wouldn't it?

(3) This was a UK civil airport. Thinking about it, civil airfields in the UK normally do what happened here - circuit traffic on QFE, transiting traffic on QNH. If it was a military airfield, they've have had us both on QFE; if this was in the USA, we'd both be on QNH. Strikes me that having two aircraft - one in the circuit on QFE, one transiting the ATZ on QNH is a recipe for cognitive errors when controllers or pilots are using declared altitudes for situational awareness. In our case I saw the other aircraft - but my much busier student didn't, and we were around 2 O'clock low to the other aeroplane (a low wing), so I doubt their pilot saw us, he certainly didn't indicate that he had. Neither my student or I had mentally processed the calls to/from the transiting traffic to a level that we were concerned - probably because "oh, 2000ft, that's 1000ft above us, not a threat" - intuitively correct, actually incorrect.


We didn't have an accident. However, if my student had drifted 100ft high, or the PA28 another 100ft low, there would have been a reasonable risk of a collision or wake turbulence incident.



It provided some great learning points in the debrief, but I thought I'd share it wider.

I may think about filing, but suspect that'll cause more trouble than it's worth, and create less good than posting it on here.


Serious problems were ultimately prevented by luck and my lookout. The potential for them were probably caused by (in my opinion, please argue with me!)

- Having aircraft on two different altimeter settings reporting in the same small ATZ.

- A VFR clearance being given "at 2000ft" rather than "not below 2000ft". (Cloudbase, incidentally, was above 3,000ft). Bearing in mind that VFR, people don't, and aren't expected to, fly precise altitudes.


G