Not quite an airprox but...
 

(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

What class of airspace?

An ATZ.

G

ATZs are not included in the Airspace Classification System. An ATZ assumes the conditions associated with the Class of Airspace in which it is situated. So what CLASS of airspace? eg. 'Class G' ? 'Class D' ?

An ATZ within class G.

G

Regardless another reason why only QNH and FLs should be used and QFE dumped from everything other than joe bloggs farmers field :ok:

No.
The CAA recommends the use of QNH at all times with QFE available on request and it's up to individual airfields to notify what pressure setting circuit traffic should use.
For instance, Fairoaks, Blackbushe and Farnborough all use QNH for visual circuit traffic. For the short period I did A/G at Dunsfold, we used QNH there too.
Suffice to say, it was up to ATC/AFIS to advise you of the transit traffic; if they didn't I think you should MOR it.

Do you have a reference for that?

My experience is that the majority of UK civil airfields use QFE for circuit and VFR arrival traffic, and QNH for departing and IFR traffic. That's what was happening here.

There are airfields using QNH for VFR circuit and arriving traffic, but in my experience, they're still in the minority.


G

G

This highlights the danger of different aircraft using different settings in close proximity and for me shows like with the OH join a reluctance to move with the times and sticking with procedures from 50 years ago :ugh: and trying to accomodate these archaic procedures in ways that they don't fit safely. Just because that is how it has always been done doesn't mean that is how it should be done

Pace

As I suggested in my first post - absolutely.

I'm pretty relaxed about the QFE .v. QNH argument in the UK myself (and use OHJs regularly at Popham, White Waltham... !), but certainly having us both on the same here would probably have concentrated a few minds on vertical separation - particularly the controller's who seems to have approved an ATZ transit through the overhead "at 2000ft" with traffic in the circuit at 1000ft QNH = 1360ft QNH. Another nominal 30ft separation lost by allowing him to use a previous QNH setting rather than the airfield's. 550ft vertiical isn't much where two VFR pilots are concerned!

I'm still struggling with a VFR transit "at 2000ft" as opposed to "not below 2,000ft" the more I think about it.

G

…which is uncontrolled airspace for which only permission to fly within is required, at best (Rule 45). Words such as 'clearance' and 'separation', and the specifying of flight in accordance with IFR or VFR therefore do not apply...

Which is a legality; I'm much more interested in the practicalities of not digging a hole in a field.

I don't recall the specific RT - it wasn't directed at me and I was busy, but the PA28 pilot certainly believed that he'd been cleared to transit the overhead at 2000ft on his lower QNH.

I was in an active circuit, and there are rules about fitting in with that - by telling him that he could route through the overhead, arguably the controller was telling the PA28 pilot that he need not fit in with the circuit. So he didn't - flew where he believed he had been "cleared", and the result thankfully wasn't nasty, but could have been with things just a few seconds and 100ft different (and poorer lookout than mine thankfully was): that interests me a lot !

G

A lot of this is compounded by the incongruity of 'control' within 'uncontrolled' airspace. Resolution of this absurd situation is long overdue.

It's probably the case that many pilots and controllers consider that "control" exists within an ATZ where the controller is higher than information or radio. That may exhibit itself more accurately as permissions "clear land, clear take-off", but progresses through "orbit right", "report before turning base" and so-on and so-forth. [Every one of which featured in this particular flight as it happens]

G

It was published in a CAA circular about 8 or 9 years ago.
I must admit my experience of airfields is more limited than yours, however all the civil airfields I've flown from recently use QNH, with QFE available on request.

Its been probably twenty years since I flew at Wycombe Air Park, but the same sort of close encounter used to happen there on a regular basis, so I wonder if that's the airfield where you experienced this not quite an airprox....

A lot of traffic used to happily power without any radio clearance of any sort over that busy hub at "2,000 feet!" thereby being in theoretical correct altitude to avoid the bottom of the Heathrow zone, and the top of WAP's airspace. To say nothing of circling gliders, returning tugs, and helicopters bouncing up and down.

I intended to go cloud flying one afternoon in a Booker glider, so for the first time, before being airtowed, set my altimeter to QNH....and then, losing situational awareness, pulled off the airtow at 2,000 feet...wondering why the ground looked closer than usual! It was, of course.

Our only defense was a keen lookout at all times, so I trust your student has learned that lesson....eyes on stalks and trust nobody!

Am I missing something here? Surely an ATZ extends to 2000 ft above the airfield datum. This is an extract from the CAA reference on establishment of an ATZ:

So by his own admission your visitor was within the ATZ by 360 ft.

And believed himself to be there with permission, yes.

390ft, as he wasn't using the airfield QNH.

G