I think this topic from my related post deserves separate discussion, as I hear as many opinions about it as number of people I talk to about it:
An ATCO mentioned that in principle you can fly right at the vertical limit of controlled airspace, as it’s
I vaguely remember such a thing from ground school, but in practice many VFR pilots seem to be very wary of flying at the limit of controlled airspace, although it might be very useful due to airspace structure (e.g., in SW France or many parts of Italy). I’ve heard various opinions about why you shouldn’t do it:
— “You might crash into a IFR traffic operating in the controlled airspace at close to the vertical limit if both slightly deviate vertically. Vertical separation at the limit is not guaranteed.”
— “The vertical limit is already controlled airspace.”
— “The vertical limit might be uncontrolled airspace in country X, but it’s not generally true in all countries or airspaces.”
— “I add a x00 feet safety margin (typically x=[3, 5]). For obvious reasons, you can’t fly accurately maintain 3000 ft. A small deviation of 50 feet might already get you a level bust.”
— “Whatever the vertical limit, you can’t usually fly VFR at it as you have to obey the semi-circular rule.”
How do you handle this topic? Do you fly at the limit if controlled airspace? Do you think it’s prudent to do?
Are the vertical limits generally excluded from the airspace? (A reference to an ICAO document or similar would be appreciated.)
Zorg wrote:
1. How do you handle this topic? 2. Do you fly at the limit if controlled airspace? 3. Do you think it’s prudent to do?
1. I fly at the limit of the controlled airspace.
2. Yes
3. Yes. Altimeters have an approved tolerance of 60ft. Added to that is a 200ft margin by which an instrument pilot must be able to maintain his assigned altitude. Those two factors are taken into account when the (vertical) airspace boundaries are designed. There is still plenty of safety margin, even when all tolerances add up in the wrong direction. And if everybody has a working transponder on board, there is even less reason to worry.
— “You might crash into a IFR traffic operating in the controlled airspace at close to the vertical limit if both slightly deviate vertically. Vertical separation at the limit is not guaranteed
In most airspaces, separation between VFR and IFR traffic is not guaranteed anyways. It’s see and avoid. Whoever brings this argument doesn’t have a clue.
I tell FIS what I am, and where, and why, and if they think that might be or become problematic they tell me. Occasionally, I have heard them to request pilots reduce altitude even OCAS – and of course I never heard anything but “glad to oblige”. The few occurrences I witnessed were West of EBBR CTR, and quite low, perhaps “please descend below 1200” or so, this might well have to do with the increase in West to East approaches to EBBR 07 both L and R, right over the city centre.
More generally, I think there could hardly ever be legal trouble about 50 or 100 feet, it would be very hard to offer decisive proof so prosecution would be difficult. But why should one not remain on the safe side?
I do use those limits if that makes my flight easier. It is allowed and If I am correct, the less restrictive rules are the one you use when flying on the border. Mostly, those are nice cruiselevels. Atc provides separation, because in almost all CAS, they have to provide separation for IFR traffic or provide traffic information between ifr and vfr. In reality, they have mostly no problem with you. Sometimes they will ask you if you want to take a another level they propose.
If I take such a level, I mostly fly on my autopilot.
According to @what_next, it must be legal in Germany, but I remember being told off by München INFORMATION for flying VFR at FL98 when it was IMC at FL95 and CAS from FL100 up.
It is allowed and If I am correct, the less restrictive rules are the one you use when flying on the border
Yes; that’s ICAO. The boundary belongs to the less restrictive airspace class.
The UK disregards that rule, however, but they haven’t published a difference 
So here “everybody” flies VFR at 2400ft / 3400ft / etc…
I remember being told off by München INFORMATION for flying VFR at FL98 when it was IMC at FL95 and CAS from FL100 up.
Well, not that ATC would care, but if you are in the clag at FL95, then going up to FL98 won’t make it legal for VFR (airspace is E up there in Germany).
ICAO says that where two classes of airspace exist at the same level/altitude, then the less restrictive one applies. So at the boundary, the less restrictive class applies.
However, not everyone sees it that way! The UK (as Peter says above) considers you inside controlled airspace if you are at the base of the controlled airspace.
The other thing to remember of course is how well can you maintain an altitude of eg 3500ft? If you try to fly 3500, can you be sure that you don’t inadvertently let it go to 3501ft? Of course not. But there is no argument that at 3501ft that you are still outside controlled airspace! You are very clearly inside at that point.
Any tolerances are for equipment errors or safety margins. They aren’t for definition purposes. So while it might be difficult to prosecute you for flying at 3550 Ft if he base is 3500ft, you are very definitely in the wrong there.
Another thought that often crossed my mind in this regard. If I have it correct, your altimeter read the altitude at the bottom of your wheels (it what you’re going to want with). So pretty much all of your aircraft is above the level of the altimeter.
Anyway, why bother with the hassle? Fly 50 ft lower and keep accurate altitude, or 200ft below and relax a little.
The limit belongs to the airspace below, but I can remember flying on the AP in the SR22T at exactly the limit in the UK and ATC telling me that I busted their controlled airspace. I told them I was flying x at QNH x on the AP and the that altitude did belong to the airspace below. The controller objected and I said I would file a complaint :-) Then it was OK. I then filed a complaint and indeed was told that formally I was correct, but it was common practise in the UK to fly a little lower.
