Vertical Separation of IFR in airspace E

reh_at Newcomer

Hi!

I work as an Air Traffic Controller in Austria and i have a question regarding vertical separation of IFR flights in airspace class E within the US.
As you probably know, EASA (the European Aviation Safety Agency) started discussion about a harmonisation of european transition altitudes.
Therefor it is interesting to know, how separation is provided by the FAA to IFR flights in airspace E.

For example:
An IFR flight departed somewhere at a controlled airport and cruises in airspace E (e.g. FL100) en route to its destination aerdrome. As it started, the local altimeter setting of the departure aerodrome was set, and in the vicinity of its destination, the altimeter setting of the destination will be set - clear so far.
But which setting does the flight have during enroute flight below FL180?
And how is vertical separation done to other IFR flights with different departure and destination aerodromes (different altimeter values)?

Thanks in advance for your response!

Greetings from Vienna, Austria.

Hannes

(@ admin
If i posted this question at the wrong section, please move my post to the correct location, thanks)

FM_Weasel Trusted Contributor

Local altimeter settings are provided to each pilot on initial radio contact with a new controller, and I believe also after every hour within the same controller's airspace. I don't work airspace large enough to know about the 1 hour rule specifically, but what I'm getting at is that pilots receive a ton of altimeter updates from controllers en route below FL180.

I think this answers your second question about separation. Pilot's operating in the same airspace will be on same or similar altimeter settings.

reh_at Newcomer

Hi!

Thanks for your response.
Ok, that means IFR flights below FL180 will receive actual "Area altimeter settings" - checked.
But if there are 2 sectors, each controlled by a different unit, and one aircraft flies from one sector to the other sector. During the transition to the other sector, no vertical separation of 1000ft can be provided because there might be another aircraft nearby operating at a different altimeter setting.

Is that correct?

Thanks,
best regards

MikeATC Retired FAA, NATCA Member Nashville TN

You can look at the 7110.65 online, the chapters that you seem interested in will be 4 through 7. IFR seperation within 40 miles of the radar antenna is 3 miles or 1000 feet, outside of 40 miles is 5 miles or 1000 feet. The only exception is if you are using a Mosaic radar (more than one radar site) then you have to use 5 miles.

As stated above as the aircraft is handed off to subsequent controllers the new altimeter setting is issued, also whenever the new hourly weather is issued the new altimeter is given.

FM_Weasel Trusted Contributor

1000 ft can still be used. The application of 1000 ft is plenty of room to account for the usually trivial differences in altimeter settings from one sector to another.

reh_at Newcomer

Thanks for your responses!

I attached an image for better understanding what i'm trying to find out how this "problem" is solved within the FAA.

If sector A transfers an aircraft to sector B at the common border and sector B transfers an aircraft to sector A, no vertical separation may be applied between those aircraft because both aircraft are under control of different ATC units and may operate under different QNH setting (even for a short period of time). Is this true?

You are right that 1000ft is noramlly enough vertical separation, but lets create a scenario:

Aircraft A flies at 12000 ft and its altimeter operates at the edge of the maximum tolerance value of 200ft.
Aircraft B flies at 13000 ft and has a similar altimeter problem.
So Aircraft A flies a true altitude of 12200 ft and aircarft B flies a true altitude of 12800 ft. Only 600 ft vertical buffer remains, totally legal.

When those aircraft are now in the "transfer" situation mentioned above, and due to thunderstorm activity between sector a and sector b, a significant altimeter setting change might be possible, the assigned vertical separation of "1000ft" could create a TCAS resolution advisory.

Anyway, our goal should not only be "to prevent collisions", it should primarily be "ensure separation minima".


Thank you very much for your answers!

Greets from Austria!

Attached Images

Example.jpg (51.1 KB, 50 views)

phillyman2633 Epic Member KDAB

Hence the FAA's defined Duty Priority: Give first priority to separating aircraft and issuing safety alerts as required in this order.

Welcome to StuckMic, cool to see people from other countries checking in.

reh_at Newcomer

@ MikeATC

I think i missed your text:
"As stated above as the aircraft is handed off to subsequent controllers the new altimeter setting is issued..."

Do you mean that sector A gives the altimeter setting of sector B to the aircraft before transfering?

Regards,

phillyman2633 Epic Member KDAB

As soon as the a/c checks on with a new controller in a new sector on a new freq, the controller gives the closest altimeter setting along the a/c's route of flight in that new controller's sector, unless the a/c's destination is in that new sector, in which case that altimeter setting is given, if available.

FM_Weasel Trusted Contributor

No it is not true. I've already said that. I'm not sure what led you to believe this.

No, this does not happen.

What maximum value are you referring to? The aircraft's altimeter must be calibrated quite precisely to be usable for IFR flight in the U.S.

It sounds like you're confusing the altitude in which the aircraft is actually flying, vs. what their transponder Mode C function indicates (this is how TCAS systems receive altitude information from intruding aircraft). If the transponder reported altitude is off by 200ft, that indicates a problem with the transponder equipment itself, and has nothing to do with the aircraft altimeter's baro setting or the aircraft's actual altitude. If Mode C was malfunctioning by more than 200ft, the pilot would be instructed to stop using Mode C by ATC.

I guess I'll need to dig into the science of this more deeply to convince you, which would probably be very educational for myself as well. What I'm trying to impart on you is that I've never seen the situation you describe cause a problem. I've transferred/received untold numbers of handoffs from/to adjacent facilities. Good weather, bad weather, thunderstorms etc... Everyone's altitude seems to be close enough for separation.