60,000' Max Alt

[theoNeth]

What is the source of the 60,000' limit on altitude? Box - govt - instruments - "That's The Way We've Always Done It"...

Some years ago in Alaska all a/c were grounded because the air pressure went outside of everybodys analog barometers.  And in Alaska you GOTTA know where the ground is in relationship to you.

Is this a leftover like "so many" 00s feet standard?

Commercial a/c generally stay under that (of course military don't) but with Polar flights becoming common in the near future, I see there would be a need.

[Fenris]

It may be that there is a limit on altitude caused by the way it is encoded, in other words height encoding unit x maximum value of altitude encoding field.

[DaveReid]

It may be that there is a limit on altitude caused by the way it is encoded, in other words height encoding unit x maximum value of altitude encoding field.

Well as far as I can see it's not an encoding limitation.  Both the Extended Squitter altitude and the Surveillance Altitude messages encode the altitude with 11 bits at either 25' or 100' resolution.

[AirNav Support]

Where is this limit?

We just checked the 3.12 Beta and the only place is in the filters (which you can turn on and off) and that's max set at 70,000ft. Which is right on the edge of what some call space.

We don't think there is any military jets breaching that height (ones that are not classified :) )

[CoastGuardJon]

Could it just be that these days, only military aircraft fly at, or exceed 60,000' - used to be Concorde's cruising height, but those days are sadly gone.

Just seen Dave's reply!

[Fenris]

The edge of space was internationally agreed in the 1950s as being 250,000 feet, or approximately 50 miles.

Glad to see there is no encoding limit at the 60,000 feet level, the only aircraft that regularly exceeded these limits are the U-2/TR-1 and SR-71, the former in the low 70,000 feet range and the latter at 80,000 feet plus. the U-2 and TR-1 still fly of course.

[theoNeth]

Exactly, the U-2s do fly daily (out of Beale at least).

I see them taking off (@2400') then rapidly increasing in alt until the alt hits 60,000 and then stays there. As if they put on the brakes. Then they stick at 60,000 for hours.

At day's end they suddenly drop out of 60k like a rock. The alt readings fall very fast.  Not what I would expect from what is said to be a very efficient glider.

It -appears- that Lockheed has provided an alt encoder that tops out at 60000.

Technically feasible of course, and would satisfy the world's Air Control authorities as currently 40,000 seems to be a commercial limit. The  GulfStreams fly higher of course.

But what about the new "Over The Poles" flights. If there were a place to encounter possible out-of-limits air pressures, there would be such an environment.

If there is someone here from the black U-2 community they will likely remain silent, but anyone here from Boeing or AirBus?

[ACW367]

In the US FL600 is the top of class A airspace, above FL600 it reverts to class E.
http://www.faa.gov/air_traffic/publications/atpubs/aim/Chap3/aim0302.html


In the UK the top of the Upper Information Region (UIR) with Class C airspace is FL660 as noted on page ENR-1-4-4 of the AIP
http://www.nats-uk.ead-it.com/aip/current/enr/EG_ENR_1_4_en.pdf

[DaveReid]

It -appears- that Lockheed has provided an alt encoder that tops out at 60,000.

Presumably this is simply intentional misinformation.

[jmhayes]

There's international agreement that Mode-S/ADS-B "can" stop reporting accurately at FL600 ... FWIW, the time I flew Concorde, we went to FL590 ... I presume that was to stay inside controlled airspace.  Even though some de-mil'd aircraft fly higher than FL600 (NASA ER-2 and WB-57 come to mind), they still don't report accurately in order to "keep the fiction alive" ... FAA radar computers likewise filter such altitudes in the feeds they give to, for instance, flightaware.  There's also a system in place to allow discussion of actual altitudes between pilots and controllers on the radio without "confirming or denying" ...

[meagain]

ADS-B (25 ft resolution) encoding is up to 50,100 ft. Above that istandard SSR Mode C encoding (up to 125,000 ft) scheme is used (100 ft resolution). Maybe the AN decoder cannot decode this properly?

[Fenris]

I think it's probably just that the encoders have been set to stop at FL600.

As for Concorde, maximum certificated altitude was 60,000 feet. Mach 2 was reached at FL500, then a cruise climb with the final altitude depending on upper air temperature as the autopilot was programmed to maintain Mach number and maximum altitude subject to the temperature at the nose probe not exceeding 127 C.

[DaveReid]

ADS-B (25 ft resolution) encoding is up to 50,100 ft. Above that istandard SSR Mode C encoding (up to 125,000 ft) scheme is used (100 ft resolution). Maybe the AN decoder cannot decode this properly?

Lots of non-ADS-B aircraft use send altitude in 100' increments.  AFAIK, RadarBox decodes those altitudes correctly.

[theoNeth]

I think the response from <jmhayes> lends credence to Lockheed including a "Altitude Governor" on the U-2 Alt Encoder.

He supplied the ICAO (?) OK for the disinformation and the military/CIA needs nothing more.

Many thanks for all the input.  A lot of valid thoughts.

Theo