• There seems to be an uptick in Political comments in recent months. Those of us who are long time members of the site know that Political and Religious content has been banned for years. Nothing has changed. Please leave all political and religious comments out of the forums.

    If you recently joined the forums you were not presented with this restriction in the terms of service. This was due to a conversion error when we went from vBulletin to Xenforo. We have updated our terms of service to reflect these corrections.

    Please note any post refering to a politician will be considered political even if it is intended to be humor. Our experience is these topics have a way of dividing the forums and causing deep resentment among members. It is a poison to the community. We appreciate compliance with the rules.

    The Staff of SOH

  • Server side Maintenance is done. We still have an update to the forum software to run but that one will have to wait for a better time.

Flight Level (FL) question

Altimeters

With the advent of RVSM(Reduced Vertical Seperation Minimums) the altimeters are even more accurate, with errors of say +/-50 ft at 40,000 depending on the airframe. The old standard was 200 ft. at 20,000. Those figures are rough numbers but you get the idea. Pitot-static tests got to be a lot more finicky with the advent of RVSM. I had to get FAA signing authority in order to sign out Jet Blue checks.

Regards, Rob:ernae:
 
After 18,000 the altimeter (barometer) becomes pretty much useless....not enough pressure to keep it going. That's why the TA in the U.S. is at that level.
Click to expand...

Err.. it's still working perfectly well, you've just set a different reference pressure. I've read elsewhere that 18,000' was chosen as it's Mt Rainer plus a bit so there's no danger of flying into the ground with 29.92 set.
 
Cold Mountains

Then I stand corrected and defer to experience -- since I'm certainly not going to go jump into my Archer II and try to climb to 45,000 to check (it would get too cold).

In my defense I have been taught this from differing sources....but I now recall that my HALO jumping buddies treat their barometers with an almost religious reverence, and I know they often exit above 18,000 feet. And the Rainer thing sounds just perfectly plausible.

BUT, Mt. Whitney is the tallest Mountain in the lower 48, at a little over 14,000 feet. Rainer is shorter (by just a few feet I think). McKinley/Denali is over 20,000 .... so if you thought you were in Iowa at 18,000 but were in fact over the Alaskan interior...you would still be at risk.

But I think if you were that lost, your main issue wouldn't be terrain. Which brings us back to an earlier theme (why 18,000 feet?).

Did we ever answer Daveroo's question?
 
One night many years ago we got a direct from Barrow to Anchorgae in C-130 (winter-night) and used the radar to home in over the summit of Denali at FL 250..... radar alt read 1200 ft.... Cold and low, look out below....

Cheers: T
 
BUT, Mt. Whitney is the tallest Mountain in the lower 48, at a little over 14,000 feet. Rainer is shorter (by just a few feet I think). McKinley/Denali is over 20,000 .... so if you thought you were in Iowa at 18,000 but were in fact over the Alaskan interior...you would still be at risk.

But I think if you were that lost, your main issue wouldn't be terrain. Which brings us back to an earlier theme (why 18,000 feet?).
Click to expand...
I think 18000 feet is chosen as it'll give you adequate clearance in the worst case scenario, i.e. flying FL180 with very low pressure, on a very cold day with an orographic low caused by wind blowing over the highest point of land, all of which could rapidly eat into the 4000' between you and the top of Mt Whitney. As in the example above.
Bear in mind if you're flying on 29.92inHg/1013mb you've no easy way of comparing your height to that of the surrounding terrain as you're no longer using the same reference (mean sea level). Once you've chosen that altitude then applying it to the whole country simplifies procedures for everyone, especially if you're flying cross country. Compare this to the UK where some areas of the country have a different TA to others, no one's ever been so lost that it's been an issue, but why overcomplicate things? There is a CAA paper on the web which proposes raising it to 10000' across the UK/Europe for that reason.
 
I spent about 15 minutes today at the office arguing the 18000 foot transition with two other pilots. What amazed both of them was that there might be a differing standard in Europe (all of us have lived in Europe for long periods of time, although we never flew there).

One thought was that there might be different altitude due to the shorter distances normally flown on the European continent.

I've no idea, but it was a nice break from work.
 
18,000 Ft

The only fact? I know of that is mayyyyybe relevant is that supposedly half the atmosphere is below you at 18,000 ft. Perhaps they thought the margin of error would increase above that.... or am I just blowing smoke.:kilroy:

Regards, Rob:ernae:
 
E.g. in Europe the TA / TL depends on the obstacles around the airport.
So even in the same country these are different.
E.g. in Austria, Innsbruck (LOWI) has a TA of 11000ft and at 1013hpa (29.92) pressure the TL is FL130.
In Linz (LOWL) the TA is 4000ft and the TL is FL60.

As a European one needs to get used to the fact that (e.g. in Canada) you sometimes switch to local pressure still 150NM away from the airport and at 32000ft (during the descent).

Regards

Bernt
 
If one is going to have a continuous descent from cruise to below the transition level it is permissible to set the governing local altimeter, however this is usually not done as an intermediate leveloff is always possible, often likely.

T.
 
You must log in or register to reply here.
Back
Top