Traffic Pattern Altitude

[Mark Hargrove]

At the KCRQ airport, the published traffic pattern altitude is 1172 AGL (for small aircraft). While flying a closed left traffic pattern on the downwind leg for Rwy 24, the terrain -- beginning at about mid-field -- rises quite noticeably and continues to rise as you fly east. Do I climb in the pattern to maintain the TPA?

-M.

[Keith Smith]

CRQ is published as having right traffic for runway 24.

That said, I'm still a little confused by the question (assuming ATC instructed you to make left traffic). Can you rephrase?

[Mark Hargrove]

Given the terrain at KCRQ, right traffic makes much more sense -- I hadn't noticed that. I was flying T&G's outside of PE there.

Here's my question, restated. It's not really an ATC issue, but more just a basic flying procedure thing:

TPA's seem to be indicated as AGL altitudes rather than MSL (which I just didn't remember from when I was flying in the real world a long time ago). That strikes me as odd, and caused me to not be sure what I was supposed to do in a traffic pattern where there a distinct change in the terrain underneath the pattern. Palomar is an example -- the terrain rises to the SE of the airport. Ramona Airport is another example -- flying right traffic for RWY 9 you're heading into rising terrain on the downwind. I'm not talking in either case about the Rocky Mountains suddenly appearing -- the ground is just getting closer while you're flying the airplane straight-and-level. TPA at Ramona isn't published (anywhere I could find), so I picked 900' AGL (which was something like 2300' MSL). By the time I turned base, I'm sure that I was effectively 100' "lower" from an AGL perspective.

So -- why are traffic pattern altitudes expressed in AGL terms? Does it mean we're supposed to maintain that AGL altitude even if the terrain changes (climbing or descending as necessary)? I can't remember EVER doing that when I was flying real-world. But if you're supposed to fly the pattern level, then why do charts express the TPA as an AGL altitude? AGL in reference to what? The highest terrain under the traffic pattern? The published altitude of the airport?

I know this doesn't come into play very many places -- airports and their immediate vicinity are usually pretty flat -- but it happened to come up for me at two airports in a row and suddenly created confusion for me.

-M.

[Pieces]

Traffic patterns are generally 1000 above the airport elevation unless otherwise specified. I believe in any case where elevation is factor the patter will be non-standard. (Right traffic or a special elevation). You would not ascend or descend with the terrain because other pilots expect you to be at a certain altitude. A good example is Agua Dulce (L70). TPA is shown as 3660 - 1000 ft above airport elevation. Other pilots will be looking for planes at 3600 indicated. There are pretty good mountains just south so runway 22 is right traffic.

[Keith Smith]

Great question, I have no idea why they're expressed in AGL. I would add the AGL figure to the published airport elevation and fly that throughout the pattern.

Short of having a radar altimeter, it's absolutely impossible, as well as operationally impractical to fly at a constant AGL, as you probably know.

[arb65912]

Another I think related question... why some airports listed in A/FD have TPA shown and others not? I do not have the exact example but I know I was running on many airports where TPA is not listed in A/FD.

Cheers, AJ

[Alex Stjepanovic]

If you are VFR then there shouldn't be one bit of an issue. Imagine flying over a hill, on a cross-country flight. Just because it goes up or down, doesn't require you to do anything, unless in your pre-flight planning you foresaw the fact that you'd get too close for safety/comfort, and as such will either need a higher altitude overall, or a change before the obstruction.

The 'ground level' in the case of TPAs' AGLs, is related to the airport, not actual ground. It's a fairly commonplace in Europe to fly patterns using the QFE setting(the airport pressure level), so you'd always reference your altitudes to the airport itself. Why? Training. It can be used operationally too, but by the time you get your license, you should know what setting you are referencing.

Whether this logic applied in the US at some point, then was forgotten, I have no idea, but it'd make sense to me if it was.

[Orest Skrypuch]

Another I think related question... why some airports listed in A/FD have TPA shown and others not? I do not have the exact example but I know I was running on many airports where TPA is not listed in A/FD.

Cheers, AJ

The reason is historic.

What is listed in the A/FD for the airport is up to the local authority, and years ago traffic pattern altitudes tended to vary, and were generally lower. Typical TPA for props was 800 ft AFE (above field elevation), turbines at 1200 ft AFE. Over time these climbed to 1000ft & 1500ft, and eventually this became the relative standard. Given the standardization you may not always a TPA in the A/FD, the TPA is implied "at standard", and the reference has been removed. Elsewhere it has not, and at some airports you may even find oddball heights still.

* Orest

[arb65912]

Thank you, Orest. I will just keep it simple, prop 1000 AGL and turbines 1500AGL. Cheers, AJ

[Keith Smith]

AJ, looking up the local TPA is an important real world skill, that's why it's mentioned in the V-1 rating. Looking up TPA's is simple, just convert AGL to MSL (you'll need to convert back and forth from AGL to MSL on a fairly regular basis).

If you try to fly a 1000ft pattern at KHWD on 28L in Norcal, for example (where I first soloed), you'll probably bust the Charlie at OAK, and you'll have a more than spectacular view of the traffic arriving on the Oakland final for rwy 29. "Pattern altitude: TPA: 600'AGL EXCEPT RWY 10L-28R 800'AGL."

Be careful not to simplify to the point of being unsafe: http://skyvector.com/?ll=37.68233186395 ... =A.K2.KHWD