If this is your first visit, be sure to check out the FAQ by clicking the link above. You may have to register before you can post: click the register link above to proceed. To start viewing messages, select the forum that you want to visit from the selection below. |
|
|
Thread Tools | Display Modes |
|
#1
|
|||
|
|||
"G.R. Patterson III" writes:
It's pretty simple, really. It doesn't matter at all if your altimeter is off by hundreds of feet at cruise altitude if everybody else at that altitude has the same error. Unless you're trying to clear mountains using an altimeter setting from a low-elevation field. It would be possibly unsafe for you to set your altimeter accurately when everyone else is setting it to the broadcast local altimeter setting. Absolutely -- no one is suggesting changing the altimeter setting. You just have to be aware of how inaccurate the altimeter is when obstacle clearance might be an issue. For example, if you are planning to clear a ridge by only 1000 ft in the winter, you might want to think again. All the best, David |
#2
|
|||
|
|||
David Megginson wrote
Not at all -- I've just been surprised at how many U.S. pilots don't seem to know about altimeter temperature errors. On mailing lists, I've actually had violent reactions from otherwise experienced and competent pilots when I casually mentioned that pressure altimeters are routinely off by hundreds of feet at cruise altitude. Yep. They forgot it, since it wasn't really relevant. As for the violent reaction, it's not a pilot thing but a people thing. There are people who are often wrong but never uncertain. It's kind of sad when an experienced pilot gets that way, but it's really terrible when an old experienced instructor falls into that mode, since at that point he's largely worthless. But think for a second - why do you suppose MEA's and OROCA's provide 1000 ft of obstacle clearace normally, but 2000 in designated mountainlous areas? If you're IFR, you're not going to be clearing that peak by less than 2000 ft, and that is going to keep you out of the rocks in even the worst case scenario. If you're VFR, then you can see the peak and don't really need the altimeter anyway. Let's not forget that the worst case temperature error at 200 ft and -50C is only 60 ft, while altimeters can be up to 75 ft off in some cases and still be legal for IFR use. What if the errors compounded? I agree that it's unlikely (and would require a very cold day), but using your numbers someone with a 75 ft altimeter error and a 60 ft temperature error could end up at only 65 ft AGL when the altimeter read 200 ft AGL. Which is still not the end of the world. In a light airplane, you can easily go missed from 65 AGL (or land, if you break out). Anything heavy and fast enought that this isn't true is probably going to have a RADAR altimeter and Cat II certification anyway. Michael |
#3
|
|||
|
|||
|
#4
|
|||
|
|||
But think for a second - why do you suppose MEA's and OROCA's provide 1000 ft of obstacle clearace normally, but 2000 in designated mountainlous areas? If you're IFR, you're not going to be clearing that peak by less than 2000 ft, and that is going to keep you out of the rocks in even the worst case scenario. If you're VFR, then you can see the peak and don't really need the altimeter anyway. I don't buy it. On a good weather day in California it is not uncommon for the alimeter setting itself to account for 500 ft altimeter errors in the mountains. If you add up non-standard lapse rate, cold air and old and distant altimeter settings you can eat into the 2000 feet rather quickly. Then deal with turbulent air and downdrafts in the mountains on top of this. I don't like it one bit. Go land on a 2000 foot runway and tell me that's plenty of room between a little airplane with poor climb performance and a big mountain that you can't see. |
#5
|
|||
|
|||
"Michael" wrote in message om... Let's not forget that the worst case temperature error at 200 ft and -50C is only 60 ft, while altimeters can be up to 75 ft off in some cases and still be legal for IFR use. There used to be a DH penalty for an inop middle marker (either at the transmitter or receiver end) but this penalty no longer applies. All this ignores the possibility that the pilost has a RADAR altimeter available. In the US, it is up to the pilot to decide whether in his particular situation, given the available equipment and his skills, he should adjust the minima as appropriate based on the expected temperature error. Michael I've never seen, nor heard of a temperatures of -68F (at low airport-type altitudes) that was not associated with an inversion. I suppose that it might happen in Anarctica in the winter but there aren't any airports there. In Alaska, when its -40F on the surface it is usually at least 0F at 1000' AGL. Mike MU-2 |
#6
|
|||
|
|||
I'm sure that in this case you are referring to the ICAO Cold Temperature Error Table, which is part of the AIM. Check it out online: http://www1.faa.gov/ATPubs/AIM/Chap7/aim0702.html#7-2-3 I don't know about you, but I have yet to meet a US flight instructor who does not require his students to have a copy of the AIM and be conversant with it. Uh. That table only showed up a couple of years ago. I bet most flight instructors have no idea its even there. Other than "hot to cold look out below" there ain't much on temperature errors in the FAA private pilot knowledge requirements. The reason that table finally showed up is because pilots familiar with Canadian and USAF procedures have been pushing the FAA to improve this area of pilot knowledge. We are not REQUIRED to do anything about those tables. If you're flying over mountains in Alaska on a cold night with an alitimeter setting from sea level? You can tell that to the granite. |
#7
|
|||
|
|||
On Tue, 23 Sep 2003 12:21:09 GMT, David Megginson
wrote: In Canada, learning about temperature errors in the altimeter is a standard part of the PPL curriculum, but I've noticed that it's not so familiar to U.S. pilots (at least not private pilots). We have tables in our AIP and other publications showing what errors to expect, and when flying IFR, we are required to add those errors to all instrument approach altitudes (MDA, DH, etc.) in very cold temperatures. There is no such "requirement" , either in Canada nor the USA. Stan |
#8
|
|||
|
|||
|
#9
|
|||
|
|||
Exaclty, the AIP is not the CARS (Canadian Regs)
Just as the AIM is not the FARS Stan On Wed, 24 Sep 2003 00:22:02 GMT, David Megginson wrote: writes: In Canada, learning about temperature errors in the altimeter is a standard part of the PPL curriculum, but I've noticed that it's not so familiar to U.S. pilots (at least not private pilots). We have tables in our AIP and other publications showing what errors to expect, and when flying IFR, we are required to add those errors to all instrument approach altitudes (MDA, DH, etc.) in very cold temperatures. There is no such "requirement" , either in Canada nor the USA. From the (Canadian) AIP, RAC 9.17.1: The calculated minimum safe altitudes must be adjusted when the ambient on the surface is much lower than that predicted by the standard atmosphere. Note the use of the term "must" rather than "should". Granted, the AIP is not the CARs or the Aeronautics Act, but it is the closest we have to a definition of what the TATC (formerly Civil Aviation Tribunal) would use for deciding whether a pilot displayed incompetence. All the best, David |
#10
|
|||
|
|||
|
Thread Tools | |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Thread Starter | Forum | Replies | Last Post |
AOPA Stall/Spin Study -- Stowell's Review (8,000 words) | Rich Stowell | Aerobatics | 28 | January 2nd 09 02:26 PM |
Terminology of New WAAS, VNAV, LPV approach types | Tarver Engineering | Instrument Flight Rules | 2 | August 5th 03 03:50 AM |
Big News -- WAAS GPS is Operational for IFR | Lockheed employee | Instrument Flight Rules | 87 | July 30th 03 02:08 AM |
Garmin Behind the Curve on WAAS GPS VNAV Approaches | Richard Kaplan | Instrument Flight Rules | 24 | July 18th 03 01:43 PM |
High Altitude operations (Turbo charge???) | Andre | Home Built | 68 | July 11th 03 11:59 PM |