Why would there be a Maximum Temperature? I know that some planes performance charts go up only so high and you aren’t allowed to ‘extrapolate’ above that. But why on an Approach Chart?
RNAV (RNP) approaches, like the LNAV/VNAV line of minima on RNAV (GPS) approaches, use barometric altimetry for calculating a glidepath. As you learned in your Private Pilot ground school, altimeters are affected by non-standard temperature. With a high temperature, it means the aircraft will be higher than the indicated altitude. So if the FAF is charted at 3000, on a hot day you may actually be at, say 3300 True altitude. This means that your descent to the runway will be steeper. At some temperature, that descent to the runway can exceed the limits for an approach for the highest Category aircraft charted on the procedure. For example Cat D on most procedures. The maximum glidepath angle for Cat D is 3.50 degrees. Therefore, at or above some temperature, the actual flown GPA will exceed 3.50 degrees, and the approach is NA above this temperature. Now, lots of notes: - The limit used to be 3.10 degrees for Cat D. Many procedures still have temperature limits based on 3.10 deg. Just in case you're doing math. - When the limit changed to 3.50 degrees, it would actually take a really high temperature to exceed 3.50 deg on a procedure designed for a nominal 3.00 deg. The maximum charted temperature by policy is 54 deg C, which effectively makes it a non-issue for most procedures, as 54 C is 129 deg F. - The note reads "For uncompensated Baro-VNAV systems...". If your system is compensated, the limits do not apply. - EVERY (*) RNAV (RNP) procedure in the U.S. will have a similar note. And ALMOST EVERY RNAV (GPS) procedure with an LNAV/VNAV line of minimums will have a similar note too, because the temperature reasons and criteria are almost identical. * There's always an exception I suppose, but I don't know of any.
The DA for the procedure is 8274. With a temp of 46C the density altitude is like 14,000 feet. The question is when the hell is it going to be 118°F at 8000 feet? That’s 146°F at sea level.
Density altitude isn’t a limit for an approach procedure, AFAIK. Its Rob ably above the performance limit for a lot of airplanes, but that would be determined from airplane charts, not the approach procedure itself.
Sorry, I reject your explanation. High temps does = high altitude, but it doesn’t effect the safety of the approach. If you are high at the FAF due to temperature, you will also be high at DA. When temps are very cold, this presents a safety issue.
The restriction is based on the limitations of barometric altimeters. The VNAV path is being determined based on baro altitude so that path moves up/down with temperature. "Hi to low, look out below". The approach is certified for the errors induced within the stated temperature range.
You can reject my explanation if you like, but if so you might like to review FAAO 8260.58C, specifically Section 3-3-4 where it discusses "Determining the critical low and high temperatures". Especially this one sentence: "The critical high temperature limit is the temp that yields an effective glidepath of 1.13 times the maximum allowed glidepath angle for the fastest designed category (not to exceed 54 degrees Celsius)." Note - I was incorrect on a detail - I stated that the maximum limit is the glidepath angle, but it's actually 1.13 times the max glidepath angle. So for Cat D, 3.5 x 1.13 = 3.955 degrees. Hey, it's been a while since I looked through these formulae in detail. But all the formulae are in there if you're interested. This section 3-3-4 is one of the most complicated in the book, and I used to teach the calculations manually. That was not much fun. Also, your statement that "If you are high at the FAF due to temperature, you will also be high at DA" is technically correct, but not the way you imply. Temperature-induced altimeter error is greater the higher you are above the reporting station. So, at the FAF which is maybe 1500 feet above the airport, the error may be a couple hundred feet on a really hot day, whereas at the DA which is only 250, 300 feet or so above the airport, the error will be almost negligible. This creates a steeper than normal glidepath for uncompensated Baro-VNAV systems on a hot day. Hence the high temperature limit (which, as I previously stated, is really no longer an issue in most cases, but used to be).
The correction is 4ft per 1°C difference from ISA per 1000ft So isa temp should be -1°c. So for 46°c you have a difference of 1500ft at the minima and 2000 at the FAF
Except that the error starts at ground level, not sea level. 47 degrees above ISA x 4 =188 feet per thousand, and DH is just over 500 feet, for an error of about 90 feet.
For others following along, the error starts at reporting station's elevation because the reported altimeter setting has already accounted for the error due to non-standard temperature from sea level up to field elevation. It is only when you continue above the reporting station's field elevation that indicated altitude starts to diverge from true altitude again.
I stand corrected. Thats the problem with coming from a small country with the max elevation is 2277ft
High temp isn’t going to get you to close to the rocks like low temp can. Getting too close to the rocks is a definite safety issue. But the high temp thing could be too. It can be argued that creating a situation where a pilot will have to fly extreme rates of descent to get down is a safety issue. Yeah, the pilot shouldn’t fly rates of descent that can get him in an unsafe situation. But why create an Approach where he is probably going to have to make that decision to descend at extreme rates of descent to get down.
This approach's vertical descent angle is already almost at the limit of 3.5° at 3.48°. The maximum for Category D is 3.5°. I imagine an OAT of 46°C would put you over the 3.5x1.13 hard limit for Baro-VNAV that Russ cited. This FAF is over 3,100 above airport elevation so the error would be much more significant.
I wouldn't worry too much. Many pilots have only a cursory understanding of altimeter errors. I didn't fully understand it until I was an ATP with nearly 8,000 hours. It's not something that came up very often, except in areas with extremely cold temperatures, before RNAV/VNAV approaches became common.
