Altimeter Errors

10 Answers

1. Best Answer
   
   

   
   

   John D. Collins on Jun 19, 2013

   Hotter conditions will cause the altimeter to read lower than the true altitude. If you are following a path which is fixed in space, such as an ILS GS, then the indicated altitude will be lower than you actually are. You aren’t closer to the ground at a specific point along the GS, but your altimeter will indicate you are. This can be problematic if you ignore the altimeter and follow the GS before reaching the GS intercept point as you may bust a minimum altitude. The encoding altimeter that is attached to your transponder mayl tattle tale and tell the ATC computer that you are too low over a step down fix. The solution is to comply with the minimum altitudes shown on the approach chart and don’t follow the GS until you reach the GS intercept altitude. After that point, there are no more step down fixes that affect the full ILS. although they will apply for flying the Localizer only procedure.

   The cold temperature table is primarily to help the pilot keep from hitting the rocks (too close to the ground). Hotter temperatures will keep you further from the ground than indicated.

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2. 
   

   
   

   Dan Chitty on Jun 19, 2013

   Thank you John for the great feedback.

   To expand further, should I adjust the indicated altitude:

   Example: A fix on a approach chart has a fix to cross at 2000 feet. Due to my calculation I show a error of 100 feet due to temperature. As a result I cross the fix at 2100 feet indicated to reflect a 2000 foot altitude on the ATC screen. Is this correct?

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3. 
   

   
   

   John D. Collins on Jun 19, 2013

   Dan, you should set the correct altimeter setting. There is no correction for temperature. If the fix says 2000, then your altimeter should read 2000. Your encoder will send the pressure altitude back to the ATC site and their computer will adjust it to the correct altimeter setting. The ATC computer will not adjust for temperature either, so ATC will also see 2000.

   Outside of the glideslope intercept point, one must fly no lower than the charted step down altitudes. Once the glideslope is intercepted at the proper altitude as shown on the approach chart with the lightning bolt symbol, there are no step downs that apply to the vertically guided portion following the GS. If you chose to follow the glideslope to descend when you have not yet reached the glideslope intercept altitude, it is possible on a warm day to be on the glideslope but below the charted step down altitude. This is not permitted, the step down minimums altitudes rule and you must abandon the GS and level off at the minimum altitude. Remember all aircraft are using their altimeters for vertical separation and to maintain the separation from this traffic, you also must follow your altimeter. So if a GS is close to a class B floor and clears it by 500 feet in some location, on a hot day, VFR aircraft below the glideslope are allowed to fly up to the altitude that forms the base of the class B and they will be higher, and in some cases be very close to the GS. If you fly the required minimum altitude, you will have vertical separation, if you fly the GS, you might not. Regardless, fly the required altitude, not the GS if the GS takes you below the minimum altitude for that segment. Doing so can earn you a pilot deviation.

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   Dan Chitty on Jun 19, 2013

   Excellent explanation John. I appreciate very much.

   In summary and assuming crossing fixes prior to the glideslope intercept point (lightning bolt symbol), what was confusing me was that the ATC computer screen would also account for a temperature variation ( and yes I would ensure the correct altimer setting) . As a result I would need to adjust my indicated altitude accordingly to reflect the correct published altitude on the ATC computer screen. Now I see thanks to your explanation how ATC sees it with temperature factors.

   Regarding crossing fixes prior to the glideslope intercept, it seems to me that I would still need to adjust my indicated altitude altitude and use caution not to follow the glideslope to cross these fixes until the FAF lightning bolt.

   Take a look at the FAA Instrument Flying Handbook page 5-6. It gives an example of a non-precision approach and correcting indicated altitude.

   Let me know your thoughts. I want to make sure I understand this as I am always seeking insight.

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   John D. Collins on Jun 20, 2013

   ATC sees what you see, so if you are assigned an altitude, don’t correct it. There is no need to correct altitudes on a hot day. On an extremely cold day, you still can’t correct ATC assigned altitudes, but if you determine that your terrain clearance will be compromised with a particular clearance, don’t accept it and request a higher altitude.

   On an approach in extremely cold conditions, you may correct the approach altitudes upward, but are required to inform ATC that you are doing so. On a hot day, there is no need to correct the approach altitudes and you must not do so. You should only be following a glideslope from the glideslope intercept altitude inbound. Further out, you are required to comply with the step down altitudes. If you follow the glideslope where it is not applicable, that doesn’t excuse you from complying with the step down minimums. It may be easier to follow the glideslope, but in this case, it can get you into trouble with the FAA.

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   Dan Chitty on Jun 21, 2013

   Thank you John for this wonderful discussion and your good explanations. Much appreciated.

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   Ed Putnam on Jun 25, 2013

   I would just like to add that it would have to be very cold before you have physically correct and “bug up” minima for altimeter errors. I fly / have flown in the arctic in part 121 scheduled jet service and as well in the tropics. Both locations present extreme temperature variations and both locations have a lot of terrain, location depending of course.

   The most usable element of altimetry correction in my opinion is if you are flying LNAV/VNAV approaches in constant descents and also to cross check your altimiter at certain check points along the approach path. If you know you are ISA plus or minus 15C, then you can expect a 10 percent variability at the fix. For example, if the check height 4NM from the threshold is 1290 feet on an ILS, and you are at ISA-15, you can expect the altimiter to read about 1400′ or so at that location (you are slightly lower than what your altimeter says on a cold day – you are still at 1290 on the GS signal, nothing changes the signal). If it’s a hot day and 30C outside, you can expect a check height on the altimeter to show about 1150′ or so. But this is an approximation. There is probably some level of non standard temperature gradient above the surface/aloft along the approach path – you are working with surface temps off of an AWOS or ATIS that are greatly affected by buildings, conrete, etc near the reading. You may, actually, read spot on crossing the fix. This is a basic reason why low and high temp altimetry has limitations and should not be over emphasized in my opinion outside of an extra level of “awareness” the pilot should consider.

   To be perfectly honest, there is only one flight operation I’ve worked for that actually applies cold weather temperature altimetry corrections and it’s only to minimums starting at ISA -15c and below for cold weather temperature corrections (on minima only) to basic raw data approaches (ILS, VOR, NDB). LNAV/VNAV approaches are already built to -25C and do not require corrections. I have flown for two global airlines, one global corporate flight department and one major airline. Just one place bothers to procedural-ize these corrections outside of “pilot awareness”. I have never applied hot weather temperature corrections and I am pretty confident there are no operators applying hot weather temperature corrections anywhere world wide, as it in effect is an error to the positive on terrain clearance. In effect, the only time you can find yourself in trouble is on an old style non precision approach with “dive and drive” step down fixes in terrain, on very, very cold days where you may be ISA-20 and also with field elevations of higher than 500 or 1000 feet MSL.

   An ILS by it’s very structure and composition will keep you safe in cold weather and also I don’t think you will be at risk for ever breaking airspace by following the GS if you are cleared for an approach that shows the GS is available from a fix that you fly the GS from. The regulatory constraints in building approaches (TERPS criteria) must also comply with warm and cold temperature altimetry to ISA+/-25C. Domestic airspace is not constructed and controlled in such a way and if there are encroachments then there would be an airspace letter of understanding between the controlling facilities to cater for the anomaly.

   What does this mean for the average FAR 91 light aircraft IFR pilot? IMHO, I think altimetry corrections in a single pilot operation in light aircraft and in moderate conditions in 99 percent of locations in the lower 48 do not require adjustments. It’s a very nice thing to know about and consider but if you are not in Canada or Alaska, the Arctic or Norway, it’s probably not going to be operationally significant.

   But just in case one may want to know, our flight operation uses a table where we specifically add 10 percent of the HAT figure (200feet for an ILS for example) to the minima on an approach of -15 to -25C below ISA. At sea level for a -10C day you can expect to add 20 feet to the minimums. So a 200 foot DA would be set for 220. If you don’t see the required visual cues at 220 feet, go around, because you are actually closer to 200 feet above touchdown. HOWEVER, as mentioned above, you do not add corrections to procedural altitudes – ATC has their own low temperature altimetry charts and minimum vectoring altitude will change for a radar sector based on temperature – it’s a seamless process for us. Furthermore, it is NOT REGULATORY to make low temperature altimetry corrections to FAR 91/121/135 minimums as such. It is my opinion after using and not using altimetry corrections over the last 20 years that pilots not specifically trained for this are better served by paying attention to flying the aircraft accurately and managing decision making, judgement calls, weather and fuel monitoring than making minutia changes to altimetry. If one is concerned over 20 feet on minimums due to temperature, perhaps a higher self imposed minimum of marginal VFR, or 1000 feet, or 500 feet should be applied so that the pilot does not operate in such conditions to begin with. This would certainly cover a casual IFR pilot for any questions regarding “careless or reckless” operations in low weather or a “fudge factor” being available.

   Results may vary, these are just my observations and I do not discount what John C. says as he is indeed very knowledgeable on the subject too.

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   Ed Putnam on Jun 29, 2013

   Oh yes, just so I may add one more thing for future viewers of the subject. I’ve been going through some of our latest approved GPS/LNAV-VNAV/RNP//LPV approaches at our operation and it is obvious we as the aviation community in general (domestic and international) are still going through some times of “transition”. In other words, much still depends on the personal equipment that is in the aircraft as far as how important altimeter errors and temperature compensation factors are for the given location. In other words, I’ve seen several new approaches only guaranteed to 2 degrees C (meaning, make no altimeter corrections and you are fine to the rated temp on the plate). Noting however, these approaches often show temperatures compatible with the typical local temperatures. So until there is more regulatory guidance on altimetry corrections in non standard temperatures, the best bet is to pay particular attention to the notes on an approach you’d like to fly. The notes are often in non standard locations depending on the approach plate, who made the plate (government, Jeppesen, NavTech, etc). It seems there is a diverse range of approved temperatures for GPS based vertical and lateral guidance approaches at this point in time (2013)

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   Dan Chitty on Jun 29, 2013

   Thank you Ed for the feedback. The comments you and John have provided are very appreciated. I have always been curious about this topic and I now have some good solid information.

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    John D. Collins on Jun 30, 2013

    Ed,

    LNAV.VNAV procedures have temperature limitations for uncorrected Baro-VNAV systems. These systems are not normally found in GA piston aircraft. The reason for the temperature limits has to do with how the GS is calculated with a Baro-VNAV system which uses the static pressure to determine the altitude of the GS at a given 2 D point. This means that the GS moves with temperature, down under cold conditions and upward under warm conditions. A WAAS derived GS does not have this issue, as the GS position in space is relatively fixed and depends on the geometry of the satellites and is independent of the temperature. The temperature limits on the approach charts do not apply to a WAAS derived glideslope when flying a LPV or LNAV/VNAV procedure.

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