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Temperature Uncompensated VNAV error on Hot days/Inversions

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Temperature Uncompensated VNAV error on Hot days/Inversions

Old 28th Nov 2021, 06:27
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Temperature Uncompensated VNAV error on Hot days/Inversions

Does anyone have any good resources to study the potential operational impacts of temperature uncompensated VNAV systems such as those used in the Boeing 747-400 and 747-8?

Of course it is understood that cold weather altitude corrections must be applied to published minimum altitudes on terminal procedures when temperatures are outside the design range. At my operator altitude adjustments to published PANS-OPS procedures must be made when the surface temperature is 0 degrees or less, and ATC is required to be informed.

How significant is the danger of one finding themselves with excessive altitude on a VNAV or IAN based approach in hot weather? Might a pilot find themselves paralelling the glideslope on the high side if they attempted to fly a VNAV cda to glideslope capture? Some pilots I work with seem so confident that this could never happen, and that VNAV will always become coincident enough with the glideslope to facilitate glideslope capture by the requisite stabilized approach gates.

Can non standard atmospheric vertical pressure gradients result in similar discrepancies? For instance, large temperature inversions resulting in the an "on glidepath" or "on VNAV path" flight deck indication resulting in excessive true altitude and potentially an unstable approach?

Is it possible that very strong surface winds in excess of 30 knots could result in significant altimetry errors affecting VNAV as well?

Thank you. Yes yes, I realize I should already be an expert on all this, but I am attempting to rectify a gap in my knowledge, so please be kind. We just do not fly non-ILS approaches often enough in the real world for this to matter very much. I am asking because I have seen some interesting VNAV path and IAN glidepath behaviors on very hot temperature days (40+ C) and days with strong winds and very strong temperature inversions (+10C degrees warmer at 2000 AGL compared to the surface).
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Old 28th Nov 2021, 07:30
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VNAV is essentially just following barometric altimeter, which is susceptible to temperature error in both directions - low and high. Just as you would end up lower than true altitude in cold weather, you will end up higher than true altitude in warm weather, if no corrections are made.

The latter very often isn't recognised as a problem, because it doesn't reduce your obstacle clearance, so all is well, right? Well, no. Unless you are following an ILS, which produces a fixed angle glideslope in the sky, you will end up high or low in non-ISA weather, regardless of AFDS mode used (V/S, LVL CHG or VNAV), as all of them are following the barometric altimeter.

On an approach in hot weather, you will end up fair amount higher than on an ISA day, so seeing 3 whites is very common - if this is combined with an already steep approach (3.5 deg or higher), it could be quite a challenge to keep the approach stabilised within normal parameters. There are generally two ways of dealing with this:
  1. Use full flap for landing, which reduces your ground speed and in turn your rate of descent. You then have a more margin to 1000ft/min, which is typically the limit for stabilised approaches. Once you are visual, disconnect the autopilot, maintain -1000ft/min until you are established on PAPI. If airline allows, you could also brief a higher rate of descent to correct.
  2. Accept that you are on 3 whites, and maintain that approach angle until flare.
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Old 28th Nov 2021, 15:37
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It doesn't seem to be common knowledge/ taught much. It would seem that a very large number of pilots haven't been taught/ wouldn't think of making an allowance to predict for higher rod from this.

From memory (Covid has grounded my career), glidepath error is +0.1 degree glidepath per 5C above ISA. Therefore, at 150kts, ISA +20C will cause an increase of about 100fpm rod.

I once flew a 787 into Kilamanjaro: from memory, it had a published GP steeper than 3 degrees, the airport has high elevation (something like 3000'), and temperature was about 25. Tricky. "It's okay if it's briefed", right?
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Old 28th Nov 2021, 16:12
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Well it was certainly something that was considered where I worked…, we had at least one wide body destination with a shortish runway/RNAV approach where on a hot day you really did need to have thought through and briefed thoroughly how, as a crew, you were going to handle the transition onto the VGSI and appropriate rates of descent.
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Old 28th Nov 2021, 18:04
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Originally Posted by RANDOMPERSON8008 November 27 2021
Does anyone have any good resources to study the potential operational impacts of temperature uncompensated VNAV systems such as those used in the Boeing 747-400 and 747-8?
Found the following in, https://www.icao.int/SAM/eDocuments/...VNAV%20eng.pdf

This essentially restates what you and others are saying but being an Advisory Circular, it provides context and guidance for VNAV with & without temperature compensation. We all know about the cold-temperature case but your post raises the high-temperature case which is rarely closely examined. But your enquiry is a good one. As "heat domes" such as the 40C to 45C temperatures last July-August experienced in the Pacific Northwest, (Washington State, British Columbia) increase, the "high temperature" case may become more relevant.

AC : 91-010
DATE : 12/10/09


This advisory circular (AC) establishes APV/baro-VNAV approval requirements (barometric vertical
navigation only) for aircraft and operators. Barometric vertical navigation may be included together with
lateral navigation in a RNP APCH approach, as established in CA 91-008. Criteria of this AC together with
criteria of AC 91-008, establish requirements for RNP APCH approach with baro-VNAV.
An operator may use alternative means of compliance, provided they are acceptable to the Civil Aviation
Administration (CAA).

Originally Posted by p.11 [b
10.3.2 Recommended functions]"a) Temperature compensation The baro-VNAV navigation system should be capable of
automatically adjusting the vertical flight path for temperature effects. The equipment should
provide the capability for entry of altimeter source temperature to compute temperature
compensation for the vertical flight path angle. The system should provide clear and distinct
indication to the flight crew of this compensation/adjustment."

Originally Posted by p.14
a) For aircraft using barometric vertical navigation without temperature compensation to conduct the
approach, cold temperature limits are reflected in the procedure design and identified along with
any high temperature limits on the charted procedure. Cold temperatures reduce the actual
glidepath angle, while high temperatures increase the glidepath angle. Aircraft using barometric
vertical navigation with temperature compensation or aircraft using an alternate means of vertical
guidance (e.g., satellite-based augmentation system (SBAS)) may disregard the temperature restrictions.
b) Since the temperature limits established in the charts are only assessed for obstacle clearance in
the final approach segment, and since temperature compensation only affects vertical guidance,
the pilot may need to adjust the minimum altitude on the initial and intermediate approach
segments, and at the decision altitude/height (DA/H)).

Note 1.- Temperature affects the indicated altitude. The effect is similar to having high and low pressure changes, but
not as significant as such changes. When the temperature is higher than standard (temperature under international
standard atmospheric (ISA) conditions)), the aircraft will be flying above the indicated altitude. When the temperature is
below the standard, the aircraft will be flying below the altitude indicated in the altimeter. For further information, refer to
altimetry errors in the aeronaut
ical information manual (AIM)

Note 2.- The ISA standard conditions at sea level are:
The standard temperature is defined as 15 Celsius (centigrade’s) or 288.15 Kelvin;
The standard pressure is defined as 29.92126 inches of mercury (Hg) or 1013.2 hectopascals (hPa); and
The standard density for these conditions is 1.225 kg/m3 or 0.002377 slugs/cubic ft. "
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Old 28th Nov 2021, 18:33
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Concur. Doc 8186 used to have the scientific equation to calculate the lo-temp correction, which however could be used to calculate the hi-temps as well.

Just remember ISA is +15, the first visible effect for cold come around -5 C (ISA-20). Hence only at SL and 35+ the same deviation will be observed on a hot day.

For our summer destinations, a nice practice was to brief going for the PAPI profile earlier - when and where appropriate - if visual, to have the angle correct. One of the 'How to avoid +2g landings at LGIR' series. Flying precise DME/ALT table with a mis-indicating altimeter was just not giving helpful results.
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Old 29th Nov 2021, 07:56
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Its quite simple actually. You determine the distance from the threshold (r) and height above obstacle clearance surface. You need to be above this surface by the following:

A = BG + ISA dev + 4/3 * sqr root(r sqrd * ANP + r sqrd * WPR + r sqrd * FTE + r sqrd * SSE + r sqrd * VAL + r sqrd * ATIS temp)

where BG = body geometry (accounts for reduced wing tip clearance in a turn), WPR = waypoint resolution (compensates for waypoint co-ordinate rounding errors), FTE = flight technical error (allow for stupid pilot who cant stay on path, nominally 65 feet), SSE = static source error, VAL = vertical angle leg (glide path angle).

Usually a half decent pilot doing the radios will constantly recalc this and advise the flying pilot if they are infringing the obstacle clearance.
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Old 29th Nov 2021, 10:16
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For a minute there I thought he was serious.

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Old 29th Nov 2021, 17:04
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Airbus has an equivalent of IAN. It's called FLS, FMS Landing System. If temperature entered by pilot on performance page is different than ISA it automatically applies the required correction. It's something IAN also should be doing. People using IAN can confirm on this.
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Old 29th Nov 2021, 18:02
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It is actually quite simple, especially for the small heights above the barometric reference (aerodrome) during most approaches.

The error can be approximated to 4ft per degree C dev from ISA for every 1000ft above the aerodrome.

The deviation from ISA should technically be the average for the air mass under the aircraft, but unless there are significant inversions, the dev from ISA at the aerodrome should suffice.

Aerodrome elev 2000ft
Aerodrome OAT +35C
OAT in ISA should be +15-4=+11C
Deviation from ISA is +24
Altimeter will under-read by 96ft per 1000ft above the aerodrome.
Call it 10% for round numbers.

I seem to recall this being an issue at Mauritius, where the GS intercept was over some high ground, and quite often in cloud. Skippers were understandably reluctant to descend below the published platform altitude, which sometimes meant that we were chasing the GS from above.

Similar problem at Mexico RW05 in the summer, with the GS intercept only about 5nm from the runway, a greater than 90 degree turn to line-up and finding oneself above the GS with only about 30secs before Stable Approach Criteria had to be fulfilled.

Our SOPs allowed for increasing the indicated altitudes to be flown in colder-than-ISA conditions, but we couldn't reduce them in warmer-than-ISA temperatures.
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