From here
Might this be the French looking over to the Alternate Flight Planning rules (FAA AIM 1-1-17 and 18) for non WAAS (EGNOS over here) equipment?
If France goes pure GPS RNP (seems unlikely) then there might be some logic?
The problem for LNAV/V+ (puddle jumpers) compared to BARO VNAV (heavy iron) is, as it says on the tin, the latter is more accurate in non ISA conditions.
My non EGNOS 530/430 combo will only be replaced when it fails.
RobertL18C wrote:
The problem for LNAV/V+ (puddle jumpers) compared to BARO VNAV (heavy iron) is, as it says on the tin, the latter is more accurate in non ISA conditions.
You mean the former is more accurate? Baro-VNAV will always give a non-standard glideslope in non-ISA conditions. (That’s why there are temperature limits on Baro-VNAV approaches.)
@Airborne_Again it shows my understanding needs improving :)
All approaches need temperature correction if below 0 degrees centigrade.
The satellites don’t know the temperature of the atmosphere so on a cold day, but above temperature correction temperatures, the vertical V+ guidance will typically show around, say a half dot fly up, in ISA minus 10o C, while it is legal to follow the CDFA profile using the altimeter.
Some SOPs, which I think didn’t understand what was going on, would suggest all GPS approaches showing vertical guidance should be temperature corrected. The problem with this is that on an ISA plus day the temperature corrected slope results in a below minima DA. Also on a warm day the vertical guidance will show a fly down relative to the published CDFA profile. More subtly on a cold day the vertical guidance is flatter, and conversely on a warmer day, steeper.
Practically the GPS is taking you to the same point of space, while a non precision CDFA is using an altimeter that is subject to temperature error. It’s a bit like an ADF, who would use it over a GPS? But in the context of IR practical test standards (+/- 50 feet on the CDFA), (within 5 degrees on the NDB – leaving you quite exposed in terms of protected area), the GPS could in theory create discrepancies.
In theory BARO VNAV is using actual atmospheric inputs within stipulated temperature limits?
This hopefully gets moved to a separate thread if it is felt worth discussing.
RobertL18C wrote:
All approaches need temperature correction if below 0 degrees centigrade.
All approaches need temperature correction for the (M)DA and stepdown altitudes, but ILS, LPV and SBAS VNAV don’t need temperature correction for the glideslope as these approach types don’t use barometric altitude to determine the glideslope. Baro-VNAV does so there the glideslope angle varies with ISA deviation.
The problem with this is that on an ISA plus day the temperature corrected slope results in a below minima DA.
I don’t understand that. The DA is what it is, it is not dependent on the slope.
Also on a warm day the vertical guidance will show a fly down relative to the published CDFA profile. More subtly on a cold day the vertical guidance is flatter, and conversely on a warmer day, steeper.
Indeed, and Baro-VNAV approaches are designed to work regardless – within stated temperature limits.
My understanding remains temperature correction only applies when temperatures fall below zero centigrade, where does it say you temperature correct on an ISA plus day, let alone a plus zero ATIS/METAR day?
All approaches need temperature correction for the (M)DA and stepdown altitudes, but ILS, LPV and SBAS VNAV don’t need temperature correction for the glideslope as these approach types don’t use barometric altitude to determine the glideslope.
Maybe means initial & intermediate segments before FAF or FAP where one fly level (procedurally) using barometric altimeter? vectoring is a different story
RobertL18C wrote:
My understanding remains temperature correction only applies when temperatures fall below zero centigrade, where does it say you temperature correct on an ISA plus day, let alone a plus zero ATIS/METAR day?
Nowhere – I didn’t said that either. What I said was that
1) the actual glideslope angle of a Baro-VNAV approach will depend on the ISA deviation, while on an SBAS VNAV (and LPV, ILS) approach it will not.
2) you don’t apply any temperature correction to the Baro-VNAV glidepath angle.
3) you do apply temperature correction to (M)DA and stepdown fixes (when needed).
In US, recently in the last 5 or so years, cold weather correction has been added at specific airports. Approach charts that are affected have a snow flake icon on them and specify a temperature at which corrections are needed. Corrections are made when the temperature is at or below the airport specific temperature. The altitudes for the initial, intermediate, final, and missed approach segment may need correction. Often the most critical segment that needs correction is the intermediate leg because it has the TERPS ROC of 500 feet and is higher above the airport altitude than the final approach segment. Altimeter error is a function of temperature, height above the airport, and distance from the point the altimeter setting is determined.
@NCYankee many thanks
@Airborne_Again I may have misunderstood your statement? Or failed to add context?
All approaches need temperature correction for the (M)DA and stepdown altitudes
Typical SOP is to only use temperature correction when the approach plate calls for it, or ATIS/METAR is below freezing?
Applying temperature correction in an ISA plus context, on for example G1000 will result in a DA below published DA?
Am using ? as it would be useful to get a consensus.
RobertL18C wrote:
Typical SOP is to only use temperature correction when the approach plate calls for it, or ATIS/METAR is below freezing?Applying temperature correction in an ISA plus context, on for example G1000 will result in a DA below published DA?
We are in agreement about this.
