LNAV/VNAV BAROVNAV APPROACH
Thread Starter
Joined: Apr 2014
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From: Tampa
LNAV/VNAV BAROVNAV APPROACH
Hello,
I have some questions on Baro vnav approaches on b737NG.
First, is it necessary to correct the DA and platform altitude for cold temperatures as long as the OAT is above the minimum published temperature?
Besides, in your airline do you add 40/50ft to comply with the CDFA technique?
If we do not correct these altitudes, is it acceptable to fly a lower angle of descent? Does the procedure guarantees obstacle margin as long as the temperature is above the minimum published temperature? this margin decreasing when we are close to the mini published T.
What happens if I decide to correct the altitudes (platform) and in consequence to fly the published angle of descent? Is that acceptable?
Finally, how does the system reacts if I put the temperature and QNH in the DESCENT FORECAST page?
I know that some FMC's will make some kind of corrections to keep optimum obstacle margin if temperature decreases close to the minimum temperature published for the procedure. Does it "update" the FAP ( making it closer to the runway)?
Thank you
I have some questions on Baro vnav approaches on b737NG.
First, is it necessary to correct the DA and platform altitude for cold temperatures as long as the OAT is above the minimum published temperature?
Besides, in your airline do you add 40/50ft to comply with the CDFA technique?
If we do not correct these altitudes, is it acceptable to fly a lower angle of descent? Does the procedure guarantees obstacle margin as long as the temperature is above the minimum published temperature? this margin decreasing when we are close to the mini published T.
What happens if I decide to correct the altitudes (platform) and in consequence to fly the published angle of descent? Is that acceptable?
Finally, how does the system reacts if I put the temperature and QNH in the DESCENT FORECAST page?
I know that some FMC's will make some kind of corrections to keep optimum obstacle margin if temperature decreases close to the minimum temperature published for the procedure. Does it "update" the FAP ( making it closer to the runway)?
Thank you
Joined: Feb 2003
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From: Blue sky
If we do not correct these altitudes, is it acceptable to fly a lower angle of descent? Does the procedure guarantees obstacle margin as long as the temperature is above the minimum published temperature? this margin decreasing when we are close to the mini published T.
Fixes and distances are never updated/changed. Glidepaths are.
Thread Starter
Joined: Apr 2014
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From: Tampa
Hello,
Ok since BARO VNAV is and APV approach and DA(H) is computed the same way as ILS approach ( height loss taking into account the most penalizing obstacle), I was wondering about the add on of 50ft.
For the DESCENT FORECAST, could you tell me how the FMC works with the datas for the approach? What does it do when you feed it with QNH and temperature?
Does it take into account for the BARO VNAV or no it will just take these datas into account for the descent part.
Eventually, for the FMCs taking into account temperature and "correcting", what do they actually do during the approach?
Ok since BARO VNAV is and APV approach and DA(H) is computed the same way as ILS approach ( height loss taking into account the most penalizing obstacle), I was wondering about the add on of 50ft.
For the DESCENT FORECAST, could you tell me how the FMC works with the datas for the approach? What does it do when you feed it with QNH and temperature?
Does it take into account for the BARO VNAV or no it will just take these datas into account for the descent part.
Eventually, for the FMCs taking into account temperature and "correcting", what do they actually do during the approach?
Joined: Jan 2002
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From: germany
Nope, no need to add 50 ft, VNAV minimas are DA/DH.
Descent forecast optimizes the T/D, no more.
Check your chart till what temperaturr the approach is approved to, if below use the LNAV minimas and correct the altitudes.
If you use IAN temp correction depends on the actual FMC software and type of approach.
Descent forecast optimizes the T/D, no more.
Check your chart till what temperaturr the approach is approved to, if below use the LNAV minimas and correct the altitudes.
If you use IAN temp correction depends on the actual FMC software and type of approach.
Joined: Jan 2000
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From: FL410
Besides, in your airline do you add 40/50ft to comply with the CDFA technique?
If we do not correct these altitudes, is it acceptable to fly a lower angle of descent?
Does the procedure guarantees obstacle margin as long as the temperature is above the minimum published temperature? this margin decreasing when we are close to the mini published T.
What happens if I decide to correct the altitudes (platform) and in consequence to fly the published angle of descent? Is that acceptable?
Finally, how does the system reacts if I put the temperature and QNH in the DESCENT FORECAST page?
For climb we want to predict a Mach Number at altitude and an ISA deviation is used to correct IAS and initial Mach number at the forecast altitude/wind/temperature/level.
For descent, the ISA DEV is the average ISA deviation during descent, something quite difficult to predict and not something provided by many operators as close to ground ISA deviation can be significantly different from aloft. This correction only has an impact when ISA deviation throughout entire decent is +15C or greater, therefore most airlines ignore this entry unless flying in very hot regions of the world.
QNH on the other hand is used to correct for difference in altitude during transition from flight levels, therefore it moves the T/D point to take advantage of this information. it also is used for non-barometric approaches like using VNAV, which make use of this information. The 737NG is a non-barometric capable aircraft only, whereas the Airbus 320 is Baro-Aided and thus does not "need" this information.
I know that some FMC's will make some kind of corrections to keep optimum obstacle margin if temperature decreases close to the minimum temperature published for the procedure. Does it "update" the FAP ( making it closer to the runway)?
ECON cruise, LR cruise...
Joined: Apr 2002
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From: MIRSI hold - give or take...
Ok - quick one.
If it's cold (ie well below 0˚C, but above MUT) - would you correct the barometric DA on an ILS?
If yes - why wouldn't you correct the DA on an APV-BARO approach? Both are 3D - both are based on a OCH (or VEB in the case of the APV-BARO) - so why would you have one type of approach where you don't correct your baro DA, but do it on all the others?
If it's cold (ie well below 0˚C, but above MUT) - would you correct the barometric DA on an ILS?
If yes - why wouldn't you correct the DA on an APV-BARO approach? Both are 3D - both are based on a OCH (or VEB in the case of the APV-BARO) - so why would you have one type of approach where you don't correct your baro DA, but do it on all the others?
Joined: Jul 2014
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From: Germany
so this seems like an appropriate thread to ask this:
is the altimeter / pressure given for an airport a value that makes the altimeter read correct at the airport elevation?
or is it the the value so that 0 ft AMSL would read correct (yes this would be a bad idea still wanted to ask)
this would also mean the temperature error for an ILS decision altitude would be very low (that's why i was thinking about it)
edit: quick check of approach plates: the ILS i checked didn't even have a temperature limit.
is the altimeter / pressure given for an airport a value that makes the altimeter read correct at the airport elevation?
or is it the the value so that 0 ft AMSL would read correct (yes this would be a bad idea still wanted to ask)
this would also mean the temperature error for an ILS decision altitude would be very low (that's why i was thinking about it)
edit: quick check of approach plates: the ILS i checked didn't even have a temperature limit.
Joined: May 2005
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From: France
QNH gives a correct reading of elevation when on the ground at the airport.
QFE makes the altimeter reads zero when on the ground.
The altimeter setting that shows correct elevation at sea level is the QFF, not really used in aviation operations but is useful for meteorological plotting of isobars, etc.
QFE makes the altimeter reads zero when on the ground.
The altimeter setting that shows correct elevation at sea level is the QFF, not really used in aviation operations but is useful for meteorological plotting of isobars, etc.
Joined: Jul 2014
Posts: 344
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From: Germany
QNH gives a correct reading of elevation when on the ground at the airport.
QFE makes the altimeter reads zero when on the ground.
The altimeter setting that shows correct elevation at sea level is the QFF, not really used in aviation operations but is useful for meteorological plotting of isobars, etc.
QFE makes the altimeter reads zero when on the ground.
The altimeter setting that shows correct elevation at sea level is the QFF, not really used in aviation operations but is useful for meteorological plotting of isobars, etc.
that also explains why some ATIS informations in the US cite Sea Level Pressure.
Though if the airport is at 11ft elevation it's kind of redundant.
Joined: May 2005
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From: France
You’re welcome!
Just to add: the QNH uses the ISA model temperature lapse rate with height, as that is also used by the mechanism in the altimeter, whereas the QFF uses the actual ambient conditions to derive the sea-level pressure.
Just to add: the QNH uses the ISA model temperature lapse rate with height, as that is also used by the mechanism in the altimeter, whereas the QFF uses the actual ambient conditions to derive the sea-level pressure.
Thread Starter
Joined: Apr 2014
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From: Tampa
B737 FCOM states:
an altitude correction due to cold temperature is not needed for the following conditions:
-under ATC vectors
- while maintaining and ATC FL
-when reported airport temperature is above 0C OR IF AIRPORT TEMPERATURE IS AT OR ABOVE THE MINIMUM PUBLISHED TEMPERATURE FOR THE PROCEDURE BEING FLOWN.
an altitude correction due to cold temperature is not needed for the following conditions:
-under ATC vectors
- while maintaining and ATC FL
-when reported airport temperature is above 0C OR IF AIRPORT TEMPERATURE IS AT OR ABOVE THE MINIMUM PUBLISHED TEMPERATURE FOR THE PROCEDURE BEING FLOWN.
ECON cruise, LR cruise...
Joined: Apr 2002
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From: MIRSI hold - give or take...
@ Raptor - that is correct. Now look at your Ops Manual Part A Ch.8 - what does that say?
Basically - if your 135/121 company decides that something needs to be done, it does not matter one iota if Boeing say 'no need to do that'.
I've also operated a type where the OEM De-icing procedure stated 'Pitch Trim - full Nose UP'. I politely enquired if that was something they had thought through? "Yes - the nose of the stab needs to be up in order for the fluid to not.... ahem.... yeah, that'd be nose down, wouldn't it.... Ahem.... SBY for an AW!" - which was issued about 5 days later.
Point is - there is what the OEM says, and there is physical reality. At the end of the day, the latter always wins ;-)
Basically - if your 135/121 company decides that something needs to be done, it does not matter one iota if Boeing say 'no need to do that'.
I've also operated a type where the OEM De-icing procedure stated 'Pitch Trim - full Nose UP'. I politely enquired if that was something they had thought through? "Yes - the nose of the stab needs to be up in order for the fluid to not.... ahem.... yeah, that'd be nose down, wouldn't it.... Ahem.... SBY for an AW!" - which was issued about 5 days later.
Point is - there is what the OEM says, and there is physical reality. At the end of the day, the latter always wins ;-)
Joined: Jun 2008
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From: Some hotel
Doc 8168 Volume 1 Flight Procedures, Part II, section 4, Chapter 1 APV/Baro-Vnav Approach Procedures:
"1.4.1 Pilots are responsible for any necessary cold temperature corrections to all published minimum altitudes/heights. This includes:
a) the altitudes/heights for the initial and intermediate segment(s)
b) the DA/H; and
c) subsequent missed approach altitudes/heights"
So basically on a Baro-Vnav approach, as long as the OAT is within the temperature limit, temperature correction is not required from and including the FAF down to and NOT including the DA/H but all other minimum altitudes are.
My outfit does add 50' to all DA/MDA on NPA yes. Jeppesen publishes minima often as DA/H without any add-on on state published OCA/H
"1.4.1 Pilots are responsible for any necessary cold temperature corrections to all published minimum altitudes/heights. This includes:
a) the altitudes/heights for the initial and intermediate segment(s)
b) the DA/H; and
c) subsequent missed approach altitudes/heights"
So basically on a Baro-Vnav approach, as long as the OAT is within the temperature limit, temperature correction is not required from and including the FAF down to and NOT including the DA/H but all other minimum altitudes are.
My outfit does add 50' to all DA/MDA on NPA yes. Jeppesen publishes minima often as DA/H without any add-on on state published OCA/H
Last edited by SR-22; 3rd April 2018 at 02:53.
Joined: Aug 2007
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From: The Netherlands
Similar
Raptor,
In december 2017 there was a similar thread on this forum, titled " Cold temp corrections on LNAV appr ".
You might want to check that one out.
In december 2017 there was a similar thread on this forum, titled " Cold temp corrections on LNAV appr ".
You might want to check that one out.
