LNAV/VNAV BAROVNAV APPROACH

[Raptor71]

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

[BraceBrace]

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?

No, never correct DA and platform altitude when using LNAV/VNAV and LNAV/VNAV minimums. Yes, we add 50ft as it is an SOP for non-ILS approaches.

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.
Yes, it's the whole goal of the minimum temperature, it is checked in advance by the authorities, and the reason why you have to use VNAV to allow the lower minimums.

What happens if I decide to correct the altitudes (platform) and in consequence to fly the published angle of descent? Is that acceptable?
Why complicate life? Don't, it is not designed to do so. Don't try to re-invent the theory.

Finally, how does the system reacts if I put the temperature and QNH in the DESCENT FORECAST page?
Use correct temperature/QNH. Descent page will have an influence, however also note descent page is valid for the complete descent (TOD to RW/MA point). It is much more important to have the correct QNH on your altimeter.

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)?
Fixes and distances are never updated/changed. Glidepaths are.

[Raptor71]

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?

[repulo]

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.

[Skyjob]

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? Not when flying LNAV/VNAV, but yes when flying LNAV or other types of non-precision approach.
Besides, in your airline do you add 40/50ft to comply with the CDFA technique? Company specific, not required in some cases.
If we do not correct these altitudes, is it acceptable to fly a lower angle of descent? Yes, that is what procedure is designed to do, keep you safe up to a certain temperature.
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.Yes as per above
What happens if I decide to correct the altitudes (platform) and in consequence to fly the published angle of descent? Is that acceptable?This is required technique below MSA when using LNAV or other non-precision approaches, not LNAV/VNAV
Finally, how does the system reacts if I put the temperature and QNH in the DESCENT FORECAST page? Temperature correction for descent page and PERF INIT are different in the sense of what is trying to be achieved.
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)?No, it does not

[Raptor71]

Great, thanks

:)

[Empty Cruise]

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?

[underfire]

Note that the parameters are "uncompensated' baro VNAV.

The temperature above/below NA includes the effects of higher/lower temperature on the glideslope for obstacle clearance.

[wiedehopf]

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.

[FlightDetent]

DA needs to be corrected for lo temp.

@Empty Cruise: spot on.

[eckhard]

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.

[wiedehopf]

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.

thank you.

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.

[eckhard]

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.

[Raptor71]

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.

[Empty Cruise]

@ 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 ;-)

[SR-22]

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

[EMIT]

Raptor,

In december 2017 there was a similar thread on this forum, titled " Cold temp corrections on LNAV appr ". (https://www.pprune.org/tech-log/601732-cold-temp-corrections-lnav-appr.html)

You might want to check that one out.