Lnav/vnav Da

[777Veritas]

QUESTION:
When flying an RNAV GPS/GNSS approach to a LNAV/VNAV DA (decision altitude), wherein the approach plate limits the Baro VNAV use to a minimum temperature of -25C (see PANC RNAV GPS RWY07R), must we still correct the DA for cold temperature? Say the OAT was -15C.

Any comments are appreciated. Thanks!

[BuzzBox]

Our FCOM states the following:

'Except for approaches with "LNAV/VNAV" minima that have specified minimum temperatures (e.g. ANC –25°C), cold weather temperature corrections shall be added to both the procedural altitudes and the DA/MDA...'

Ergo, if a minimum temperature IS specified, then temperature corrections don't need to be applied provided the temperature is at or above the minimum specified temperature. If the temperature is below the minimum specified temperature, then the approach may be flown to the higher "LNAV" minima, provided temperature corrections are added to the procedural altitudes and the LNAV MDA.

[FE Hoppy]

conccur with BB. Just been though this with our training team. If there is a published temperature limit then no correction when operating within it.

[DFC]

The minimum temperature secified on the chart simply ensures that the VPA which of course changes with temperature does not get too low.

Pilots still must make the appropriate temperature corrections for the various promulgated altitudes/heights in the normal way.

To explain further:

If the VPA is 3deg at 15C

It will be only 2.5deg at -31C

So as the temperature decreases the VPA gets more shallow and the minimum temperature limits how shallow the VPA can get.

Imagine an ILS GP that changed from 3deg at 15C to only 2.5deg at -31C

With a lowering VPA then the obstacle surface must also lower and there will be a limit on how low the obctacle limitation surface can go.

The movement of the VPA does not remove the requirement to ensure that the actual level of the aircraft at for example the DA(H) (indicated level corrected for temperature) is not below the true level of the DA(H).

Hope that explains the two different requirements.

Regards,

DFC

[777Veritas]

I tend to agree with DFC's explanation. Now then. Where can we find appropriate regulation that clearly defines this. I have been through AC 120-29A several times, and yes it mentions that the baro VNAV can be used up to a temp. of -25C, but it does not clarify (I think), if the DA should be corrected for cold temperature.

The DA for PANC RNAV GPS 07R shows [ "DA(H) 660' (530') ]. To my understanding, terrain at the DA is still a concern, and even though you descend on a corrected VNAV path, the DA still should clear the terrain appropriately.

[FE Hoppy]

Correction to my previous. I must be getting senile!!!

Operational Constraints
• Pilots responsible for cold temperature corrections to all
published minimum altitudes/heights including initial and
intermediate segments, DA/H and missed approach.
• Baro-VNAV not permitted below minimum aerodrome
temperature for procedure. LNAV may still be used provided
a conventional RNAV procedure exists and pilot applies all
temperature corrections to minimum altitudes/heights
• Baro-VNAV only with local altimeter source available and
QFE/QNH as appropriate.
• Vertical path excursion less than +100 ft, -50 ft.

The above is PANS OPS.

Sorry for the confusion.:confused:

[LLLK]

DFC and FE Hoppy have it - the procedure is designed to protect aircraft down to a minimum temperature - if you want to fly the optimal vertical path then you should make allowance for the low temperature - if you don't make allowance for the temperature you are still protected.

[DFC]

if you don't make allowance for the temperature you are still protected.


Not quite.

Assume the temperature is say -31 (ISA-46).

You are happily flying down the presented VPA and have it pegged exactly in the middle - not even 1ft of deviation. You are not worried that the VPA has reduced from 3 to 2.5 degrees because you know that the obstacle surface will have reduced also and will be OK.

However, When you are on the VPA and the altimeter says you are at 350ft, what is the true altitude of the aircraft?

Without bothering to work it out, it is a lot lower than 350ft and if that was your DA then you are actually well below it unlesss you have made the appropriate temperature correction.

So to sumarise - as the temperature drops the VPA decreases. As the VPA decreases, the obstacle limitation surface also decreases. There will be a limit of how shallow an approach you can make and this limit will be reached as the temperature lowers.

So the minimum temperature ensures that the VPA does not get too shallow.

A separate issue is that unless the DA is corrected for cold temperature then when DA is indicated on the altimeter the aircraft will actually be well below the DA............a long way below it at -31C!!.

To satisfy yourself use -31C and sea level. The VPA will be 2.5deg. With a DA of 350ft - how high will the aircraft actually be and at that level how far away from the threshold?

So you are lower than you think you are and closer to the threshold - do you really what to be there when you have to go missed on a limiting missed approach procedure?

Ask yourself - If you are flying an ILS with a 2.5deg glideslope (provides the required obstacle clearance) and the temperature is -31 - would you not bother to do a temperature correction to the DA simply because you know that the glideslope keeps you clear of obstacles?

Regards,

DFC

[LLLK]

The cold temperature effect at DA is less than at the FAP - the higher you are above the elevation of the altimeter setting source (in the case of LNAV/BAro VNAV this is the aerodrome) then the greater the cold temperature effect. So, at -30C, the height difference at 300ft above aerodrome is 60ft, while at 1500ft above aerodrome it is 280ft. Furthermore, the Obstacle Assessment Surface is set back an additional 0.3NM from the point at which the 'MOC surface' meets the ground so you have at least 322 ft of vertical clearance, after taking account of depressed VPAs, at any point along the final approach.

[LLLK]

Moreover, PANS-OPS Vol 1 requires :In all cases, regardless of the flight technique used, cold temperature correction must be applied to all
minimum altitudes (see Part III, Section 1, Chapter 4, 4.3, “Temperature correction”).

[LLLK]

Moreover, PANS-OPS Vol 1 requires :In all cases, regardless of the flight technique used, cold temperature correction must be applied to all
minimum altitudes (see Part III, Section 1, Chapter 4, 4.3, “Temperature correction”).