Let pose the issue like this. You can do a VOR or NDB approach in a 777, i suppose and in a 737 with LNAV V/S or LNAV FPA or LNAV/VNAV, with the precondition of doing the altitude corrections if the temperature is 0 C or bellow. When you start to descend from the FAF the altitude you read at your altimeter will be the corrected. The next altitude , let say a fix or outer marker will be the corrected etc. until the corrected MDA or DA. You read on the altimeter the altitudes and verify if you are on profile or not . But you use LNAV/VNAV. You have pressed the LNAV button and the VNAV button haha. The same is if you do an RNAV GNSS approach. You use LNAV/VNAV. The RNP AR is an other story. I don't ask for RNP. At an RNP if you are not in the temperature or wind envelope you cannot do it and you do not do any corrections there. My question is why if you do the altitude corrections at the specific approach at Frankfurt and insert them in the FMS you can't use VNAV. You will correct the FAF REDGO,the 7.0 NM 2650', the 2NM 1060' . These are in the FMS. You will check if the 3 deg descend angle is there after REDGO in your FMS , you will validate the approach and you will descend to the LNAV corrected minima. With LNAV FPA you ' ll do the same thing. You will follow the 3 deg descend angle and you are legal to do it, but in LNAV/ VNAV you are doing the same thing but you are illegal .

If the approach flown is 2D APCH (LNAV only) to the LNAV (only) minima - which I suspect you do say so - I do not see why that would be forbidden or what's wrong about it.

As long as you calculate the corrections and observe them, using the applicable techniques for the different FD guidance modes, it makes no difference IMHO. V/S, FPA, VNAV (FD mode). Can you do VOR overlay with LNAV-VNAV FD mode, assuming corrections are made to fixes and minima? I'd say yes and would not see 2D APCH (LNAV only) to be any different.


Some reservations however about the wiggy's quoted "no cold temp corrections required to the coded vertical profile on 2D APCH (LNAV only) if within ISA-25" (hope the paraphrase is not oversimplyfying). I am loosely aware there are some cold temp considerations in the design of 2D APCH (LNAV only) procedures, but still:

Attachment 1976

This page has 3D APCH (LNAV+VNAV) constrained to ISA-10. The above mentioned guidance seem to suggest that it would be acceptable to shoot 2D APCH (LNAV only) with the indentical profile uncorrected down to ISA-25. Does not sound in tune.

In practical terms that is if Egypt freezes over, I know.:rolleyes:

Underfire,

Not sure about your background. Quite knowledgeable you seem from other posts but now you've completely misunderstood. There is a difference beteende LNAV+VNAV autoflight modes and LNAV+VNAV minima. In the 737 at least you can use LNAV+VNAV autoflight modes to LNAV minima at any temperature, as long as initial, any step down and decision altitude is corrected.

MIght be my poor phrasing/syntax FD. For clarity:

If there is no minimum temperature on the chart our Company backstop is no use of VNAV on approach if below -25 ISA. FWIW most if not all of our fleets have uncompensated baro VNAV, no option to enter a surface temp for corrections, and we are not allowed to alter FMC altitudes either, hence the restriction.

Yes, I agree we would need to apply corrections on a 2D LNAV (and e.g VS or FPA) approach in cold temp conditions.

Hope that makes more sense.

wiggy: understood. Quite likely that's what you had said already but I was not patient enough to read properly into it.

An extract from our ops (company) manual -

VNAV approaches are based on the use of barometric vertical path computations and as a result subject to the effect of temperature deviation from the standard. Under extreme cold temperature conditions (ISA-45deg), the vertical path angle can decrease by as much as 0.5 deg. Applying a correction to FAF crossing altitude will not correct this problem. Hence it may not be possible to use VNAV under extreme temperature conditions.

So the FD mode of the VNAV cannot be utilized to try to achieve a FMC computed/guided vertical descent to an LNAV minima (below ISA-45 atleast). Might as well use VS FPA to LNAV minima

Between the lowest temp limit published on the chart (when the design procedure becomes 2D)
to ISA-45 (different for diff carriers I am assuming), probably use FD (VNAV) with cold altitude corrections and ensuring all true altitudes are at or above the min altitude restrictions from IAF to touchdown.

That's my understanding after reading the above post for uncompensated BARO FMC systems.

From the FCTM:
Once modified you can then use VNAV to fly the LNAV approach.

Obviously if that's your FCTM, so is correct for you.... but it highlights something fundamental about this whole debate: Some operators allow changes to the alts on the FMC legs page, others may not.

The OPs answer lies in his/her own Ops manual.

IIRC, in Bombardier products with Temp Comp, the FMS altitude constraints on approaches can only be changed by using the Temp Comp entries. Basically, one enters the current temp and the FMS makes the calculates and displays the corrected indicated altitude constraints.

It is simple, the procedure states NA below for VNAV.

When you apply correction as you stated above, do you inform ATC of your corrections?

Will ATC maintain minimal sep between the aircraft that have the ability to correct? Do you expect ATC to manage corrected and non-corrected altitudes?

It matters not if you can correct, or if you cannot, it matters with ATC....

Again, the plate referenced is simple. NA below.

underfire,

Are you flying airplanes or designing approach procedures? From a pilot point of view doing an approach to LNAV minima, it shouldn't matter what vertical navigation mode you use as long as you stay above the minimum altitudes (+ temp correction). What the aircraft is capable of probably depends on aircraft type. Different operators may have different SOPs. I can tell you in the two different airlines I have been the SOP has been to fly LNAV and VNAV autoflight modes to LNAV minima with temperature correction to all altitudes below MSA. This regardless of OAT. If using LNAV/VNAV minima charted temperature restriction has applied.

Different Coy SOPs, Aircraft Certifications & NAV Environment
 

Just to add to the information, or confusion...

Part of the reason why there might be different company SOPs/FMs with regard use of LNAV/VNAV minima are dependent on whether or not the aircraft have certificated final approach temp compensation embedded or not.
Further if operating in an SBAS environment (eg, where WAAS available within required minimum integrity levels - ie, no alerts), then the VNAV minima can also be used because the vertical guidance is not relying on the baro but instead getting the feed from SBAS-related avionics.

Refer to the attached extract from the ICAO 8168 PANS-OPS procedure design criteria for Baro-VNAV procedures

So, if final APCH auto temp compensation or SBAS not available, then the LNAV minima must be used if the temp is BLW the NA Temp published on the plate.