LNAV/VNAV

[toseg]

Example is EDDF RNAV(GPS) Z Rwy 25C. If the temp is bellow -15 C then baro VNAV is not authorised. So you use LNAV minima. If you have temp -18 C and do the temperature corrections according with the FCOM and enter the corrected altitudes in the FMS. During the approach can you use LNAV/VNAV? ( with the corrected altitudes and descend to LNAV minima)? Many people say that you have to do it LNAV and FPA or V/S because baro LNAV/VNAV minima are not authorised. My opinion is that you can use LNAV/VNAV during the approach with the corrected altitudes. Thank you in advance

[172_driver]

You can do it in any vertical speed mode as you like as long as the minimum altitudes are adhered to. Your company may have some SOP/restrictions but we do it in LNAV/VNAV yes.

[galaxy flyer]

If you temp comp, yes LNAV/VNAV because the limits are there to "protect" the aircraft from descending outside the altitude limits. By compensating, you are flying true altitudes.

[oggers]

toseg, I believe what you are saying is correct. If you have an uncompensated baro VNAV system you cannot use the LNAV/VNAV DA outside the temperature limits. You can still use the baro VNAV for vertical guidance but you have to fly to the LNAV MDA (and apply temperature correction to each segment altitude including MDA). If you have compensated baro VNAV you can use the LNAV/VNAV DA but you must still apply temperature correction to each altitude including DA. Ref AIM 5-4-5 and AC 90-105A App B.4.2

[RUMBEAR]

The FCOM states that the approach procedure cannot be modified from FAF to MAPt, so adjusting altitudes to compensate the final segment for temp is not possible. Our company policy describes this as the reason LNAV/VNAV minima cannot be used when below the minimum temp. FCOM also says a vertical selected mode is available and consequently the minima will change to the published LNAV only minima. ( That's our company guidance anyway )

[galaxy flyer]

Can you enter the temp into a temp comp FMS entry and have the FMS compensate for temps?

[FlightDetent]

GF: the Airbus kit does not have TEMP correction feature.

The only way to do it is to manually modify the altitudes over waypoints which:
a) is rejected by the FMS as 3° profile is the basis of construction
b) is explicitly forbidden by VNAV OPS approval.

So, no (in our case). If temps are below what's charted then no (uncompensated) VNAV. Manual correction is not an approved TEMP compensation standard and would invalidate the coding.

[reynoldsno1]

It should be borne in mind that the methodology used for obstacle protection for an LNAV/VNAV approach is quite different to that for an LNAV procedure. The VNAV functionality in the aircraft is not directly related to the procedure design.

[underfire]

The chart is very specific Baro-VNAV operations N/A below -15C...non-comp or comp N/A

http://i67.tinypic.com/21e86qx.jpg

My opinion is that you can use LNAV/VNAV during the approach with the corrected altitudes.

You can do it in any vertical speed mode as you like as long as the minimum altitudes are adhered to. Your company may have some SOP/restrictions but we do it in LNAV/VNAV yes.

If you temp comp, yes LNAV/VNAV because the limits are there to "protect" the aircraft from descending outside the altitude limits. By compensating, you are flying true altitudes.

Pretty amazing the understanding of flight procedures exhibited here on PPRUNE....Care to revise?


So, no (in our case). If temps are below what's charted then no (uncompensated) VNAV. Manual correction is not an approved TEMP compensation standard and would invalidate the coding.

The FCOM states that the approach procedure cannot be modified from FAF to MAPt, so adjusting altitudes to compensate the final segment for temp is not possible

Thank you.

[flite idol]

The FCOM states that the approach procedure cannot be modified from FAF to MAPt, so adjusting altitudes to compensate the final segment for temp is not possible. Our company policy describes this as the reason LNAV/VNAV minima cannot be used when below the minimum temp. FCOM also says a vertical selected mode is available and consequently the minima will change to the published LNAV only minima. ( That's our company guidance anyway )

Bingo......Thats what the FCOM I'm bound to follow says too. Corrections apply but VNAV cannot be used as the vertical mode.

[underfire]

The procedure is NA. Simple as that, below -15C, use the LNAV parameters

[Lancelot de boyles]

Now, I thought/understood this was slightly different.
The LNAV part of the procedure remains the same.

The VNAV part is where the anomalies arise.
You may use VNAV as your descent profile/method, with any altitude/temperature corrections up to, but not inside the FAF.
Within the published temperature (o/p gives -15c as the lower temperature limit) constraint of the procedure, then VNAV minima apply; LNAV/VNAV.
once outside of the published temperature constraint, the LNAV only minima apply, which are likely higher. This does not mean that VNAV cannot be used; it simply means that your minima are higher.

[toseg]

First i would like to thank you all very much for your answers . At FCTM is written the following : "Boeing airplanes have uncompensated Baro-VNAV systems and are prohibited from using LNAV/VNAV minima on approach charts when operating outside of published temperature restriction limits. However, if cold temperature altitude corrections are applied as described in the Cold Temperature Operations Supplementary Procedures chapter of the FCOM, descent to the corrected LNAV (MDA) minima is allowed." It says for minima not for VNAV mode.
Is mentioning to descent to LNAV minima but does not clarify if you can use VNAV mode. At the part C of my company, if you are above -15 C you correct the FAF altitude and the DA only by 10% .Not all the altitudes between. Bellow -15 C you correct all the altitudes and the MDA from the classic table. In the FMS you don't insert new points. Just correct the altitude of the existed ones in the FMS.

[hawk37]

Like Galaxy Flyer says, if you can enter the temperature in the FMS, then it will compensate for the altitudes in the approach procedure. The IF, and FAF will then have increased minimum altitudes, and then you must increase your LNAV or LNAV/VNAV minimums. Then compensate for the IAF altitude (talk to ATC if necessary). For us, ATC always says to cross the IAF "at or above" 3500 feet so that's not a problem.
With a cold temperature input to the FMS, the FMS adjusts the VNAV angle up a bit to compensate for the reduced altitude clearances that would exits if you used the somewhat lower altitudes that the altimeter reads on cold days.
If the approach says NA, then it's NA, though I can only surmise that is old school before temp compensation was allowed by the designers.
aTerpster will probably chime in.

[toseg]

The next approach at the same airport is RNAV (GPS) Y Rwy 25C. Does not mention any temperature restriction and has LNAV minima. In this one why you can use the VNAV mode? With the same logic you shouldn't. You do only the altitude corrections to ensure the 3 deg. descend angle and use LNAV/VNAV. Any objection?

[wiggy]

*** completely re - edited having reread your question again and having read FDs excellent post below: There is no minimum temp on the chart you mention because it is not a "VNAV procedure" , IMHO the chart/minima is drawn on the basis of you using LNAV and VS or FPA having applied the appropriate corrections i.e. It is what FD describes as a 2D approach. Can you substitute VNAV? What does your ops manual say (we can in certain circumstances)?

On a more general point if no minimum temp is published for a "VNAV procedure" our ops manual says, to paraphrase "In the case of an RNAV approach where no minimum temperature is published....... pilots should assume a minimum temperature of ISA -25 for the use...of VNAV."

FWIW We have uncompensated baro VNAV and are not allowed to alter RNAV altitudes in the FMC.

[FlightDetent]

toseq: there seems to be still a little confusion over the terminology because VNAV is used for two independent things. Let me try to split them so we can be certain of reading the same page. Most likely I'd be preaching to the converted, reader's patience is appreciated.

a) The LNAV+VNAV sub-version of RNP APCH. A stand-alone type of instrument approach procedure. Different from the others in its own manner, similar as with CAT II being different from a simple CAT I approach. Procedures for RNP LNAV+VNAV which is a 3D approach type (=with vertical guidance) require specific design, need to be published as such and operator must be authorized by their CAA to fly them, with all the bells and whistles normally associated such as training for the specifics of the operation.

Maybe it is possible to call this "3D APCH (LNAV+VNAV)" for the purpose of this conversation.

Note 1: The a) above is somewhat obscured by the fact that the printed chart is normally combined for "3D APCH (LNAV+VNAV) and "2D APCH(LNAV only)", the only visible difference being the two boxes of different minima. But they are fundamentally different, just like ILS and LOC approaches that also commonly share one page.

b) The VNAV mode of autoflight vertical guidance. If the on-board system is capable of generating a vertical profile, a mathematical glide path for the final approach, the Flight Director may be set up to follow that guidance and provide steering commands for AP / human at controls.

I'd call this VERTICAL GUIDANCE OF FMS/FD FOR FINAL APPROACH SEGMENT but it is such a mouthful. Settle for VNAV (FD mode)?

Note: This b) is a widespread technology, commonly used to fly NDB and VOR approaches for decades. We let the machine calculate the profile and follow it, whilst checking from the underlying decisive NAVAID approach chart that the altitude / distance is reasonable. Just let the clever box sort out the guesswork of target and desired vertical speed values.


These points should be visible now:


For 3D APCH (LNAV+VNAV) the availability and use of VNAV (FD mode) is a technical pre-requisite. Many other rules apply as well.
A 3D APCH (LNAV+VNAV) may only be flown within the published temperature envelope which actually is an integral and critical part of the procedure's design. In certain cases, only if temperature compensation feature is embedded into the internal logic of VNAV (FD mode), it may be permissible fly 3D APCH (VNAV+LNAV) below the procedure's LO TEMP limit. Or maybe not, rules do vary.
There is no temperature envelope for 2D APCH (LNAV only), similar to VOR APCH. But cold temp corrections, the ordinary variety with ground OAT <= 0 should be applied to all minimum altitudes.
It is entirely possible and actually desirable to fly 2D APCH (LNAV only) using the VNAV (FD mode) feature.



If I read the thread and your questions correctly:

Topic 1 (closed): Can you perform a 3D APCH (VNAV+LNAV) approach outside temp envelope? ->> NO.

Topic 2 (your last posted question): Please re-phrase.

[Skyjob]

To add to confusion, it depends on software logic loaded into FMS as well...
VNAV can be activated/loaded in several methods, depending on which it will fly (e.g. geometric is what is loaded into our FMS).
Geometric will allow VNAV to be used to fly a vertical profile as long as corrections are made.
Note: though VNAV can be flow their minima cannot be used

[agg_karan]

Flight detent has simplified the post very amazingly. THANKYOU.

[oggers]

toseg

First i would like to thank you all very much for your answers . At FCTM is written the following : "Boeing airplanes have uncompensated Baro-VNAV systems and are prohibited from using LNAV/VNAV minima on approach charts when operating outside of published temperature restriction limits. However, if cold temperature altitude corrections are applied as described in the Cold Temperature Operations Supplementary Procedures chapter of the FCOM, descent to the corrected LNAV (MDA) minima is allowed." It says for minima not for VNAV mode.
Is mentioning to descent to LNAV minima but does not clarify if you can use VNAV mode. At the part C of my company, if you are above -15 C you correct the FAF altitude and the DA only by 10% .Not all the altitudes between. Bellow -15 C you correct all the altitudes and the MDA from the classic table. In the FMS you don't insert new points. Just correct the altitude of the existed ones in the FMS.

Your company rules seem to be compliant with the regs. In your example, due to the temperature restriction you must use the LNAV minima (and apply temperature correction to it). But as a commercial operator you must make a CDFA so what vertical mode you use depends on your particular system and SOPs.

However, if you had WAAS (or equivalent type of SBAS) you CAN still use the LNAV/VNAV minima (and apply temperature correction to it).

[toseg]

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 .

[FlightDetent]

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:

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:

[172_driver]

Underfire,

Pretty amazing the understanding of flight procedures exhibited here on PPRuNe....Care to revise?


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.

[wiggy]

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,..

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.

[FlightDetent]

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

[agg_karan]

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.

[Bug Smasher Smasher]

From the FCTM:
For approaches where an RNP is specified, or approaches where a DA(H) is used, the waypoints in the navigation database from the FAF onward may not be modified except to add a cold temperature correction, when appropriate, to the waypoint altitude constraints.

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

[wiggy]

From the FCTM

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.

[galaxy flyer]

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.

[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

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.

[172_driver]

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.

[roulette]

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.