B744F off the runway in YHZ
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This was definitely a landing where margins were tight. According to the report, despite all the notams, the longer runway(runway 23), which was into wind, actually did have an available approach for landing(NDB and LNAV). Being nighttime with the weather at non-precision minima and a wet surface, this would seem ideal except for one thing, the threshold was displaced by 1767 feet, something that would seem unusual for any large aircraft to land on, especially a 747. That being said, a 757 did land on the long runway earlier in the night.
With the decision to land on the 7700’ runway 14 with a 20 knot crosswind having been made, there was little room for error. But an accumulation of errors was made, starting with a belief that there was a reasonable margin for this landing, a landing that was not at a particularly light weight. It seems surprising that the captain would choose a flaps 25 landing versus flaps 30. While new on type, the report states that he “had more than 1000 hours PIC experience flying large aircraft worldwide” leading one to think that he would recognize that this was a short runway for an aircraft like the 747. The choice of flaps 25 instead of flaps 30 added 6 knots to the approach speed.
Unlike the other two 747’s in the fleet, this aircraft had performance charts that showed unfactored landing distances, meaning that there was no 15% safety margin added to the actual landing distance values. This information was mentioned on the performance chart but only in the fine print below the calculation table. In addition, the fine print mentions an air time of only 4.22 seconds from crossing the threshold to touchdown, something that’s unlikely to happen in the real world and is for Boeing test pilots(I believe this allowance was removed when TALPA came into effect). It is a good example of how valuable it can be to read the fine print which frequently has critical information that is not obvious.
The crew calculated a landing distance of 6000’ while the TSB calculated 6375’. What adjustment that was missed by the crew is not entirely known. There was an error that catches some pilots, an unnecessary wind additive to the approach speed. There was no headwind component, only a crosswind at 90° when the landing performance calculation was made. Yet the crew added another 5 extra knots to the approach making the Unfactored landing distance 6735’. Even when a 5 knot gust was reported later on, it need not have been considered as the crosswind had become a tailwind and gusts do not affect approach speed calculation with a tailwind. Therefore, with the flap 25 choice and the adjustment for the winds, there were eleven extra, unnecessary knots on the calculated approach speed.
Upon reaching the final approach, the wind had changed direction to become a quartering tailwind, adding close to another 500’ to the landing distance. In reality, they had an approach that had almost no room for error for what were certification test performance landing numbers. They confirmed that they were unaware of the small performance margin as they did discuss the new tailwind but appeared satisfied to continue on.
According to the report, there was ‘heightened anxiety’ in the comments made by the crew on short final but that was related to the crosswind. Unfortunately, their indicated speed was 9 knots faster than even the planned, unnecessarily high approach speed and also were slightly high by the time they crossed the threshold. That being said, they were able to touch down at 1350’ past the threshold which maybe could have somehow allowed a stop just by the end of the runway.
However, despite only a 15 knot crosswind(perhaps the wind did pick up with reports averaging it out), the landing was obviously not handled well and they touched down ‘firmly’, crabbed almost 5° and likely struck one of the nacelles. Although Boeing allows a crabbed touchdown, there appears to have been enough of a lateral displacement after touchdown that the report states ‘preventing a runway side excursion became their priority’. Perhaps the aircraft was drifting to the right upon touchdown due to a corrective action after being blown downwind of the centerline.
The control difficulties on landing led to other events on the rollout resulting in decelerating devices not being activated promptly. The No. 1 thrust lever which had been closed, was advanced forward of idle. One wonders if this was due to the pilot accidentally knocking with his hand in a rush for reverse while trying to control the aircraft. Because of this, no reverse was initially activated on the No. 1 engine and the speedbrakes retracted. The air-ground logic switches in the landing gear changed momentarily to Air Mode during this time period. The No. 1 thrust lever was brought to idle which allowed the speedbrakes to deploy and its reverser to be deployed but then the autobrakes disengaged for reasons unexplained, although large rudder pedal displacements were being used. Maximum reverse was selected but the captain was initially unaware of the autobrake disengagement. Manual brake application began 8 seconds after touchdown, but maximum braking effort did not occur until 15 seconds later, when the aircraft was 800 feet from the end of the runway.
Perhaps in the end, one needs to recognize when an approach has little margin for error, which may only be detailed in the fine print. Once that is recognized, one needs to ensure that in such situations, if any factor moves to a negative performance result, the approach is abandoned. In this case, a new tailwind with the approach speed faster than planned while slightly high on approach should have been enough to trigger a go-around. As for the touchdown and rollout, that is a pilot skill requirement. In this case, it needed a better level of proficiency, something that 110 hours of long haul flying on type may not give you.
With the decision to land on the 7700’ runway 14 with a 20 knot crosswind having been made, there was little room for error. But an accumulation of errors was made, starting with a belief that there was a reasonable margin for this landing, a landing that was not at a particularly light weight. It seems surprising that the captain would choose a flaps 25 landing versus flaps 30. While new on type, the report states that he “had more than 1000 hours PIC experience flying large aircraft worldwide” leading one to think that he would recognize that this was a short runway for an aircraft like the 747. The choice of flaps 25 instead of flaps 30 added 6 knots to the approach speed.
Unlike the other two 747’s in the fleet, this aircraft had performance charts that showed unfactored landing distances, meaning that there was no 15% safety margin added to the actual landing distance values. This information was mentioned on the performance chart but only in the fine print below the calculation table. In addition, the fine print mentions an air time of only 4.22 seconds from crossing the threshold to touchdown, something that’s unlikely to happen in the real world and is for Boeing test pilots(I believe this allowance was removed when TALPA came into effect). It is a good example of how valuable it can be to read the fine print which frequently has critical information that is not obvious.
The crew calculated a landing distance of 6000’ while the TSB calculated 6375’. What adjustment that was missed by the crew is not entirely known. There was an error that catches some pilots, an unnecessary wind additive to the approach speed. There was no headwind component, only a crosswind at 90° when the landing performance calculation was made. Yet the crew added another 5 extra knots to the approach making the Unfactored landing distance 6735’. Even when a 5 knot gust was reported later on, it need not have been considered as the crosswind had become a tailwind and gusts do not affect approach speed calculation with a tailwind. Therefore, with the flap 25 choice and the adjustment for the winds, there were eleven extra, unnecessary knots on the calculated approach speed.
Upon reaching the final approach, the wind had changed direction to become a quartering tailwind, adding close to another 500’ to the landing distance. In reality, they had an approach that had almost no room for error for what were certification test performance landing numbers. They confirmed that they were unaware of the small performance margin as they did discuss the new tailwind but appeared satisfied to continue on.
According to the report, there was ‘heightened anxiety’ in the comments made by the crew on short final but that was related to the crosswind. Unfortunately, their indicated speed was 9 knots faster than even the planned, unnecessarily high approach speed and also were slightly high by the time they crossed the threshold. That being said, they were able to touch down at 1350’ past the threshold which maybe could have somehow allowed a stop just by the end of the runway.
However, despite only a 15 knot crosswind(perhaps the wind did pick up with reports averaging it out), the landing was obviously not handled well and they touched down ‘firmly’, crabbed almost 5° and likely struck one of the nacelles. Although Boeing allows a crabbed touchdown, there appears to have been enough of a lateral displacement after touchdown that the report states ‘preventing a runway side excursion became their priority’. Perhaps the aircraft was drifting to the right upon touchdown due to a corrective action after being blown downwind of the centerline.
The control difficulties on landing led to other events on the rollout resulting in decelerating devices not being activated promptly. The No. 1 thrust lever which had been closed, was advanced forward of idle. One wonders if this was due to the pilot accidentally knocking with his hand in a rush for reverse while trying to control the aircraft. Because of this, no reverse was initially activated on the No. 1 engine and the speedbrakes retracted. The air-ground logic switches in the landing gear changed momentarily to Air Mode during this time period. The No. 1 thrust lever was brought to idle which allowed the speedbrakes to deploy and its reverser to be deployed but then the autobrakes disengaged for reasons unexplained, although large rudder pedal displacements were being used. Maximum reverse was selected but the captain was initially unaware of the autobrake disengagement. Manual brake application began 8 seconds after touchdown, but maximum braking effort did not occur until 15 seconds later, when the aircraft was 800 feet from the end of the runway.
Perhaps in the end, one needs to recognize when an approach has little margin for error, which may only be detailed in the fine print. Once that is recognized, one needs to ensure that in such situations, if any factor moves to a negative performance result, the approach is abandoned. In this case, a new tailwind with the approach speed faster than planned while slightly high on approach should have been enough to trigger a go-around. As for the touchdown and rollout, that is a pilot skill requirement. In this case, it needed a better level of proficiency, something that 110 hours of long haul flying on type may not give you.
Last edited by punkalouver; 28th Nov 2021 at 12:29.
Only half a speed-brake
Much appreciated. A tale of many swiss-cheese holes.
There doesn't seem to be any single master error of the type you would not get away with on a normal day.
More like a snow-ball of stinky choices. Those where the TRE plucks his hair but cannot really fail a candidate for any single one of them.
Or? Myself not a checker...
There doesn't seem to be any single master error of the type you would not get away with on a normal day.
More like a snow-ball of stinky choices. Those where the TRE plucks his hair but cannot really fail a candidate for any single one of them.
Or? Myself not a checker...
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I would be curious to know if any 747 drivers out there have an opinion on the flying the other approach to runway 23 with an almost 2000 foot displaced threshold.
Would you have chosen that approach that night based on the conditions at that time(500 foot ceiling at minimums at night and possibly raining).
Would you have chosen that approach that night based on the conditions at that time(500 foot ceiling at minimums at night and possibly raining).
Originally Posted by punkalouver;11148011.....
Being nighttime with the weather at non-precision minima and a wet surface, this would seem ideal except for one thing, the threshold was displaced by 1767 feet, something that would seem unusual for any large aircraft to land on, especially a 747. That being said, a 757 did land on the long runway earlier in the night.
Could someone explain the significance of the displaced threshold in this discussion ? Are you saying it's OK to land before the displaced threshold under such circumstances? If so that seems odd, presumably the threshold is displaced for a reason. If not, how does the fact that the threshold is displaced affect the discussion - the landing distance available is the landing distance available.
I don’t think anybody here is suggesting that.I think the real significance of the “displaced threshold” comment is in the context of the NOTAMS (3 pages just for that runway) - it may well have been one of the several factors that led to the crew constructing a seemingly unshakeable mental model
that 23 was simply not available…From the performances POV even taking the displaced threshold into account there was just over 8700 feet of landing distance available on 23..verses the 7700 feet on the accident runway…and of course 23 would have had them landing into wind albeit having flown a non-precision approach, if I may use that term…
Rules regarding hindsight of course apply…
ETA, having worked back up the thread…re F25 vs. F30…we know that use of F30 should have been a given because of performance even before the wind turned out to be not as forecast. With that in mind from a human factors POV it would be very interesting to know what the company culture was, if there was any, to landing flap setting?
For flap life reasons F25 was pretty much the routine setting at the company I worked at and we only used F30 when conditions demanded it. I wonder how often the accident PF, 1000 hours on type, had performed a F30 landing?
Last edited by wiggy; 29th Nov 2021 at 12:19.
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But the report does discuss that runway 23 with its 1787' displacement was available for landing as if it was an obvious option for the crew. However, there is no discussion about whether it was realistic to expect a 747 to be landed on a runway like this. The weather was at minimums and it was nighttime. If this approach(LNAV or NDB) was to be flown, I would assume the crew would use VNAV with a 3° descent angle, which would put them in position for landing as if there was no displaced threshold. At 500' above the airport, the crew would have to disconnect the autopilot and then do some sort of level off or reduced descent rate maneuver to land further down the runway.
Perhaps no big deal, but I am just wondering if anybody in the larger aircraft types has done this sort of thing.........or if they were going into Halifax that night would they have chosen runway 14 even if they knew 23 was available.
Only half a speed-brake
Although my type fits inside the 74F,
Standard circle-to-land / Canarsie / KaiTak requires rolling out on the centre line around 500.
Performance wise, nominal touchdown point is typically 1500 ft past the threshold. Especially with heavies, observation shows around 2000' is the daily bread.
To compensate for the displacement discussed, adjusted profile is 130 ft i.s.o. 50 over the fence.
Does not sound too much of an issue, really.
Standard circle-to-land / Canarsie / KaiTak requires rolling out on the centre line around 500.
Performance wise, nominal touchdown point is typically 1500 ft past the threshold. Especially with heavies, observation shows around 2000' is the daily bread.
To compensate for the displacement discussed, adjusted profile is 130 ft i.s.o. 50 over the fence.
Does not sound too much of an issue, really.
“ I would assume the crew would use VNAV with a 3° descent angle, which would put them in position for landing as if there was no displaced threshold. At 500' above the airport, the crew would have to disconnect the autopilot and then do some sort of level off or reduced descent rate maneuver to land further down the runway”
Given the NOTAMS I wouldn’t assume they would use VNAV….
It would LNAV and probably some other vertical mode such as V/S to fly a continuous descent…
I have not looked at the plates for what was available for 23 on the night but agree with FlightDetent’s thoughts..
BTW going back to my previous thoughts/comments re F25 vs F30 - whilst F30 almost certainly wouldn’t have saved things on the night it’s perhaps worth considering the choice of F25 and not F30 on a limiting”ish runway was a major source of comment in the report published post the Qantas 744 Bangkok gardening exercise about 20 years ago.
Given the NOTAMS I wouldn’t assume they would use VNAV….
It would LNAV and probably some other vertical mode such as V/S to fly a continuous descent…
I have not looked at the plates for what was available for 23 on the night but agree with FlightDetent’s thoughts..
BTW going back to my previous thoughts/comments re F25 vs F30 - whilst F30 almost certainly wouldn’t have saved things on the night it’s perhaps worth considering the choice of F25 and not F30 on a limiting”ish runway was a major source of comment in the report published post the Qantas 744 Bangkok gardening exercise about 20 years ago.
Last edited by wiggy; 30th Nov 2021 at 07:02.
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As they have flown the freighter close to MLW before I am sure they have experience using F30. F30 or F25 wouldn’t make a huge difference anyhow. A couple of kts and a hardly noticeable change in attitude compared to types with bigger differences in landing flaps. Maybe they opted for F25 due to x-wind of 15 kts, who knows…
Ultimately who knows, there was a lot suboptimal that night…still a F30 with a resultant 6 knot drop in Vref might have been worth having if the crew had really been aware edgy things were but they didn’t ( though the pre-departure landing check must have rung bells).
As you say there’s not much attitude difference between the two settings, must admit never had an issue with F30 in crosswinds…
FWIW as I mentioned upthread the whole subject of F25/F30 got a bit of a wringing out in this…though mostly in the context of contaminated runways.
BKK overrun report
Page 33 onwards and a whole annex to itself as well, Annex H…
As you say there’s not much attitude difference between the two settings, must admit never had an issue with F30 in crosswinds…
FWIW as I mentioned upthread the whole subject of F25/F30 got a bit of a wringing out in this…though mostly in the context of contaminated runways.
BKK overrun report
Page 33 onwards and a whole annex to itself as well, Annex H…
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According to the report, flaps 25 was used due to the belief that they had sufficient margin, not due to crosswind considerations. 15 knots at flaps 30 is not an issue.
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The difference was 6 knots, which is significant in a situation like this, even if the crew was unaware of this.
According to the report, flaps 25 was used due to the belief that they had sufficient margin, not due to crosswind considerations. 15 knots at flaps 30 is not an issue.
According to the report, flaps 25 was used due to the belief that they had sufficient margin, not due to crosswind considerations. 15 knots at flaps 30 is not an issue.
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“ I would assume the crew would use VNAV with a 3° descent angle, which would put them in position for landing as if there was no displaced threshold. At 500' above the airport, the crew would have to disconnect the autopilot and then do some sort of level off or reduced descent rate maneuver to land further down the runway”
Given the NOTAMS I wouldn’t assume they would use VNAV….
It would LNAV and probably some other vertical mode such as V/S to fly a continuous descent…
Given the NOTAMS I wouldn’t assume they would use VNAV….
It would LNAV and probably some other vertical mode such as V/S to fly a continuous descent…
Would anybody build their own VNAV descent complying with all crossing altitudes and perhaps that 130' threshold crossing altitude that Flight Detent mentioned.
SaulG …Agreed.
punkalouver - I’d be pretty sure the availability of approaches played into this but what the incident crew were faced with wasn’t that unusual. I suspect many of us here have looked at NOTAMs and seen situations at destinations or alternates where due WIP you’ve potentially got no ILS, reduced landing distance and either no PAPIs or a temporary set..
At the end of the day a traditional non-precision approach is flyable in the 74 and routinely demonstrated on recurrent checks…In many instances LNAV can be used to assist the lateral tracking leaving a bit more capacity to handle the vertical profile using e.g. V/S.
As for VNAV - as I recall it on the likes of the 744/777 you weren’t allowed to build your own VNAV procedure, amending preprogrammed altitude constraints was not allowed.
punkalouver - I’d be pretty sure the availability of approaches played into this but what the incident crew were faced with wasn’t that unusual. I suspect many of us here have looked at NOTAMs and seen situations at destinations or alternates where due WIP you’ve potentially got no ILS, reduced landing distance and either no PAPIs or a temporary set..
At the end of the day a traditional non-precision approach is flyable in the 74 and routinely demonstrated on recurrent checks…In many instances LNAV can be used to assist the lateral tracking leaving a bit more capacity to handle the vertical profile using e.g. V/S.
As for VNAV - as I recall it on the likes of the 744/777 you weren’t allowed to build your own VNAV procedure, amending preprogrammed altitude constraints was not allowed.
Last edited by wiggy; 1st Dec 2021 at 08:02.
Notwithstanding that there may have been some catastrophic failure of one or several retardation systems: would it be reasonable to assume that this crew had had ample time during the cruise to read the NOTAMS* and calculate their LDR in the conditions at Halifax - especially given that their desired runway, (with an ILS), was not the longest available?
Would it also be reasonable to assume they had EFBs, giving them a pretty accurate LDR and margin.? **
I ask because the decision of the Captain to continue to use the out-of wind runway - right on the limits - and use less than maximum landing flap, rather than an NPA onto a more into-wind runway, seems very questionable to me.
Tailwinds are very insiduous - even a few knots may sound insignificant, but as we know can have a large effect on the LDR.
*NOTAMS are often criticised for their number, relevance and awkward presentation: Perhaps ICAO should consider reviewing the format to present clearer information, more easily assimilated. For example, on runway related NOTAMS, add a simple statement at the beginning of each to state that "RWY XX is open but subject to the following restrictions":............
** Maybe EFBs could be modified to calculate a maximum wind acceptable in the given landing conditions; so a crew would instantly have a displayed go/no-go wind figure for when ATC gives the final wind with the landing clearance. Maybe some EFBs do, but the ones I used, (in three airlines) did not AFAIK.
Would it also be reasonable to assume they had EFBs, giving them a pretty accurate LDR and margin.? **
I ask because the decision of the Captain to continue to use the out-of wind runway - right on the limits - and use less than maximum landing flap, rather than an NPA onto a more into-wind runway, seems very questionable to me.
Tailwinds are very insiduous - even a few knots may sound insignificant, but as we know can have a large effect on the LDR.
*NOTAMS are often criticised for their number, relevance and awkward presentation: Perhaps ICAO should consider reviewing the format to present clearer information, more easily assimilated. For example, on runway related NOTAMS, add a simple statement at the beginning of each to state that "RWY XX is open but subject to the following restrictions":............
** Maybe EFBs could be modified to calculate a maximum wind acceptable in the given landing conditions; so a crew would instantly have a displayed go/no-go wind figure for when ATC gives the final wind with the landing clearance. Maybe some EFBs do, but the ones I used, (in three airlines) did not AFAIK.
Last edited by Uplinker; 1st Dec 2021 at 10:56.
Only half a speed-brake
punkalouver There is a lot of confusion about what VNAV and LNAV actually are and what are they not.
The VNAV declared not available by ANSP is the result of the profile not taking you to the touchdown point. That invalidates the Instrument Approach Procedure, which for VNAV by definition must bring you with a constant slope to 50' over the threshold.
During VNAV+LNAV I.A.P you don't need(*) to check ALT-DIST as the sensor-instrument-autoflight-database combo is certified to be trusted. Just like an ILS approach (with its own set of pre-conditions). On the day the VNAV would still work fine only bring them to the original threshold.
With LNAV you rely on lateral guidance from the FMC but the vertical profile becomes your task to solve, that's what the ALT-DIST table is for. Then you fly a target 3° profile along the charted values same as any other 2D IAP. Think of LOC-only approach.
The ILS v.s. LOC analogy is helpful since, in a similar fashion,
- the 2 separate (VNAV+LNAV) and (LNAV) procedures are printed on the same sheet of paper
- the 2 separate (VNAV+LNAV) and (LNAV) procedures are completely identical laterally and vertically - trajectory wise
- the 2 separate (VNAV+LNAV) and (LNAV) procedures have different minima (DA/DDA/MDA+buffer/MDA)
- the 2 separate (VNAV+LNAV) and (LNAV) procedures require different authorisation (crew training, on-board installation, quality assurance of the database)
There's a small catch about the vertical guidance provided on the flight deck. For a LOC-only approach you would not the GP (most cases). Since the lateral and vertical profiles are both generated by the FMC onboard a VNAV+LNAV capable ship, when you fly LNAV-only, none of the displays, data and automation modes normally associated with VNAV is degraded in any way. Here the difference between
- flying the certified computer-generated profile and trusting it without cross-check (VNAV) and
- flying the ALT-DIST table with your own brainwork, using the FMC distance information to assure obstacle clearance (LNAV) - while the verical profile for VNAV remains visible and accurate
gets very very thin. But it is there! Yes, unless we talk low temperatures outside the IAP envelope, flying LNAV on a VNAV+LNAV capable aeroplane is pretty much only a state of mind.
This comes BEFORE we discuss which automation modes are available for each of the two options. Their overlap is brutal and the confusion is made worse by the fact that the most advanced vertical guidance mode of AP/FD is also called 'VNAV'.
(*)+(**) any explanation on this probably deserves a standalone thread.
- - - - - - - part 2- - - - - - - - - - - - - - - - - - - - - - - - - -
What wiggy explains about the no-modification rule for VNAV is a certification/authorisation requirement, breach of system integrity.
The easiest, most straightforward ways of flying an LNAV approach to a displaced threshold are to
a) Use the VNAV autopilot mode to drive the aeroplane's vertical trajectory along the self-generated profile, while the brain is manually checking the ALT-DIST table (LNAV rules!!), as if nothing was amiss. Once visual, disconnect and manually deviate 80' above the profile, then fly the airplane towards the desired touchdown point.
b) Use the V/S autopilot and while manually checking the ALT-DIST table (LNAV rules) deliberately stay 80' above the checkpoint altitudes. This takes you down the perfect profile for the displaced landing.
Obviously, had there been a TEMPO LNAV procedure with officially published corrected altitudes, the 'A' above would be illegal.
The VNAV declared not available by ANSP is the result of the profile not taking you to the touchdown point. That invalidates the Instrument Approach Procedure, which for VNAV by definition must bring you with a constant slope to 50' over the threshold.
During VNAV+LNAV I.A.P you don't need(*) to check ALT-DIST as the sensor-instrument-autoflight-database combo is certified to be trusted. Just like an ILS approach (with its own set of pre-conditions). On the day the VNAV would still work fine only bring them to the original threshold.
With LNAV you rely on lateral guidance from the FMC but the vertical profile becomes your task to solve, that's what the ALT-DIST table is for. Then you fly a target 3° profile along the charted values same as any other 2D IAP. Think of LOC-only approach.
The ILS v.s. LOC analogy is helpful since, in a similar fashion,
- the 2 separate (VNAV+LNAV) and (LNAV) procedures are printed on the same sheet of paper
- the 2 separate (VNAV+LNAV) and (LNAV) procedures are completely identical laterally and vertically - trajectory wise
- the 2 separate (VNAV+LNAV) and (LNAV) procedures have different minima (DA/DDA/MDA+buffer/MDA)
- the 2 separate (VNAV+LNAV) and (LNAV) procedures require different authorisation (crew training, on-board installation, quality assurance of the database)
There's a small catch about the vertical guidance provided on the flight deck. For a LOC-only approach you would not the GP (most cases). Since the lateral and vertical profiles are both generated by the FMC onboard a VNAV+LNAV capable ship, when you fly LNAV-only, none of the displays, data and automation modes normally associated with VNAV is degraded in any way. Here the difference between
- flying the certified computer-generated profile and trusting it without cross-check (VNAV) and
- flying the ALT-DIST table with your own brainwork, using the FMC distance information to assure obstacle clearance (LNAV) - while the verical profile for VNAV remains visible and accurate
gets very very thin. But it is there! Yes, unless we talk low temperatures outside the IAP envelope, flying LNAV on a VNAV+LNAV capable aeroplane is pretty much only a state of mind.
This comes BEFORE we discuss which automation modes are available for each of the two options. Their overlap is brutal and the confusion is made worse by the fact that the most advanced vertical guidance mode of AP/FD is also called 'VNAV'.
(*)+(**) any explanation on this probably deserves a standalone thread.
- - - - - - - part 2- - - - - - - - - - - - - - - - - - - - - - - - - -
What wiggy explains about the no-modification rule for VNAV is a certification/authorisation requirement, breach of system integrity.
The easiest, most straightforward ways of flying an LNAV approach to a displaced threshold are to
a) Use the VNAV autopilot mode to drive the aeroplane's vertical trajectory along the self-generated profile, while the brain is manually checking the ALT-DIST table (LNAV rules!!), as if nothing was amiss. Once visual, disconnect and manually deviate 80' above the profile, then fly the airplane towards the desired touchdown point.
b) Use the V/S autopilot and while manually checking the ALT-DIST table (LNAV rules) deliberately stay 80' above the checkpoint altitudes. This takes you down the perfect profile for the displaced landing.
Obviously, had there been a TEMPO LNAV procedure with officially published corrected altitudes, the 'A' above would be illegal.
Last edited by FlightDetent; 1st Dec 2021 at 18:11.
