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Temperature correction on Baro-VNAV approach

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Temperature correction on Baro-VNAV approach

Old 4th Nov 2018, 07:43
  #41 (permalink)  

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Originally Posted by InSoMnIaC View Post
Some here must be quite happy to fly into terrain as long as they can quote the relevant AMC when asked at the pearly gates.
I came from the same team, but is this not below the waistline?
  • If there is no obstacle, then there is no nothing to hit.
  • If there is one, or essential terrain, it would be properly taken in an account in the OCA/H value, calculated down to the low limits of the temperature bracket for a "lowest denominator pilot" i.e. uncompensated flight.
With the two in mind

Should you raise the (D/M)DA for an uncompensated, flattened profile, you move the point of decision further from the THR and that
- invalidates your RVR minima values
- makes it harder or less likely to achieve the required visual reference in inclement weather (and uncovers vulnerability to get-homeitis)
- achieves no geometrical purpose in the terms of terrain clearance, since the underlying OCA has been calculated with a non-corrected altimeter in mind.

Simpler is safer, assuming the math has been done. The AMC achieves both goals.

At the same time, 172_driver shows a nice case where simple=safer has them do it the old way, while Sepp above explains the how to get it approved.

What are the loose ends?

Last edited by FlightDetent; 4th Nov 2018 at 09:37.
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Old 4th Nov 2018, 08:58
  #42 (permalink)  
 
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When adjusted, they came in high and hot???? oh well, they landed....That seems okay?
Taken slightly out of context here.

The reason for the terrain warning was obviously high descent rate combined with terrain. They were always above charted altitudes. A correction, had they done so (they weren't required by SOP at the time) would have mitigated the problem.

So don't blame 'fiddling with the FMC' in this case, at least
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Old 4th Nov 2018, 09:21
  #43 (permalink)  
 
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We go through this conversation every year.
I’m now told we do correct the DA for cold, even if within the temp limits.thought that was the whole point of having a temp band ?
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Old 6th Nov 2018, 14:25
  #44 (permalink)  
 
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The problem guys is not the final segment as that has been taken into consideration by the procedure makers wih EASA rules. The problem is the way you get to the final segment. Flying for a certain Red Nosed operator to the most northern parts of Europe it is crucial with cold temperature compensations. So even if the final segment is protected down to the temperature that is published on the chart the way from the initial fix via the intermediate segment is not protected and thus the operator must ensure terrain clearance.

Since the 737 in our case has no way to intercept a glide path from a corrected intermediate segment it will start a descent down to the final approach fix (point) and thus penetrating the terrain clearance. That is why we temp correct all altitudes below temp corrected MSA. What people unfamiliar with flying in cold weather tend to forget is that the true height of the aircraft when flying temp. Corrcted altitudes are the same as in ISA STD atmosphere and actually more correct.

As I saw someone saying they add 10% to all altitudes can be conservative or not enough. 10% is only safe to -10 degress anything bellw that has to be corrected more. And then there is of course the effect wind can have on minimun altitudes. I suggest looking at RNAV X in ENEV and start playing with wind and temperature and you will quickly find the need to correct, otherwise cumulus granitus might appear.
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Old 6th Nov 2018, 15:34
  #45 (permalink)  
 
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Originally Posted by Denti View Post
Completely different case though, the ILS is a fixed geometric path in space, the VNAV approach is not. And if you, in direct breach of the relevant AMC, adjust your DA, you put it quite a bit farther away from the runway, whereas an adjusted DA on an ILS simply puts it where it should be anyway. And yes, the DA is taken into account for the minimum temperature of the procedure, in fact it is one major point there, choose a higher temperature and you can accept a lower DA, chose a lower temperature and you have to choose a higher one.
The source of path is irrelevant, ILS or VNAV, the regulations require you to initiate a go around if not visual with the runway at a given altitude, and this altitude needs to be corrected for cold temperature.
This applies to any type of approach.
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Old 6th Nov 2018, 21:36
  #46 (permalink)  
 
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and this altitude needs to be corrected for cold temperature.
This applies to any type of approach.
I argued in a similar way with my flight ops team several years ago.

Apparently the real DA is correct at the minimum temperature published. The real DA will be higher during warmer temperature operations - hence still safe.
The only downside is there may be less success in summer.
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Old 7th Nov 2018, 01:04
  #47 (permalink)  

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LEVC, are you trolling, good sir? The binding EASA regulation (unless alternative compliance means approved and notified) requires you NOT correct DA on baroVNAV. Valid for 2 years, scrutinized via the NPA process and voted for. Because it is safer. Your last post makes no sense.
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Old 7th Nov 2018, 07:42
  #48 (permalink)  
 
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Originally Posted by LEVC View Post
The source of path is irrelevant, ILS or VNAV, the regulations require you to initiate a go around if not visual with the runway at a given altitude, and this altitude needs to be corrected for cold temperature.
This applies to any type of approach.
Well, that is not true, if you work under EASA rules. And there is a very good reason for that. But then, the only one you hurt by correcting the DA on a Baro VNAV approach is your own airline by making it much less likely in marginal weather to successfully land, and stupid competition is always the best kind. So carry on
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