VNAV in QFE
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VNAV in QFE
Dear Fellows Aviators,
I've had an interesting discussion with a colleague of mine regarding the use of VNAV in QFE. We fly in Russia, where approaches are conducted in reference to QFE. We use Collins FMS 6000 .
My understanding is that LNAV/VNAV approaches are not to be used in QFE,because the NAV database in most FMS is referred to altitudes mean sea level (for obstacle clearance purpose). I might be wrong,so please correct me.
My colleague's opinion is that, we can manually change the altitude constrains in LEGS PAGE to match the height in QFE ,as long as we have set our altimeters to QFE.
I am attaching an approach plate at UUWW. Lets take a look at NDB rwy19 for example?
http://
If we set the approach on our FMS, it will show all altitude constrains in QNH by default i.e all altitudes such as those starting after point OSTIS would be shown as 2610',2280',1950' up to FAF and then 1410 as an MDA.
Now if we set QFE and manually change these altitudes in LEGS PAGE into height, such as 1976',1646',1316' up to FAF and then 776' as for MDA ,how will this affect our profile?
What are your thoughts on this?
I am getting confused on this , so any help explaining would be greatly appreciated.
Many thanks and Happy Holidays!
cskafan
I've had an interesting discussion with a colleague of mine regarding the use of VNAV in QFE. We fly in Russia, where approaches are conducted in reference to QFE. We use Collins FMS 6000 .
My understanding is that LNAV/VNAV approaches are not to be used in QFE,because the NAV database in most FMS is referred to altitudes mean sea level (for obstacle clearance purpose). I might be wrong,so please correct me.
My colleague's opinion is that, we can manually change the altitude constrains in LEGS PAGE to match the height in QFE ,as long as we have set our altimeters to QFE.
I am attaching an approach plate at UUWW. Lets take a look at NDB rwy19 for example?
http://
If we set the approach on our FMS, it will show all altitude constrains in QNH by default i.e all altitudes such as those starting after point OSTIS would be shown as 2610',2280',1950' up to FAF and then 1410 as an MDA.
Now if we set QFE and manually change these altitudes in LEGS PAGE into height, such as 1976',1646',1316' up to FAF and then 776' as for MDA ,how will this affect our profile?
What are your thoughts on this?
I am getting confused on this , so any help explaining would be greatly appreciated.
Many thanks and Happy Holidays!
cskafan
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Might depend on the legal framework in which you work (state of registry), for us it is quite clear, for any approach we fly out of the database we may not change any altitudes after the final approach fix. Therefore the method you mention would be illegal. However, that is our EU OPS take on it. We still fly QNH based in Russia, not QFE.
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c123, not sure which aircraft you're referring to but on airbus equipped with QFE option it's possible. However beware of wrong MDH in case of secondary FPL activation on Honneywell FMS. In case of QNH pin programmed software ONLY it's absolutely a NO GO. In this case selected vertical guidance must be used along with AMSL altitudes. 
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This is an easy one for Boeing. The answer is no. LNAV and/or VNAV is not to be used below transition level/alt with QFE ops. (Boeing FCTM)
Its not acceptable to construct you're own VNAV approach.
Not sure why you carnt ask for a QNH approach though. The Russians are pretty good with that these days and you have everything you need for a QNH approach on that chart
Its not acceptable to construct you're own VNAV approach.
Not sure why you carnt ask for a QNH approach though. The Russians are pretty good with that these days and you have everything you need for a QNH approach on that chart
Last edited by 8che; 20th Dec 2012 at 10:02.
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The question is not whether to ask for a QNH approach,rather if you are to use QFE,can you legally change LEGS PAGE altitudes to those as height?!
And from what I assume, the answer is NOT. However I would love to see what backs up this statement.
cskafan
And from what I assume, the answer is NOT. However I would love to see what backs up this statement.
cskafan
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you must change all the altitude constrains to height for the arrival and departure and missed approach, so that if any turn is based on altitude the fms will handle it correctly.
approach, from faf to threshold are prohibited and non-sense, as the fms calculates the approach path to 50ft to the threshold based on qnh.
most of the times you will get an msg to check fpl altitude as in the fms logic you are going to fly below the airport altitude.
approach, from faf to threshold are prohibited and non-sense, as the fms calculates the approach path to 50ft to the threshold based on qnh.
most of the times you will get an msg to check fpl altitude as in the fms logic you are going to fly below the airport altitude.
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Pilots in Boeing aircrafts are not allowed LNAV/VNAV when using QFE below transition level/altitude...there are possibilities of turns and manoeuvres based on altitudes achieved as conditional waypoints which are " hardwired " and cannot be changed through FMC LSK manipulation.
One can argue that where altitude based conditional waypoints are absent, there is nothing preventing pilots from altering the altitude constraints to enable LNAV/VNAV operation below transition level/altitude but I guess Boeing and the FMC manufacturers are just covering their arses just in case.
The bad and pesky thing about altitude based conditional waypoints is that one also cannot alter them to factor in cold temperature altitude correction. Hopefully I guess the chart designers had already factored that in conservatively when they promulgated their procedures. Strangely, they had never had the caveat that certain SID/STAR not be flown using LNAV/VNAV below certain temperature unlike certain RNAV/GPS minimums that cannot be used as such.
One can argue that where altitude based conditional waypoints are absent, there is nothing preventing pilots from altering the altitude constraints to enable LNAV/VNAV operation below transition level/altitude but I guess Boeing and the FMC manufacturers are just covering their arses just in case.
The bad and pesky thing about altitude based conditional waypoints is that one also cannot alter them to factor in cold temperature altitude correction. Hopefully I guess the chart designers had already factored that in conservatively when they promulgated their procedures. Strangely, they had never had the caveat that certain SID/STAR not be flown using LNAV/VNAV below certain temperature unlike certain RNAV/GPS minimums that cannot be used as such.
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The AFM limitation for Challengers is clear--no VNAV using QFE altimetry. Why is this in doubt?
In my opinion the reason not to use VNAV in QFE operations is ,that because it will violate obstacle clearance of each waypoint. That is why Collins put that into the limitation.
I wanted to see others opinions on that as well. I am really interested in how the FMS was programmed.
Again,I am just thinking out loud here,so if I'm wrong please correct me.
Happy Holidays,
Cskafan
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Sorry, but that is not correct. All of the FMS and data on the charts is based on WGS84 ellipsoid.
The coordinate origin of WGS 84 is meant to be located at the Earth's center of mass; the error is believed to be less than 2 cm.[2]
Here is the difference between WGS84 and MSL....(up to 85m difference)
Ellipsoid Calculator...choose a spot in the ocean if you like...
This will tell you the difference at a location between MSL and the ellipsoid..
From 8260.52 procedure design..
this added..
In the US, the standard used is WGS84 +30.644 meters = MSL
The coordinate origin of WGS 84 is meant to be located at the Earth's center of mass; the error is believed to be less than 2 cm.[2]
Here is the difference between WGS84 and MSL....(up to 85m difference)
Ellipsoid Calculator...choose a spot in the ocean if you like...
This will tell you the difference at a location between MSL and the ellipsoid..
From 8260.52 procedure design..
this added..
In the US, the standard used is WGS84 +30.644 meters = MSL
Last edited by FlightPathOBN; 23rd Dec 2012 at 23:30.
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The concept of a "mean sea level" is in itself rather artificial, because it is not possible to determine a figure for mean sea level for the entire planet, and it varies quite a lot even on a much smaller scale. This is because the sea is in constant motion, affected by the high and low pressure zones above it, the tides, local gravitational differences, and so forth. The best one can do is to pick a spot and calculate the mean sea level at that point and use it as a datum. For example, the Ordnance Survey uses a height datum based on the measurements of mean sea level at a particular gauge at Newlyn, Cornwall from 1915 to 1921[1] for their maps of Great Britain, and this datum is actually some 80 cm different from the mean sea level reading obtained on the other side of the country. An alternative is to base height measurements on an ellipsoid of the entire earth, which is what systems such as GPS do. In aviation, the ellipsoid known as World Geodetic System 84 is increasingly used to define mean sea level. Another alternative is to use a geoid based datum such as NAVD88.
Above mean sea level - Wikipedia, the free encyclopedia
it's the same for practical puposes in aviation.
Above mean sea level - Wikipedia, the free encyclopedia
it's the same for practical puposes in aviation.
Last edited by 9.G; 24th Dec 2012 at 09:34.
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cskfafan,
Sorry to hear you have these kind of pilots in your ranks.
Id be pissed to know, if i were paying big dough, that i would get get monkeys upfront ,especially captains.
Sorry to hear you have these kind of pilots in your ranks.
Id be pissed to know, if i were paying big dough, that i would get get monkeys upfront ,especially captains.
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WGS-84 vs. PZ-90
Just to add additional confusion, in Russia there is AIC 03/08 that discusses the use of the PZ-90 reference datum instead of WGS-84. http://www.caiga.ru/common/AirInter/...009-04_eng.pdf
PZ-90 is so close to WGS-84, but there is a question about whether that is good enough for Airbus and Boeing (and maybe other OEMs?). Those two OEMs have specific guidance in the manuals about the use of other than WGS-84.
More info on PZ-90: http://www.unoosa.org/pdf/icg/2012/t...PZ90-02_v2.pdf
PZ-90 is so close to WGS-84, but there is a question about whether that is good enough for Airbus and Boeing (and maybe other OEMs?). Those two OEMs have specific guidance in the manuals about the use of other than WGS-84.
More info on PZ-90: http://www.unoosa.org/pdf/icg/2012/t...PZ90-02_v2.pdf