Heathrow Thread
Only half a speed-brake
Joined: Apr 2003
Posts: 4,459
Likes: 136
From: Commuting not home
There is a system problem for departures from 27L. Modern aircraft fly on lateral navigation modes sequencing waypoints one by one. Dover (DVR4G) and Detling SIDs out of 27L are prescribed as follows: Climb straight ahead, passing DME1 ILL (associated with ILS 27L) turn left to intercept track 140 deg.
In UK it is common to have only one DME transciever for both ILS on one piece of concrete (two runways). Such dme station will be electronically biased to indicate value 0 (zero) at runway thresholds, the position of the transciever being at the exact midpoint between thresholds.
Now, the DVR/DET SID is perfactly flyable on any aircraft equipped with DME interrogator, starting with C-150. However modern jet transprts equipped with FMS navigation fly waypoint chain as described above. Unlike on other SIDs based on ILS dme turn, (except Gatwick 26L) UK AIP does NOT provide the cooridantes of the initial turn point. Hence, the database provider (Jepp in my outfit) will try to re-code the coordinates into a waypoint themselves.
What happened in this very case, is that the first waypoint is coded as D274A - first hint!!! It essentially means that the fix is referenced to a radio station, bearing 274 (rwy heading) and distance 1 NM. But wait, ILL dme is electronically biased!
Indeed, 09R and 27L pavement is 3660m long, i.e. 2 NM. DME reads zero at thresholds, the bias is approx 0,9 NM. (09R threshold displaced by approx 300m). The DVR/DET sid procedure ask pilots to turn at 1 DME reading, this being approximately 1,9 NM from the radio aid position.
My theory is that Jepp people missed the bias and their waypoint D724A is in fact 1 NM from the DME position.
Analysis:
1 degree of longitude at equator is 1855.4m, quite close to 1852m = 1 NM. At 51 deg latitude (LHR 27L, essentially east/west direction) 1 degree represents 1 NM * cos (51deg). This gives 1'35,4'' per NM.
Coordinates of 09R threshold (where dme 09R - IBB reads zero, the same radio station as ILL) are W000 28' 56,49''. Add to this one mile distance (truning point as per procedure) and the turning point coordinates are 000 30' 31'29''.
On the other hand, when you add one mile to the DME ILL/IBB coordinates, you get W000 29' 08,26''. (about 0,2 NM past threshold).
What are the coordinates of D274A? According to Jepp database W000 29' 06.88''. (Honeywell Airbus database, Honeywell Boeing coordinates are different, but VERY close)
Based on my less than precise geometry, D274A is about 1700m short of turn-point coordinates as calculated above, but only 180m (about three times runway width) past the DME station+1NM position.
Case study:
about a week ago my A320 tried to turn at D274A waypoint, but the DME indication was only 0,2D.
Conclusion:
UK AIP does not provide cooridnates of 1 DME ILL turn out of EGLL27L, the waypoint coding is incorrect. Until Jepp realises the error (message on the way) or UK publish coordinates to remove any ambiguity (in most cases they do, apart from 27L LHR and 26L GTW) lateral navigation modes based on Jepp coding must not be used for first turn on DVR/DET departures 27L Heathrow.
Always at your disposal,
FD
(the Ivan from East Europe)
professional replies please here: http://www.pprune.org/forums/showthr...40#post2904040
In UK it is common to have only one DME transciever for both ILS on one piece of concrete (two runways). Such dme station will be electronically biased to indicate value 0 (zero) at runway thresholds, the position of the transciever being at the exact midpoint between thresholds.
Now, the DVR/DET SID is perfactly flyable on any aircraft equipped with DME interrogator, starting with C-150. However modern jet transprts equipped with FMS navigation fly waypoint chain as described above. Unlike on other SIDs based on ILS dme turn, (except Gatwick 26L) UK AIP does NOT provide the cooridantes of the initial turn point. Hence, the database provider (Jepp in my outfit) will try to re-code the coordinates into a waypoint themselves.
What happened in this very case, is that the first waypoint is coded as D274A - first hint!!! It essentially means that the fix is referenced to a radio station, bearing 274 (rwy heading) and distance 1 NM. But wait, ILL dme is electronically biased!
Indeed, 09R and 27L pavement is 3660m long, i.e. 2 NM. DME reads zero at thresholds, the bias is approx 0,9 NM. (09R threshold displaced by approx 300m). The DVR/DET sid procedure ask pilots to turn at 1 DME reading, this being approximately 1,9 NM from the radio aid position.
My theory is that Jepp people missed the bias and their waypoint D724A is in fact 1 NM from the DME position.
Analysis:
1 degree of longitude at equator is 1855.4m, quite close to 1852m = 1 NM. At 51 deg latitude (LHR 27L, essentially east/west direction) 1 degree represents 1 NM * cos (51deg). This gives 1'35,4'' per NM.
Coordinates of 09R threshold (where dme 09R - IBB reads zero, the same radio station as ILL) are W000 28' 56,49''. Add to this one mile distance (truning point as per procedure) and the turning point coordinates are 000 30' 31'29''.
On the other hand, when you add one mile to the DME ILL/IBB coordinates, you get W000 29' 08,26''. (about 0,2 NM past threshold).
What are the coordinates of D274A? According to Jepp database W000 29' 06.88''. (Honeywell Airbus database, Honeywell Boeing coordinates are different, but VERY close)
Based on my less than precise geometry, D274A is about 1700m short of turn-point coordinates as calculated above, but only 180m (about three times runway width) past the DME station+1NM position.
Case study:
about a week ago my A320 tried to turn at D274A waypoint, but the DME indication was only 0,2D.
Conclusion:
UK AIP does not provide cooridnates of 1 DME ILL turn out of EGLL27L, the waypoint coding is incorrect. Until Jepp realises the error (message on the way) or UK publish coordinates to remove any ambiguity (in most cases they do, apart from 27L LHR and 26L GTW) lateral navigation modes based on Jepp coding must not be used for first turn on DVR/DET departures 27L Heathrow.
Always at your disposal,
FD
(the Ivan from East Europe)
professional replies please here: http://www.pprune.org/forums/showthr...40#post2904040
Last edited by FlightDetent; 12th October 2006 at 10:29. Reason: link to tech log
Per Ardua ad Astraeus
Joined: Mar 2000
Posts: 18,575
Likes: 4
From: UK
Cannot comment on the core of your post, but remember that unless the WP is coded in your database as a 'flyover' an FMC will turn early with a turn of 130 degrees for the next track. This is a very common problem with those who insist on LNAV handling for complicated and close-in turns.
HD - I think FD is right! The a/c 'interrogates' the station and the distance is measured on response return time.
HD - I think FD is right! The a/c 'interrogates' the station and the distance is measured on response return time.
Guest
Posts: n/a
HEATHROW DIRECTOR - I knew it was something like that, the cells are a little rusty. Thanks for reminding me. Not a pilot but worked in Brymon Ops years ago and sat on the jumpseat many times climbing out towards the setting sun and launching from two thirds down the length. One Capt got reprimanded @ CDG after being given immediate takeoff clearance whilst on the high speed turnoff and was airborne before entering the runway! Good ol days!
Last edited by Dash-7 lover; 12th October 2006 at 22:09.
Only half a speed-brake
Joined: Apr 2003
Posts: 4,459
Likes: 136
From: Commuting not home
The waypoint is fly-over, left turn, course-to-fix and lateral navigation guidance follows the track precisely in this case. Incorrect waypoint position seems to be the problem here. The plotting proof is excelent idea - however I cannot do it on-line.
Take the map, plot the waypoint, and see how far it is from the threshold. DME reads about 0,2 in aircraft, I calculate it is 0,11 NM past 09R displaced threshold.
Take the map, plot the waypoint, and see how far it is from the threshold. DME reads about 0,2 in aircraft, I calculate it is 0,11 NM past 09R displaced threshold.
Joined: Oct 2004
Posts: 332
Likes: 0
From: bedlam
Finnair MD11 today Heathrow
We were given the pleasure of OH-LGG today passing thru LHR on the way to Paris. It departed around 1000hrs local this morning as Finnair 4831.
Was this a charter? Odd routing, Odd flightnumber and strange to see Finnair MD11s over here!
Was this a charter? Odd routing, Odd flightnumber and strange to see Finnair MD11s over here!
Joined: Mar 2001
Posts: 2,335
Likes: 0
From: Wet Coast
Finnair is having to rejig its flights due to the cabin crew strike. Flights are being combined and or cancelled to ensure there are enough crew. My guess is the MD11 was one of these 'combined' flights.
Joined: Oct 2005
Posts: 570
Likes: 0
From: UK
Wouldn't be surprised if it was flying Beaujolais into the UK.