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)
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http://www.pprune.org/forums/showthr...40#post2904040