I've asked these questions before but not got a satisfactory answer, so I'd be grateful for any answers.The following thread illustrates my question:http://forums.avsim.net/dcboard.php?az=sho..._id=10707&page=It has two flight plans from Heathrow (EGLL) to the same destination.One begins "DCT WOBUN" which I interpret as meaning fly direct to the WOBUN waypoint: the other begins "..WOBUN.. " which I intepret as meaning use a WOBUN SID.(1) Is my interpretation correct? If not what is the difference between them?The UK Manual of Air Traffic Control Services Part 1 (MATS) says "At aerodromes where standard instrument departure routes (SIDs) are designated, any filed flight plan which specifies a non-standard route should be referred back to the originator for correction". My interpretation is that where there is a SID it must be included in the filed flight plan.BAA Heathrow's Flight Evaluation Report 2004/2005 (the latest I can find) states:"Aircraft departing from Heathrow are required to follow one of a number of Noise Preferential Routes (NPRs) which are overlaid on the Standard Instrument Departure Routes (SIDs). ... All aircraft are required to follow an NPR unless instructed by ATC. NPRs lead from the take-off runways to the main UK air traffic routes and form the first part of the SIDs. Compliance with NPR procedures at Heathrow is required up to an altitude of 4,000ft. Once this altitude is reached, an aircraft may be redirected (vectored) off the NPR/SID by ATC to provide it with the most approrpriate routing in the contect of other flights in the area at that time and destination. ATC may also decide to leave the aircraft on the SID."This confirms my interpretation that, generally, all aircraft departing Heathrow must begin by following a SID initially. I assume that the phrase "unless instructed by ATC is simply for completeness and that, because of noise implications, ATC will rarely use it.(2) Should not all filed flight plans for Heathrow departures begin with a SID?

Not having had a reply I've looked further into this.The Heathrow Airport - London (Noise Abatement Rgulations) Notice was made by the Secretary of State of the Department for Transport under his powers under the Civil Aviation Act and has the power of law. Section 2 (8) says "... the aircraft shall follow the Noise Preferential Routeing Procedure....which relates to the ATC clearance previously given...". For flights via Burnham /WOBUN for Rwy27L this is:"Straight ahead to be established on BUR NDB QDM 301 by LON DME 3. At LON DME 7 turn right to follow BUR NDB QDR 359 to abeam BNN VOR (LON DME 16)". This is the first part of the WOBUN 2G SID. The SID has a note stating "SIDs reflect Noise Preferential Routeings."Note 9 to the Regulations is: (9) "Aircraft which have attained an altitude of 4000 ft (Heathrow QNH) may be directed by air traffic controllers onto a different heading...".(My emphasis on shall and may.)My conclusion is that all aircraft leaving Heathrow must follow a SID to an altitude of at least 4000ft.Any comments?Incidentally Note 8 requires aircraft to remain with a lateral swathe of +/- 1.5km of the route centreline in order to be deemed to comply with the NPR.