Originally Posted by
treadigraph
Aerospark, I have always assumed they are held low when necessary to get underneath the Epsom hold and occasionally the Biggin hold, and the traffic from both routing to join the approach. Presumably Gatwick traffic also can have an effect on outbound altitudes and routings which seem to vary considerably throughout the day.
Incidentally, I was awake very early this morning and had a squizz at Flight Tracker; seemed to be more than usual amount of heavy traffic in all four Heathrow holds; wonder if there was a delay or perhaps some of the trans-Atlantics had very favourable winds...
What also might be a factor is that the end point of the DET SID is relatively a long way from LHR, so modern FMSs will moderate the climb rate. If the end point of the SID was brought closer to the airport (truncations, which have already occurred with WOBUN/BUZAD changing to UMLAT/ULTIB and SAM changing to GOGSI/GASGU) then this might result in steeper climb gradients.