Cazatou
How could you possibly get so confused – the 80 was a suggestion if they had had another ½ mile or so in that power regime – and was a prompt for people with hands on experience with a Mk1 or 2 to estimate a better figure.
Pls read post again and for an understanding of the air speed analysis, I recommend downloading the Boeing document (NOT the simulation) which is freely available on the web:
8-7D20- DSS-0306, Enclosure 4
Dated: June 18, 2002
“Mull of Kintyre – Analysis of Available Data”
Prepared by James Mitchel
Technical Fellow
The Boeing Company
There are detailed calculations therein giving specific values from where I got the 20 kts reduction figure for true air speed but the following extract from the conclusions should answer your query:
<<An average ground speed for the flight, from the Aldergrove ATC fix until the point of impact, was determined by simply dividing distance traveled by time taken. This ground speed was converted to true airspeed by correcting for the effects of wind. The resulting value for average true airspeed was at the higher end of the normal cruise airspeed range.
A more precise, local, value of true airspeed just prior to the point of impact was determined from data retrieved from the SuperTANS that was related to the last steering calculations the unit had performed. This value was lower than the average value determined from the time and distance calculations, and was more consistent with the airspeed value normally used for a cruise climb profile. This leads to a conclusion that the airspeed was reduced as the landmass of the Mull was approached; from a relatively high cruise airspeed to a more normal airspeed as a cruise climb was initiated. It was also determined from the same set of data that, even with this lower airspeed, the aircraft ground speed remained essentially constant as the aircraft approached the Mull. It was concluded that this was due to the effects of the increase in wind strength at the Mull when compared with the wind strength over Northern Ireland and during the sea crossing. >>