Syc
One of the baro alts had a subscale setting that could have been the QFE at Aldergrove; this could have been noted before departure and used for crossing the shoreline at way point A where there is a known LZ and in any case the level swathe of
ground that starts there is at the same elevation as Aldergrove. (This is one of the indicators I used to deduce a plan to cross there, either landing at the LZ or passing over it whilst initiating a turn to swing around the back of the lighthouse and thence back out to the 028 track to Corran – a scenario I believe I have explained some time ago.) I would have thought that the distance was not so great nor the time as to diminish the value of not having to interpret forecasts but to set the subscale to QFE, note it, and use it later for reference to the ground of the same level; of course, this value would/should not have been left on that subscale over the first long leg (Antrim hills, etc) suggesting the local activity.
RC & others
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the clearance for the flight was understood to be 'not above 500 feet on the regional QNH>>
Not getting confused with VFR “500 feet
above the ground”, are you?
You are not seriously suggesting that they went around the Antrim hills, are you? The math is simple enough - do it properly - the most obvious scenario fits - nothing other than a straight leg from Aldergrove to waypoint A gets near a fit; remember also that the all up weight was constrained so as to allow single engine flight (in emergency obviously) which left this flight fuel critical - further, they were short of duty time - together points to straight lines at max comfy cruise which analysis supports.
Had the weather been a bit worse, the alternative route Ald to H (Curran) to B (Corran) was a safe option (low and clear all the way) but with the geometry of the routes (remember that little map I posted yonks back?) going Ald to A (Mull) then back out to that HB track would have been exactly the same fuel and timewise.