Possibly an intelligent guess at that time
I asked my VC10 base trainer and he seemed to think we were told that it was to do with windshield heat and strength which could be the answer as the heating capacity was probably limited and with decreasing oat the screens became slightly less impact resistant, he quoted the adverse with Concorde which had a vne of 300 knots at sea level which increased to 400 knots BUT
one needs to realise the experimental nature of designing post war aircraft and old wives tales.
My father in law was stationed at Luftfahrtforschungsanstalt Volkenrode after liberating Belsen.
The allies divied up the German wind tunnels which were far in advance of ours. I met a guy last year who worked on the supersonic one designing the inlet doors of concorde. (RAS Toulouse.)
In my gliding days we reduced VNE with altitude due to reducing flutter speed wrt IAS. Imho that could be the reason for the reduction up to 20,000ft but it could still be a risk assessment of bird strike with altitude.
Even posters here have no realistic idea of how high birds fly and there was an accident with a bird strike in cruise above 30,000ft over Africa iirc.
Typical misinformation is on a thread concerning aquaplaning speeds calculated with tyre pressure.
I was on the BALPA tech committee in 1974, it was obvious that the pressure theory was relevant to smooth tyres and the speed that we slammed Tridents onto a wet runway was way above the then theoretical aquaplaning speed. Hence the procedure was a waste of time but 20 years on the myth had been adopted by Big Airlines. I don't know whether they still do it but its a waste. Proof is that I did a bike track day in torrential rain near Marseilles with less than 30psi and chickened out at 200+kph - without aquaplaning. (The technique was unheard of in Swissair).
On a sideways note a VC10 went way above VNE after a badly handled smoke incident.
Last edited by blind pew; 23rd Jan 2020 at 22:10.