The limits had in fact been found in the wind tunnel.
From the official accident report -
"The technique followed in the VC10 stalling programme consisted of taking the aircraft up to or just beyond the angle of incidence at which wind tunnel tests had shown CL
max to occur, so that experience and information would be built up gradually. During the intial stalling tests of the One-Eleven, however, the angles of incidence based upon wind tunnel tests, which were provided as a guide to the test pilots,were considerable exceeded, no allowance for scale effect had been made when establishung these incidence values. Nevertheless if the VC10 stall investigation technique had been closely followed in this case the One-Eleven stalling tests would not have been taken so far, so fast.........................It seems reasonable to conclude, therefore, that as by 25 degs in the wind tunnel tests the nose-down in the pitching moment gave way to a nose-up tendancy, and as the firm had a general background knowledge of stalling problems that had arisen with T-tail aircraft, stalling tests should have been more cautiously approached, more closly controlled and more carefully correlated with wind tunnel and flight recorder data."
The tail parachute -
" Consideration had been given in the case of the One-Eleven, as in that of the VC10, to the fitting of a tail parachute. The matter was being kept under review and no final decision had been made, although it had been intended that a parachute should be fitted before the aircraft made a dymanic stall, thus significantly exceeding the stalling incidence............................Wind tunnel tests carried out by BAC since the accident indicate that with the elevators in effect locked up and with the aircraft in astable stall, a tail parachute of the type it was intended to fit would not have given sufficient pitching moment to provide for recovery."
All the 7 crew wore parachutes. Once stalled the a/c descended virtually horizontal at 10800 fpm with pilots trying recover. " Although it may be expected that there was considerable alarm at the rapid loss of height, it seems reasonable to accept that no question of abandoning the a/c arose until all possibilities of recovery, culminating in the application of full power, had been attempted. When this had been done the a/c was probably at just under 5,000 Ft with less than 30 seconds to go before impact. There is evidence that some attempt was made to abandon the a/c at a very low height, probably far less then 5,000 Ft, since
a) witnesses heard a sharp report, which could have been the firing of the explosive bolts on the forward escape hatch, when they estimated the a/c to be a few hundred feet; after the crash the door was found trapped between the fuselage and the ground in an inverted position still partly covering the door opening and two of the occupants were near this exit.
b) althought the rear ventral door (second escape exit) was in position, two of the occupants were some distance towards it."
The accident site revealed the a/c impacted the ground at a very high ROD and low forward speed, only moving forward 70 Ft.
How do you escape from below an a/c descending at a rate of 10,000 FPM with low fwd airspeed?