1,225,691. Flight-path computers. CSFCOMPAGNIE GENERALE DE TELEGRAPHIE SANS FIL. 31 Jan., 1968 [8 Feb., 1967], No. 5090/68. Heading G4G. In computing apparatus determining deviation between the angular flight-path of an aircraft and a selected descent or blimb path AS (Fig. 1) angled to the horizontal when the aircraft is at required height H and angular distance D from a DME transmitter E algebraically spaced C from the intersection of the selected flight-path with the ground at S, it is shown that H#(C+D)#, so that (Fig. 3, not shown) a resistive potentiometer energized from an A.C. reference source and set in accordance with # energizes a transformer having centre tapped and earthed secondary energizing a resistive potentiometer set algebraically in accordance with C, and a further resistive potentiometer set in accordance with D from a DME receiver. The potentiometer outputs are added to produce signal representing required altitude H which is compared with an actual altitude signal from a pressure altimeter to produce an error indication. When transmitter E is displaced from the vertical plane through AS (Fig. 2) so that EAS = #, the azimuth angle of AS is R m and the azimuth angle of AE is p; it is shown that H#[D cos (p - R m Œ#)+C<SP>1</SP>]# where C<SP>1</SP> - E<SP>1</SP>S and E<SP>1</SP> is the projection of E on AS. In Fig. 4, potentiometer 2 is set to 0, potentiometer 4 is set algebraically to C<SP>1</SP>, and potentiometer 5 is set to D. The output D# of the latter energizes a resolver 12 whose rotor is angularly positioned to p - R m Œ# by shaft 13 for multiplication by cos 0, and whose output is added at 7 to that of potentiometer 4 to a signal which represents H, for comparison at 10 with an actual altitude signal H R from 11. Shaft 13 is driven through (p - R m ) by motor 15 together with damping generator 16 and control transformer 14 the outputs of which are combined in amplifier 17 to control the motor; transformer 14, also receiving a signal p - R m from a differential transmitter 18 adjustable in accordance with R m and receiving a signal p from transmitter 20 driven from VOR receiver 22. In a modification (Fig. 5, not shown) an adder receives signals p from VOR receiver, R m from a potentiometer, and control signals from a damping generator and a resistive or inductive potentiometer driven by the shaft, to control the motor driving the same. In a further modification (Fig. 6, not shown), an operational amplifier produces an output u; energizing potentiometers producing signals C<SP>1</SP>u and Du; the latter being multiplied by (p - R m Œ#) in a resolver energized thereby. The resultant signals are combined in the operational amplifier with a signal from an altimeter representing -H, and at balance for substantially null input C<SP>1</SP>u+Du cos(p-R m Œ#)-H=0 so that u represents #<SP>1</SP> of the actual aircraft path, which is compared with a preset signal representing the required path. Other types of computer may be employed, and the difference signal may control an autopilot.