Process and device for supplying the turbojets of an aircraft during automatic flight

1. A process for supplying fuel to the turbojets (M 1 to M 4 ) of a multijet aircraft (A) flying in automatic pilot mode, according to which process the fuel supply to each of said turbojets is slaved to a preset value EPRt, common to all said turbojets and corresponding to a particular value of the ratio EPR of the gas pressures at the outlet and at the inlet of the turbojets, said preset value EPRt depending on the automatic thrust order and on the flight conditions of the aircraft,

wherein, when said aircraft (A) is in level flight at steady altitude, said common preset value EPRt is converted into a preset value N 1 t related to the speed of rotation N 1 of the fan of said turbojets and wherein the fuel supply to all said turbojets is slaved to said preset value N 1 t.

2. The process as claimed in claim 1 ,

wherein the replacement of the preset value EPRt by the preset value N 1 t is moreover subject to the condition that the Mach number of the aircraft (A) is greater than a first threshold short of which said turbojets could operate below their performance.

3. The process as claimed in claim 2 ,

wherein said first threshold is at least approximately equal to Mach 0.45.

4. The process as claimed in claim 1 ,

wherein the replacement of the preset value EPRt by the preset value N 1 t is moreover subject to the condition that the differences between, on the one hand, the maximum value of the ratio EPR at which said turbojets (M 1 to M 4 ) can operate and, on the other hand, each of the current values (EPRm. 1 to EPRm. 4 ) of this ratio, measured for each of said turbojets, are greater than a second threshold short of which said turbojets could not maintain the speed of said aircraft.

5. The process as claimed in claim 4 ,

wherein said second threshold is at least approximately equal to 0.02.

6. A system for supplying fuel to the turbojets (M 1 to M 4 ) of a multijet aircraft (A) flying in automatic pilot mode, the driving parameter of each of said turbojets being the ratio EPR of the gas pressures at the outlet and at the inlet of the corresponding turbojet and said aircraft comprising:

means ( 13 to 15 ) for measuring the current value (EPRm. 1 to EPRm. 4 ) of said driving parameter EPR for each of said turbojets;

means ( 16 ) for measuring the current value (N 1 m. 1 to N 1 m. 4 ) of the speed of rotation N 1 of the fan of each of said turbojets,

while said supply system consists of a plurality of supply devices (D 1 to D 4 ) each of which is associated with one of said turbojets and comprises:

a first computer ( 20 . 1 to 20 . 4 ) producing, on the basis of the automatic thrust order and of the current flight conditions, a preset value EPRt for said driving parameter EPR, said preset value EPRt being identical for all the turbojets (M 1 to M 4 );

a slaving device ( 23 . 1 to 23 . 4 ) receiving said preset value EPRt and the corresponding current measured value (EPRm. 1 to EPRm. 4 ) of said driving parameter EPR and tending to align said current measured value with said preset value EPRt;

a second computer ( 26 . 1 to 26 . 4 ) controlled by said slaving device ( 23 . 1 to 23 . 4 ) and computing the flow rate of fuel to be fed to said engine so as to align said current measured value of the driving parameter with the preset value; and

fuel supply means ( 27 . 1 to 27 . 4 ) controlled by said second computer and delivering to said engine the flow rate of fuel computed by the latter,

wherein:

each supply device (D 1 to D 4 ) comprises, in parallel with said device ( 23 . 1 to 23 . 4 ) for slaving to said corresponding preset value EPRt, the arrangement in series:

of a conversion table (T), identical for all said supply devices (D 1 to D 4 ) and connected to said first computer ( 20 . 1 to 20 . 4 ) so as to receive said preset value EPRt, said conversion table (T) being able to convert said preset value EPRt into a preset value N 1 t of said speed of rotation N 1 ; and

of a slaving device ( 24 . 1 to 24 . 4 ) receiving said preset value N 1 t and the corresponding current measured value (N 1 m. 1 to N 1 m. 4 ) of said speed of rotation N 1 intended to align this current measured value with said preset value N 1 t ; and

each supply device (D 1 to D 4 ) comprises, between said devices ( 23 . 1 to 23 . 4 – 24 . 1 to 24 . 4 ) for slaving to the preset values EPRt and N 1 t , on the one hand, and said second computer ( 26 . 1 to 26 . 4 ), on the other hand, a switching means ( 25 . 1 to 25 . 4 ) able to connect to the latter, in a first state, said slaving device ( 23 . 1 to 23 . 4 ) associated with the preset value EPRt, and, in a second state, said slaving device ( 24 . 1 to 24 . 4 ) associated with the preset value N 1 t ; and

said switching means ( 25 . 1 to 25 . 4 ) are controlled in common by a control device ( 29 ) so as to toggle from said first state to said second state when the aircraft (A) is in automatic level flight at steady altitude.

7. The system as claimed in claim 6 ,

wherein said control device ( 29 ) is of the AND gate type, delivering a switching order to said switching means ( 25 . 1 to 25 . 4 ) only when all its inputs ( 30 . a to 30 . d ) are supplied.

8. The system as claimed in claim 6 ,

wherein a timeout device ( 31 ) is disposed between said control device ( 29 ) and said switching means ( 25 . 1 to 25 . 4 ).