Method and system for monitoring a turbojet engine

1. A method of monitoring a turbojet, the method comprising:

acquiring a signal representative of a vibratory level of a rotor during operation of the turbojet from a vibration sensor which is connected to an engine monitoring unit;

acquiring a speed of rotation of the rotor in operation from a tachometer probe mounted on the rotor and connected to the engine monitoring unit;

comparing, using the engine monitoring unit, an amplitude of the vibratory level signal acquired from the vibration sensor with at least one vibratory threshold that is predetermined as a function of the speed of rotation of the rotor; and

in event of the threshold being crossed by an amplitude peak, analyzing, using the engine monitoring unit, the vibratory level signal acquired from the vibration sensor over a defined time window on either side of the amplitude peak to determine whether a phenomenon giving rise to the amplitude peak is a mechanical impact against the turbojet rotor or an electronic disturbance of the vibratory level signal acquired from the vibration sensor by analyzing symmetry of the vibratory level signal or by determining whether the vibratory level signal presents damping following the amplitude peak,

wherein the phenomenon is the mechanical impact if at least one of the amplitude peak being substantially symmetrical or the vibratory level signal presenting damping following the amplitude peak is satisfied, and

wherein the phenomenon is the electronic disturbance if at least one of the amplitude peak being not symmetrical or the vibratory level signal does not present damping following the amplitude peak is satisfied.

2. A method according to claim 1 , wherein the vibratory level signal presenting damping following the amplitude peak is determined by modeling a decrease in the vibratory level signal after the amplitude peak.

3. A method according to claim 2 , wherein the analyzing the signal over the time window comprises calculating a difference between a curve plotting averages of amplitudes of a spectrogram of the signal at each instant over the time window, with an exponential model of that the curve plotting averages of amplitudes.

4. A method according to claim 1 , wherein the predetermined vibration threshold comprises high and low values for the signal over different ranges of rotor rotation speed.

5. A method according to claim 1 , further comprising, after determining that there has been a mechanical impact against the rotor of the turbojet, determining whether an unbalance has appeared in the rotor following the mechanical impact.

6. A method according to claim 5 , wherein the determining that an unbalance has appeared in the rotor comprises:

making a spectrogram of the signal over a time window;

calculating an average of amplitudes of the spectrogram at each instant;

calculating a difference in levels of the average before and after the amplitude peak; and

comparing the difference in level with a predetermined level threshold.

7. A method according to claim 1 , wherein the acquiring the signal and the acquiring the speed of rotation of the rotor are performed continuously during an operating cycle of the turbojet.

8. A method according to claim 1 , wherein the acquiring the signal, the acquiring the speed of rotation of the rotor, and the comparing the amplitude of the signal are performed in real time, while the analyzing the signal is performed in deferred time.

9. A method according to claim 1 , further comprising issuing a maintenance message if it is determined that the phenomenon giving rise to the vibratory event is a mechanical impact against the rotor of the turbojet.

10. A method according to claim 1 , wherein the amplitude peak is determined to be substantially symmetrical if the condition

S

=

Min

⁢

⁢

(



M



,



m

)



Max

⁢

⁢

(



M



,



m

)



≥

c

is satisfied, where M is a positive maximum value taken by the amplitude peak, m is a minimum negative value taken by the amplitude peak, and c is a predetermined parameter.