METHOD AND DEVICE FOR NOTIFYING AUTHORIZATION TO
COMPLETELY SHUT DOWN AN AIRCRAFT GAS TURBINE ENGINE
Background of the invention
The present invention relates to the general field
of aircraft gas turbine engines for which it is necessary
for the engine to idle for a certain duration on the
ground before causing the engine to be shut down
completely.
In the field of aviation, it is common practice in
utilization manuals for aircraft pilots to indicate that
the engine needs to run at an operating speed known as
''idling'' for a certain duration before causing the engine
to be shut down completely. This duration is referred to
15 as "stabilizing the engine'' and it is associated mainly
with the risk of the oil or fuel in the engine coking on
hot portions of the engine (e.g. on injectors for
injecting fuel into the combustion chamber of the
engine) . In order to avoid excessive coking of such hot
20 portions of the engine, it is thus recommended to leave
the engine operating at an idling speed for a certain
duration before shutting it down completely.
The duration of this engine stabilization stage is
generally predefined as a function of the type of engine
25 and it typically lies in the range 30 seconds to
2 minutes.
However, imposing a fixed stabilization duration
presents numerous drawbacks. Specifically, if it is
desired to cover the worst possible thermal situation for
30 the engine, then the stabilization duration will
necessarily be long, which imposes a waiting time that is
penalizing for the crew of the aircraft before they can
shut the engine down completely. In contrast, if it is
desired to minimize this constraint for the crew, the
35 stabilization duration should be as short as possible,
which can expose the engine to significant coking
2
phenomena that, in the long run, lead to significant
maintenance costs and times.
There therefore exists a need for a method of
determining a stabilization duration at idling speed for
5 an aircraft gas turbine engine prior to complete shutdown
that does not present the above-mentioned drawbacks that
are associated with determining a fixed stabilization
duration.
10 Object and summary of the invention
The present invention satisfies this need in
particular by providing a method of notifying an
authorization to shut down completely an aircraft gas
turbine engine, the method being applied after detecting
15 that the engine has passed to an idling speed, and
comprising:
a) an evaluation step of using a value of a first
operating parameter of the engine to evaluate a value for
a second parameter characterizing a thermal behavior of a
20 part of the engine that might be subjected to coking,
this evaluation being performed by means of a thermal
behavior model for said part;
b) a comparison step of comparing the value of the
second parameter with a predefined threshold value
25 corresponding to a value of the second parameter that
does not lead to coking of said part; and
c) a notification step of notifying authorization to
shut down the engine completely if the value of the
second parameter is lower than the value of the
30 predefined threshold;
else reiterating steps a) to c).
Correspondingly, the invention also provides a
device for notifying authorization to shut down
completely an aircraft gas turbine engine, the device
35 comprising:
evaluation means for using a value of a first
operating parameter of the engine to evaluate a value for
3
a second parameter characterizing a thermal behavior of a
part of the engine that might be subjected to coking,
this evaluation being performed by means of a thermal
behavior model for said part;
5 comparator means for comparing the value of the
second parameter with a predefined threshold value
corresponding to a value of the second parameter that
does not lead to coking of said part; and
notification means for notifying authorization to
10 shut down the engine' completely.
The invention thus provides a sound mechanism for
adapting the stabilization duration of the engine as a
function of the actual use of the engine. By using a
model of the thermal behavior for a part of the engine
15 that might be subjected to coking, ·and merely by having
knowledge of an operating parameter of the engine, this
mechanism (which may be integrated directly in the
electronic computer of the engine) thus makes it possible
in effective manner to determine the necessary duration
20 that the pilot needs to apply prior to shutting down the
engine completely. This mechanism is simple to perform
(it does not require special tools or measurement
instruments, since the operating parameter that it uses
is a parameter that is already monitored for other
25 monitoring functions of the engine), and it makes it
possible to calculate accurately the duration needed to
avoid any excessive coking of the hot portions of the
engine.
By avoiding any risk of excessive coking, the
30 mechanism of the invention thus makes it possible to
reduce the costs and the time required for maintaining
the engine. Furthermore, the mechanism makes it possible
to optimize piloting and flying procedures by making this
engine stabilization stage as simple as possible for the
35 pilot. Finally, with such a mechanism, it is possible to
anticipate maintenance operations if stabilization
durations on the ground have not been complied with, and
4
to keep account of the number of stabilization stages
that have been of long duration.
Advantageously, a value for the second parameter may
be evaluated by calculating a mean value of the first
5 operating parameter of the engine over a moving time
window.
Also advantageously, the method may further comprise
triggering a timer on detecting that the engine has
passed to an idling speed, and notifying authorization to
10 shut down the engine completely when the time that has
elapsed since triggering the timer exceeds a
predetermined threshold duration.
Typically, the predetermined threshold duration is
selected to be a duration for which it is known by
15 experience that it covers the worst possible thermal:
situation for the engine. For example, this threshold
duration may be equal to 2 minutes. Thus, this
additional step makes it possible to shorten the
stabilization duration if the mean value of the operating
20 parameter of the engine takes time to come down below the
predefined threshold value.
The first parameter may be selected from the
following operating parameters of the engine: a
temperature measured in a flow channel for a gas stream
25 between two turbine stages; an ambient temperature
measured at the inlet of the engine; an oil temperature
of the engine; a fuel temperature of the engine; and an
operating speed of the engine.
In addition, the notification authorizing complete
30 shutdown of the engine may consist in a visual display
informing the pilot of one or more of the following
items: authorization to shut down the engine completely;
complete shutdown of the engine without risk of coking;
displaying a previously estimated waiting duration
35 necessary prior to authorizing complete shutdown of the
engine.
5
5
The passage of the engine to an idling speed may be
detected as a result of the pilot of the aircraft
actuating a switch or as a result of detecting an
operating speed of the engine that corresponds to an
idling speed, or a related thermal state.
In a particular embodiment, the various steps of the
method are determined by computer program instructions.
Consequently, the invention also provides a computer
program on a data medium, the program being suitable for
10 being performed in a notification device or more
generally in a computer, the program including
instructions adapted to performing steps of a method as
described above.
The program may use any programming language and be
15 in the form of source code, object code, ... or code
intermediate between source code and object code, such as
.in a partially compiled form, or in any other desirable
form.
The invention also provides a computer readable data
20 medium including computer program instructions as
mentioned above.
The data medium may be any entity or device capable
of storing the program. By way of example, the medium
may comprise storage means, such as a read only memory
25 (ROM), e.g. a compact disk (CD) ROM or a microelectronic
circuit ROM, or indeed magnetic recording means, e.g. a
floppy disk or a hard disk.
Furthermore, the data medium may be a transmissible
medium such as an electrical or optical signal that can
30 be conveyed via an electrical or optical cable, by radio,
or by other means. The program of the invention may in
particular be downloaded from an Internet type network.
Alternatively, the data medium may be an integrated
circuit in which the program is incorporated, the circuit
35 being adapted to execute or to be used in the execution
of the method in question.
6
The invention also provides an aircraft gas turbine
engine including a device as defined above.
Brief description of the drawings
5 Other characteristics and advantages of the present
invention appear from the following description made with
reference to the accompanying drawings, which show an
implementation having no limiting character. In the
figures:
10 · Figure 1 is a flow chart showing .the main steps of
15
a method of the invention for determining a duration for
stabilizing idling; and
· Figure 2 is a graph showing an implementation of
the method of the invention.
Detailed description of the invention
Figure 1 is a flow chart showing the main steps of a
method of the invention for notifying an authorization to
shut down completely an aircraft gas turbine engine.
20 Authorization to shut down completely an aircraft
gas turbine engine, e.g. a commercial airplane turbojet,
takes place after a flight and following stages of
landing an aircraft and taxiing on the ground. The
purpose of such authorization to shut down completely is
25 to ensure that the engine operates for a certain duration
while idling before the engine is shut down completely in
order to avoid any risk of oil or fuel coking hot
portions of the engine, such as for example injectors for
injector fuel into the combustion chamber.
30 The purpose of the method of the invention is to
define a duration for idling operation of the engine
before it is shut down completely, which duration varies
as a function of certain operating parameters of the
engine characterizing the thermal behavior of a part of
35 the engine that might be subjected to coking.
7
To this end, and as shown in Figure 1, the method of
the invention comprises a first step ElO that consists in
detecting whether the engine has passed to idling speed.
This passage to idling speed can be detected as a
5 result of the aircraft pilot actuating a switch, or as a
result of detecting a speed of operation of the engine
that corresponds to an idling speed, where such detection
can be performed on the basis of one of the parameters
seen by the computer, such as an indication of the
10 collective pitch, of the speed of the gas generator, of
the outlet temperature of the turbine, etc.
Once it has been detected that the engine is
operating at idling speed, the method of the invention
makes provision during a step E20 to use a value of a
15 first operating parameter of the engine to evaluate a
value for a second parameter that characterizes the
thermal behavior of a part of the engine that might be
subjected to coking.
The value of the first parameter can be calculated
20 continuously or as a result of an event indicating that
the engine is going to be shut down.
The first operating parameter of the engine used
during this evaluation step E20 typically corresponds to
a temperature measured in the engine in the vicinity of
25 the part that might be subjected to coking.
By way of example, this first operating parameter
may be selected from the following parameters: a
temperature measured in a flow channel for a gas stream
between two turbine stages (referred to below as T45);
30 ambient temperature measured at the inlet of the engine;
an oil temperature of the engine; a fuel temperature of
the engine; an operating speed of the engine; etc.
These examples have the advantage of being
parameters that are already monitored during a mission of
35 an aircraft. Typically, the measured values for these
parameters are delivered continuously during the mission
- .-;
8
to the computer of the engine regulator device in order
to be analyzed therein.
From a model of the thermal behavior of the part for
which the first parameter is measured, a value is then
5 evaluated for a second parameter characterizing a thermal
behavior of that part.
This model of thermal behavior is a function (also
known as integral operator) that makes it possible on the
basis of an input value for an operating parameter (such
10 as a temperature measured in the engine in the~vicinity
of the part), to model the thermal behavior of the part.
15
In an applied implementation of the invention in
which the first operating parameter is the temperature
T45 measured in a flow channel for a gas stream between
two turbine stages of the engine, the thermal behavior
model may be calculating the mean T45MG of the values for
the temperature T45 as measured over a moving time window
of predetermined duration (e.g. of the order of a few
minutes).
20 The evaluation step E20 may be performed throughout
the mission, from starting to complete shut down of the
engine. It may be performed by calculation software
means embedded in the engine computer, thus making it
possible to obtain continuously a value for the second
25 parameter characterizing a thermal behavior of the part
that might be subjected to coking (in the example shown:
T45MG).
In parallel with this evaluation step E20, the
method may make provision in a step E20' to trigger a
30 timer CM from the moment at which it is detected that the
engine has passed to an idling speed.
The following step E30 consists in comparing the
value of the second parameter as obtained in step E20 (in
this example the mean T45MG) with a predefined threshold
35 value (in this example T45thresh). This comparison step
is performed by calculation software embedded in the
engine computer.
9
The threshold value T45thresh corresponds to a value
of the second parameter for which it has previously been
established that coking of the part will not take place.
This threshold value T45thresh is established
5 beforehand from a multi-entry table (e.g. with the
temperature and the pressure at the inlet of the engine,
the tempera·ture of the oil, the temperature of the fuel,
etc.), and as a function of measurements of the engine at
the time it is detected that the engine has passed to an
10 idling speed. Such a table is typically prepared by
experience feedback on the basis of data obtained in
earlier missions for engines of the same family as the
engine in question.
At the end of this comparison step E30, if it is
15 established during a step E40.that the value of the
second parameter T45MG is less than the predefined
threshold value T45thresh, the engine computer notifies
the pilot of authorization to shut down the engine
completely (step E50).
20 In contrast, if the value of the second parameter
T45MG is higher than the predefined threshold value
T45thresh, then steps E30 and E40 are repeated. In other
words, if the value of the second parameter T45MG is
higher than the predefined threshold value T45thresh, the
25 engine computer performs a new comparison between the
mean T45MG as calculated from new values of the
temperature T45 measured over the moving time window and
the predefined threshold value (T45thresh), which value
may possibly vary as a function of time.
30 This process of evaluating the mean T45MG and
comparing it with the predefined threshold value
T45thresh is reiterated until the value of the mean T45MG
becomes lower than the predefined threshold value
T45thresh, whereupon the computer notifies the pilot of
35 authorization to shut down the engine completely.
Furthermore, when a timer CM is started during the
step E20', the method makes provision during the step E40
10
to verify whether the time that has elapsed since the
timer was started exceeds a predetermined threshold
duration (typically of the order of 2 minutes), whereupon
the computer of the engine notifies the pilot of
5 authorization to shut down the engine completely (step
E50). In contrast, if the elapsed time is shorter than
the predetermined threshold duration, then steps E20 to
E40 are repeated, as described above.
The step E50 of notifying the pilot of an
10 authorization to shut dmm the engine completely may take
various forms. In particular, it may consist in a visual
display informing the pilot of one or more of the
following items: authorization to shut down the engine
completely; complete engine shutdown possible without
15 risk of coking (the risk is mentioned·so longcas the.
stabilization prior to shutdown is not sufficient);
displaying a previously estimated waiting time needed
before authorization to shut down the engine completely.
With reference to Figure 2, there follows a
20 description of an implementation of the above-described
steps E20 and E30 of the method.
In this example, the first operating parameter of
the engine is the temperature T45 measured in a flow
channel for a gas stream between two turbine stages of
25 the engine, and the second parameter characterizing a
thermal behavior of a part of the engine that might be
subjected to coking is a mean T45MG of the values of the
temperature T45 as measured in a moving time window of
predetermined duration.
30 Figure 2 plots temperature curves as a function of
time. In this example, there can be seen the curve for
the measured temperature T45 plotted as a continuous
line. On the basis of this temperature T45, the engine
computer calculates a mean value T45MG of the temperature
35 over a moving window, corresponding in this example to
the five most recent measurements taken of the
temperature. The curve for this mean value T45MG is also
5
11
plotted in Figure 2 (this mean value T45MG is calculated
continuously from starting to shutting down the engine)
The mean value T45MG is then compared with a
threshold temperature T45thresh, which in this example is
constant over the entire duration of the stage of
shutting down the engine and is equal to about 940°C. As
soon as the mean value T45MG becomes lower than this
threshold temperature T45thresh, in this example at a
time tstop equal to about 4300 seconds, the engine computer
10 sends a notification to the pilot authori:.cing complete
shutdown of the engine. Thus, the engine stabilization
duration prior to the engine being shut down is given by
tstop-t0 , which is about 25 seconds.

CLAIMS
1. A method of notifying an authorization to shut down
completely an aircraft gas turbine engine, the method
being applied after detecting (E10) that the engine has
5 passed to an idling speed, and comprising:
a) an evaluation step (E20) of using a value of a
first operating parameter of the engine (T45) to evaluate
a value for a second parameter (T45MG) characterizing a
thermal behavior of a part of the engine that might be
10 subjected to coking, this evaluation being performed by
means of a thermal behavior model for said part;
b) a comparison step (E30) of comparing the value of
the second parameter (T45MG) with a predefined threshold
value (T45thresh) corresponding to a value of the second
15 parameter that. does r1ot lead to coking of said part; ar1d
20
c) a notification step (E50) of notifying
authorization to shut down the engine completely if the
value of the second parameter (T45MG) is lower than the
value of the predefined threshold (T45thresh);
else reiterating steps a) to c).
2. A method according to claim 1, wherein a value for the
second parameter is evaluated by calculating a mean value
of the first operating parameter of the engine over a
25 moving time window.
3. A method according to claim 1 or claim 2, further
comprising:
triggering (E20') a timer (CM) on detecting that the
30 engine has passed to an idling speed; and
35
notifying (E50) authorization to shut down the
engine completely when the time that has elapsed since
triggering the timer exceeds a predetermined threshold
duration.
4. A method according to any one of claims 1 to 3,
wherein the first parameter is selected from the
13
following operating parameters of the engine: a
temperature measured in a flow channel for a gas stream
between two turbine stages; an ambient temperature
measured at the inlet of the engine; an oil temperature
5 of the engine; a fuel temperature of the engine; and an
operating speed of the engine.
5. A method according to any one of claims 1 to 4,
wherein the notification authorizing complete shutdown of
10 th