Rotating round dial aircraft engine instruments

An apparatus and method for displaying an engine performance parameter are provided wherein the scale for reading the value of the engine performance parameter is rotated as required in order that a pointer showing the value of the engine performance parameter appears at a desired clock position on a display when the value of the engine performance parameter is equal to a predetermined value, such as a takeoff thrust value.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention pertains in general to aircraft engine instruments and more 
particularly to an apparatus and method for rotating the scale of such 
instruments to provide alignment of the pointers for a particular thrust 
setting in order to enable detection of erroneous readings. 
1. Description of the Prior Art 
Aircraft turbine engine performance parameters such as engine pressure 
ratio, EPR, exhaust gas temperature, EGT, and revolutions per minute, RPM, 
are commonly represented on round gauges wherein the reading is obtained 
by noting the position of a pointer symbol with respect to a scale which 
is displayed around the circumference of the circular dial. Depending on 
the particular thrust setting selected, the angular position, or clock 
position, of the needle or pointer with respect to the circular scale for 
one engine parameter, such as EPR, may or may not coincide with the 
angular position of the pointer with respect to the scale for another 
performance parameter such as EGT. 
Various color bands and marker bugs have been used to indicate the desired 
reading of a particular performance parameter for a given thrust setting, 
such as a bug for marking the target EPR setting for takeoff. In order to 
determine engine performance using these circular dial instruments 
separate readings must be made for each performance parameter with respect 
to its corresponding scale. Under certain failure modes an erroneous 
reading from one performance parameter may not be recognizable without 
specific detailed reference to the remaining readings with respect to 
their corresponding scales. During critical flight modes such as takeoff, 
the time required to discern an erroneous engine parameter reading may be 
sufficient to cause a delayed reaction to the erroneous reading, and in 
some cases may be a significant cause of an aircraft accident. 
It would therefore be desirable to have an aircraft engine instrument 
system wherein the angular position, or relative clock position, of the 
needles or pointers for the various aircraft engine performance parameters 
are aligned to be the same relative clock position for a particular thrust 
setting, thereby allowing an early detection of a failed engine or 
erroneous reading by a quick glance at the needles which should be aligned 
under normal conditions. 
SUMMARY OF THE INVENTION 
In accordance with the present invention an aircraft engine instrument 
display apparatus is provided wherein a sensor means for generating a 
signal representative of an engine performance parameter and a reference 
means for generating a predetermined desired value of the engine 
performance parameter are coupled to a symbol generator for generating 
drive signals representative of the performance parameter value and the 
predetermined desired value. The symbol generator means is coupled to a 
display means for displaying the performance parameter symbol and a 
reference scale, wherein the reference scale is rotated as required to 
provide the predetermined desired value at a predetermined clock position 
with respect to the reference scale. 
The invention will be described more fully by way of example with respect 
to the accompanying drawings wherein:

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 2 illustrates a typical set of engine instrument readings utilizing a 
prior art system wherein the EPR readings of 2.12 appear to align with the 
target setting as illustrated by the alignment of pointer 36 with target 
symbol 32. However, the remaining engine instrument readings are not those 
which would correspond to an actual EPR reading of 2.12. The only method 
of determining this erroneous EPR reading is to look at the actual 
readings in question, that is EGT equal to 500, N1 equal to 88.6, N2 equal 
to 80.0 and realize that these readings are significantly lower then they 
should be for an EPR reading of 2.12. 
Utilizing the method and system of the present invention, the outer scales 
or reference scales of the various engine instruments are rotated so that 
at a particular thrust setting the corresponding pointer positions for 
needles 36, 46, 56 and 66 are at the same relative clock position for the 
corresponding instruments as shown in FIG. 4. Utilizing this system the 
pilot may recognize with a very brief glance that all of the engine 
performance parameters are approximately correct for the given desired EPR 
or thrust setting. 
As can be seen in FIG. 3, the particular failure in question such as an 
erroneously high EPR reading may be easily detected by noting the relative 
clock positions of pointers 46, 56, and 66. These remaining pointers are 
substantially different from the desired 12 o'clock position for the 
respective pointers. This system therefore provides a rapid means for the 
pilot to detect an erroneous engine instrument reading at a glance without 
having to determine a specific numerical reading with respect to a scale 
for any given case. 
The example illustrated in FIGS. 2 and 3 represents a case where the EPR 
reading is erroneously high, such as for example in the case where the 
inlet probe for the engine has been blocked by ice or other foreign matter 
accumulations. In this case the engine instrument representative of engine 
pressure ratio or EPR reads the desired target thrust setting of 2.12 at a 
lower actual thrust output setting for the engine. This can be detected by 
observing the remaining performance instruments such as EGT, N1, and N2. 
However, the erroneous reading may not be easily discernable utilizing the 
prior art system of FIG. 2, as the specific readings themselves must be 
compared to a numerical desired value in order to detect the erroneous EPR 
reading. The system of the present invention, as illustrated in FIG. 3, 
clearly shows that the EPR reading of 2.12 is not correct when compared to 
the remaining performance instrument readings of EGT, N1 and N2. This 
difference is discerned not by determining that a specific numerical 
reading is incorrect, but by observing that the clock positions of 
pointers 46, 56, and 66 are not at the desired 12 o'clock position which 
they should be to correspond with the 12 o'clock position of needle 36 
which indicates the desired EPR reading. 
Normal full thrust takeoff engine instrument readings are shown in FIGS. 5A 
and 5B. As shown in FIG. 5A, the actual appropriate or desired readings on 
the various engine instruments do not coincide with a particular relative 
clock position. The system of the present invention, as illustrated in 
FIG. 5B essentially rotates the outer scales in order that the desired and 
normal respective instrument readings result in the identical, or nearly 
identical, relative clock position of the needles showing the EPR, EGT, N1 
and N2 readings. 
A further illustration is shown in FIGS. 6A and 6B for a normal derated 
takeoff. Again, the various engine instrument readings, although normal, 
produce different relative clock positions of the specific pointers under 
the prior art system of FIG. 6A. The system of the present invention, on 
the other hand, again rotates the scales of the individual instruments to 
provide the same desired 12 o'clock position of the needles for the 
different engine instrument parameters for the desired thrust setting. 
With appropriate calculations to provide the desired instrument readings, 
the same relative clock position, such as the 12 o'clock position, may be 
obtained for any desired thrust setting. For example, the full thrust 
readings of FIG. 5B appear to be the same as the derated thrust setting 
readings of FIG. 6B, even though the numerical readings are in fact 
different for the two thrust settings. As with present engine instrument 
systems, the desired thrust setting may be either pilot selected or 
determined based on engine parameters by an external system. 
As illustrated in FIG. 1, the system of the present invention includes 
engine sensors 15 coupled to an electronic instrument system symbol 
generator 20, as well as a flight management or other computer system 10 
coupled to the electronic instrument system symbol generator 20. The 
electronic instrument system symbol generator 20 provides the required 
computations for generating both the actual instrument reading for the 
needles in question as well as the computations required to rotate the 
outer scales the desired amount to result in the alignment of the needles 
to the same clock position for the various instruments in question. Symbol 
generator 20 provides the required signals to display system 25. 
While the system and method of the present invention have been described 
for a particular application, this description is not to be taken as a 
limitation of the claims. Alternative embodiments may be implemented using 
other desired clock positions, engine parameters or flight modes without 
departing from the scope of the invention.