Baro data indicator

Avionics apparatus for providing to a pilot readings of temperature, pressure altitude, and density altitude. Sensors provide electrical signals responsive to ambient temperature and pressure, and a functional module operates on signals derived from said sensors to provide signals indicative of the pressure altitude and density altitude. A multiplexer and display respond to a mode selection switch to provide visual readouts of temperature in degrees Fahrenheit and in degrees Celsius, and altitude.

This invention relates generally to avionics apparatus, and more 
particularly the invention relates to apparatus for facilitating the 
determination of aircraft performance criteria based on the combined 
effects of ambient temperature and pressure measurements. 
Conventional avionics apparatus includes separate ambient pressure and 
temperature sensors. Typically, an altimeter is provided which senses 
static or ambient pressure and provides an indicated altitude readout 
calibrated in hundreds, thousands and ten-thousands of feet. Standard sea 
level pressure is defined as 29.92 inches of mercury or 1013.25 millibars, 
and the operation of the altimeter is based on this standard pressure. 
When the altimeter baro-set is set to this standard sea level pressure, 
the readout is called "pressure altitude." The altimeter can be adjusted 
or set to correct for changes in local barometric pressure also. Thus, an 
"indicated altitude" is read when the altimeter is set to the local 
barometric pressure. After an aircraft takes off and begins climbing, the 
atmospheric pressure decreases and the increased altitude is read out. The 
indicated altitude will correspond to pressure altitude only when the 
local pressure corresponds to the standard pressure, 29.92 inches of 
mercury or 1013.25 millibars. Also, typically, a temperature sensing probe 
is provided, usually a direct-reading twisted bi-metal type, mounted in 
the windshield for the purpose of reading outside air temperature (OAT) in 
degrees F. or degrees C. 
Aircraft performance is dictated by the density of the ambient air, and air 
density is dependent on both altitude and ambient temperature. "Density 
altitude" is the pressure altitude modified by local ambient temperature. 
Conventionally, density altitude is established by first adjusting the 
altimeter to standard sea level pressure (i.e., 29.92 inches of mercury or 
1013.25 millibars) and reading the pressure altitude. Then, conversion 
charts based on pressure altitude and ambient temperature (called outside 
air temperature or OAT) are referenced to provide the corresponding 
density altitude. 
Density altitude is necessary in calculating true air speed (TAS), and 
importantly, in calculating runway length for a high temperature and/or 
high altitude takeoff or landing. The flight manual for a particular 
aircraft gives charts or tables for takeoff and landing distances at 
various density altitudes and for various aircraft loads. 
The necessity for adjusting the altimeter from local barometric pressure to 
standard pressure in order to determine pressure altitude, next 
determining density altitude from a flight manual based on pressure 
altitude and temperature, and finally determining takeoff distance from 
the density altitude and known aircraft load often leads to erroneous 
calculations and possible disastrous consequences, especially in high 
altitude and/or high temperature environments. 
Accordingly, an object of the present invention is avionics apparatus for 
facilitating aircraft performance calculations. 
Still another object of the invention is avionics apparatus having multiple 
modes of operation. Briefly, apparatus in accordance with the invention 
includes pressure means for measuring ambient pressure and providing an 
electrical signal indicative of pressure; temperature means for measuring 
temperature and providing an electrical signal indicative of temperature; 
and altitude means for receiving the electrical signal indicating pressure 
and the electrical signal indicating temperature and providing therefrom a 
measure of pressure altitude and a measure of density altitude. Mode 
selection means is provided for selectively providing the pressure 
altitude and density altitude outputs. In addition, display means 
including a multiplexer receives the electrical signals and output signals 
and selectively provides a display of temperature in degrees Fahrenheit 
and in degrees Celsius, pressure altitude, and density altitude, one at a 
time. 
More particularly, the altitude means responds to the pressure signals and 
provides a pressure altitude signal in accordance with the following 
equation: 
##EQU1## 
Pressure altitude combined with a temperature signal from the temperature 
means provides a density altitude signal in accordance with the following 
equation: 
##EQU2## 
where P.sub.o =29.92 in. Hg=2116.22 lb/ft.sup.2 =1013.25 millibars 
P=measured pressure 
Z.sub.o =145367 ft=44,308 meters 
n=5.2568 
T.sub.o =59.degree. F.=15.degree. C.=288.18.degree. K.=standard ICAO 
atmospheric temperature 
T=measured temperature (absolute) The apparatus can be employed with 
additional processor means to give a direct indication of runway length 
for takeoff and landing, and vertical speed indication, for example.

Referring now to the drawings, FIG. 1 is a functional block diagram 
illustrating one embodiment of the invention. The apparatus includes a 
pressure transducer 10 and a temperature transducer 12 which respond to 
ambient conditions and generate electrical signals indicative thereof. For 
example, a commercially available Foxboro 1800A pressure transducer will 
generate an electrical signal between 0 and 100 millivolts and a 
conventional temperature transducer such as the Analog Devices AD 590 
generates an electrical signal of one microamp per degree Kelvin. 
The voltage signal from pressure transducer 10 is applied to preamplifier 
and bias circuitry 14 which generates an adjusted output voltage more 
suitable for processing. In preferred embodiments, the output signal from 
the circuitry 14 is a voltage from 0 to 10 volts corresponding to the 
pressure transducer signal from 0 to 100 millivolts. Similarly, 
preamplifier bias circuitry 16 receives the signal from temperature 
transducer 12 and generates an output voltage from 0 to 10 volts 
corresponding to the temperature transducer output signal. 
In a preferred embodiment, the apparatus has four modes of operation: 
namely, temperature in degrees Fahrenheit, temperature in degrees Celsius, 
density altitude, and pressure altitude. A mode switch 71 is included so 
that only one readout of the four modes can be selected at a time. In 
deriving the first two modes of operation, the output signal from 
preamplifier bias circuit 16, which corresponds to degree Kelvin, is 
applied to converters 18 and 20 which convert the signal from degrees 
Kelvin to degrees Fahrenheit and degrees Celsius, respectively. The 
outputs from converters 18 and 20 are then applied through multiplexer 22 
to an analog to digital converter and display driver 24 and thence to 
display 26 where temperature in accordance with mode 1 and mode 2 of 
operation is displayed. As will be described further hereinbelow, a mode 
switch controls multiplexer 22 and also provides a decimal point and 
either Celsius or Fahrenheit designation for display 26. 
In mode 3 and 4 of operation, the pressure signal from circuitry 14 is 
applied to a programmable function module 30 along with an input from 
multiplexer 32. Multiplexer 32 is controlled by the mode switch whereby 
the temperature signal from circuitry 16 is applied to the programmable 
function module 30 in the mode 3 operation for deriving density altitude, 
and multiplexer 32 applies a reference voltage (e.g., +10 volts) for mode 
4 operation in deriving pressure altitude. The mode switch also controls 
the exponent selection for module 30. The programmed module 30, such as a 
National Semiconductor LH0094 device, is programmed to operate on the 
input signals to give an output signal which is a function of both 
pressure and temperature in mode 3 and which is a function of pressure 
only in mode 4. The signals from module 30 are then applied to a summing 
amplifier 36 which operates on the signals from module 30 to provide 
output signals in accordance with the following equations: 
##EQU3## 
The elements of the equations are given hereinabove, and the equations 
assume an ideal gas law and dry air. 
The output signal from summing amplifier 36 is applied to multiplexer 22 
and in response to the mode selection switch is passed to the display 
driver 24 and to display 26. 
In addition, the pressure altitude and density altitude signals can be 
applied to a processor 40 along with specific aircraft criteria and other 
inputs such as load of the aircraft whereby a direct reading of runway 
length, hover capacity (for helicopters) and the like are provided. 
Thus, a pilot preparing for takeoff need only select the proper mode of the 
apparatus to obtain density altitude without the necessity for adjusting 
the pressure setting of the altimeter to obtain pressure altitude and then 
referring to charts to obtain density altitude from pressure altitude and 
ambient temperature, as in conventional practice. Consequently, the 
pilot's procedures in takeoff and landing are simplified, and errors in 
obtaining density altitude and attendant information, such as runway 
length, are minimized. 
Apparatus in accordance with the present invention can be readily packaged 
in a compact assembly for cockpit mounting. FIG. 2 is a perspective view 
of one embodiment of the apparatus which has been assembled on three 
printed circuit boards measuring 2.250 inches by 2.250 inches and which 
are stacked in array measuring 4.0 in depth. As will be described further 
hereinbelow, the preamplifier and bias circuitry and temperature 
conversion circuitry are mounted on board 44, the programmable function 
module, multiplexer, and mode switch are mounted on board 46, and the 
display driver and display are mounted on board 48. 
In one embodiment a liquid crystal display having six 7-segment units and a 
decimal point was employed. FIG. 3A illustrates the display in mode 1 with 
an illustrative readout of 41.0 degrees Fahrenheit. In FIG. 3B, the mode 2 
operation is illustrated in which the corresponding Celsius readout is 
5.0.degree. C. In mode 3 operation, as illustrated in FIG. 3C, a density 
altitude of 3750 feet is displayed, while in FIG. 3D the corresponding 
pressure altitude of 4000 feet is illustrated. 
A detailed schematic of one embodiment of the apparatus of FIG. 1 is shown 
in FIGS. 4,5,6 which correspond to boards 44, 46 and 48, respectively, of 
FIG. 2. 
In FIG. 4, a pressure transducer shown generally at 50 comprising a silicon 
resistor bridge network such as a Foxboro 1800A transducer is 
interconnected between bias circuitry comprising differential amplifier 51 
and differential amplifier 52, with the output of the bridge circuitry 
applied to differential amplifier 53 which provides an output signal from 
0 to 10 volts corresponding to a pressure transducer signal from 0 to 100 
millivolts. Potentiometer 54 provides a pressure zero adjust and 
potentiometer 55 provides a pressure gain adjust. 
The temperature transducer 60 is serially connected with a resistive 
network including a temperature adjust potentiometer 61 and generates an 
electrical signal of one microampere per degree Kelvin which is applied to 
a differential amplifier 62. The output of amplifier 62 is a signal 
indicative of temperature in degrees Kelvin which is applied to the 
resistive network shown generally at 63 which converts the signal in 
degrees Kelvin to a signal in degrees Celsius. Similarly, the output 
signal from amplifier 62 is applied to the resistive network 64 which 
converts the signal from degrees Kelvin to a signal in degrees Fahrenheit, 
as indicated. In addition, the signal from amplifier 62 is applied to the 
multiplexer 32 of FIG. 1 which is mounted on the borad 46 of FIG. 2. 
FIG. 5 is an electrical schematic of the board 46 of FIG. 2 which includes 
the programmable functional module 70 in which terminal 9 receives the 
pressure output signal from amplifier 53 of FIG. 4, and in which the 
exponent select is determined by the mode switch 71 which controls the 
inputs to terminal 10, 14, and 3 of module 30 through the voltage divider 
network shown generally at 72. Multiplexer 74 (which corresponds to 
multiplexer 32 of FIG. 1) receives the temperature signal from amplifier 
62 of FIG. 4 and a reference voltage (+10 volts) and in response to the 
mode switch applies an input to terminal 13 of module 70. Module 70 acts 
on the inputs as above described and provides output signals at terminal 1 
which is a function of pressure and temperature (mode 3 operation) or a 
function of pressure alone (mode 4 operation). The output signal is 
applied through biasing circuitry shown generally at 75 which adjusts the 
mode 4 signal for application to multiplexer 76. Similarly, the output 
signal is applied through the biasing circuitry 77 which adjusts the mode 
3 signal which is then applied also to multiplexer 76. Multiplexer 76 
responds to the mode switch and selectively applied signals from biasing 
circuitry 75, 77 to a summing amplifier 78. The output from summing 
amplifier 78 is applied through a voltage divider shown generally at 79 to 
provide an "altitude/50" signal. The "ALT/50" signal from summing 
amplifier 78 is applied along with the temperature signals through 
multiplexer 80 to the display driver circuitry. 
Also mounted on board 46 is the multiplexer control circuitry shown 
generally at 82 which responds to the setting of mode switch 83 and 
provides control signals to the multiplexers as indicated. FIG. 6 is a 
schematic of the circuitry on board 48 of FIG. 2 and includes an A to D 
converter and display driver unit 90 such as an Intersil 7106 device. The 
display driver 90 is biased as illustrated with pin 31 receiving the 
output of multiplexer 80 of FIG. 5. Multiplexer 80 corresponds to 
multiplexer 22 of FIG. 1. The outputs from driver 90 are connected to the 
inputs of a liquid crystal display 92, for instance, a Crystaloid 4360 LCD 
with the logic circuitry shown generally at 94 providing the decimal 
point, Celsius, Fahrenheit and negative designations, when applicable. 
Apparatus in accordance with the present invention has simplified the 
procedures of a pilot in calculating the performance characteristics of 
his aircraft. The embodiment of the invention illustrated schematically in 
FIGS. 4-6 has been embodied in the compact module shown perspectively in 
FIG. 2. However, the embodiment is illustrative of the invention and is 
not to be construed as limiting the invention. Various modifications and 
applications will occur to those skilled in the art without departing from 
the true spirit and scope of the invention as defined by the appended 
claims.