TECHNOLOGY
BY KIM WIOLLAND PORTER-STRAIT INSTRUMENT CO. INC
ir Data Computers have been The Beginning with us for many years now Initially, the main air data sensing A and have become increasingly application that concerned us was for more important, never more so then an altitude hold capability with the now as the RVSM mandate deadline autopilot. This “Altitude Capsule,” a approaches. The conventional aneroid simple aneroid just like an altimeter, pressure altimeter has been around is interfaced to a locking solenoid that for decades and is surprisingly accu- allows an error signal to be gener- Rockwell rate for a mechanical instrument. This Collinsʼ ated as the diaphragm changes with ADC-3000 instrument however will slowly lose altitude. All these capsules used gears, accuracy with increasing altitude. This cams, potentiometers and solenoids to eliminated these mechanical concerns. scale error is why they will not meet maintain a given altitude when com- Solid state pressure sensors and digital todayʼs stringent RVSM accuracy manded. In those days if the aircraft instruments are much more forgiving. requirements. The history of RVSM held within +/-100 feet, that was con- Current generation air data computers goes back further then you think, it sidered nominal, but then the accuracy have evolved into a separate sensor/ was first proposed in the mid–1950s would also vary at different altitudes. amplifier that provides a multitude of and again in 1973, and both times was These mechanically sensing instru- functions and information. rejected. With RVSM going into effect ments have at times been problems for The first generation of a Central Air this month, it will provide six new us all, failing to maintain proper pres- Data Computer (CADC) evolved out flight levels, increase airspace capac- sure rates on them during normal rou- of the Navy F14A Tomcat program in ity and most likely save hundreds of tine maintenance would cause undo the 1967-1969 time period. This com- millions in fuel burn each year. stress or even failure. puter employed Quartz Sensors, 20 bit Electrically driven altimeters, when Letʼs look at the airspeed indica- analog to digital and digital to analog paired with current generation air data tor as an example. We have found devices and a microprocessor chipset computers, provide a much improved that by keeping the airspeed “alive” designed by American Microsystems. and more accurate system. The air data while performing any pitot-static sys- Although this computer was also computer has become most important tem checks is the only way to ensure designed to manage some of the con- in maintaining optimum performance the integrity of the instrument as well trol surfaces of the aircraft, it was a of the aircraft in this ever-demanding as provide a visual reference of the major achievement in its time. environment we call the national air- applied pressure differential in the space system. These computers have system. For example, several type air- The Sensor made it possible to actually implement speeds in Learjetʼs would tend to drop The heart of any air data computer the RVSM mandate that we now face a pin positioned internally between is the pressure sensor itself. The accu- as well as providing additional infor- the aneroid and the bi-metal calibra- racy of the entire system is based on mation to other aircraft systems. tion arm when subjected to abnormal the sensor. The two types of pressure rates. The dislocation of this pin then sensors used are absolute sensor for would cause the instrument to read the static port and a differential sensor high above 120 knots. This of course for the pitot system. There are three required a tear down of the airspeed common sensor designs employed and Typical mechanical in order to re-install the pin. Todayʼs they are: bonded strain gauge, depos- airspeed in an early air data computers with solid state ited or ion implanted piezoresistive Learjet manufac- tured by Bendix transducers integrated with electrical elements, and capacitive. These pres- Aviation Corp. instruments or digital displays have sure transducers are simply an electro-
46 AVIONICS NEWS • JANUARY 2005 TECHNOLOGY
mechanical device for translating fluid Ideally you want undisturbed pitot- pressure values into voltages across static pressure to arrive at the comput- a high-impedance. The piezoresistive er however, some influences do exist. sensor design employs a fully active Placement and sizing of the pitot- Wheatstone Bridge. Applied pres- static probes and ports is also critically sure presents a distributed load to the important in order to help minimize diaphragm, which provides bending these influences. The CIA found out stresses. This stress creates a strain Honeywell Air Data and altimeter removed after they lost an A-12 aircraft on May proportional to the applied pressure, from a CitationJet to install RVSM equipment. 24, 1963, just how critical this can be. which results in a bridge imbalance. fication calls for a tolerance of +/-25 The computer must be capable of With an applied voltage across the feet (+/-0.012 psi) at 5,000 feet and providing a stable and extremely accu- bridge, the unbalance produces a +/-125 feet (+/-0.010 psi) at 50,000 rate measurement over long sustained millivolt output. The capacitive type feet. The Collins ADC-3000 system periods of flight. Pressure altitude is is believed to provide the highest for example lists a resolution of one the altitude referenced to a standard sensitivity, least aging effects and foot and an accuracy of +/-40 feet at sea level pressure of 29.92Hg. Since provide very accurate low pressure FL410 to maintain RVSM envelope barometric pressure varies locally as sensing. This capacitive sensing tech- tolerances. One common method used a function of the weather conditions, a nique measures atmospheric pressure to digitize the sensor output is to allow local correction to the measured pres- through the change in voltage across it to control a voltage controlled oscil- sure is required. This Baro-Corrected the capacitive element, one plate of lator (VCO). The oscillator output or Baro-Altitude is referenced to the which deflects slightly with changes then is a digital value from which all local pressure and this is used below in applied pressure. other computations can take place. The FL180. The pilot controls the amount The sensitivity of a transducer is more common technique now is an of correction applied to the measured defined as the ratio of its electrical analog to digital interface directly to pressure altitude simply by dialing in output to its mechanical input. These the microprocessor. The microproces- the local pressure on the altimeter or transducers are very accurate but they sor then can also monitor and control the display control panel. This correc- do have some imperfections in that the transducer temperature to ensure a tion is sent to the air data computer in their calibration curves do not repre- predictable output. one of several forms: analog voltage, sent a straight line (linear) relation- synchro format or digitally. The final ship between the pressure applied and Air Data Functions measurement that must be considered its output. The air data computer can The modern air data computers and and one that influences many calcula- correct for this by storing the correc- their associated instrumentation are tions is the air temperature. The total tion curve parameters in permanent used to measure a number of criti- air temperature (TAT) probe is one memory in the form of a look-up table cal air mass properties. The computer method while a simple outside air tem- or put into flash memory, for example, must track pressure changes as the perature (OAT) probe is another. to allow for re-writing the curve as aircraft climbs, descends, accelerates, Letʼs look at the total air tempera- aging occurs. This curve is critical decelerates and then accurately pre- ture method. The probe compresses to masking out the non-linearity and dict, for example, capture points for the impacting air to zero speed and the achieving maximum accuracy. the autopilot. The atmospheric pres- resulting temperature causes a change The predictability and repeatabil- sure or force per unit area of the air in in the resistance of the sensing element. ity of the transducerʼs performance, the earthʼs atmosphere will decrease The air data then converts this resis- regardless of type used, will greatly monotonically with the distance above tance to temperature. The TAT probe effect system performance. Controlling the surface. There are three critical design could be of two types: aspirated the transducerʼs temperature will help measurements for the computer. The and non-aspirated. The air temperature ensure this consistent performance. two fundamental pressure measure- is used to calibrate the impact pressure In order to appreciate the sensitivities ments required are impact pressure as well as in determining air density. required letʼs look at what the SAE has (Qc) as measured at the pitot tube and The air density will affect the force defined as accuracyʼs required by the the ambient static air pressure (Qs) of the air. This measured temperature air data system. An earlier SAE speci- sensed at the static ports. Continued on following page
AVIONICS NEWS • JANUARY 2005 47 AIR DATA COMPUTERS ports may differ. This is an impor- sents groundspeed also. True air- Continued from page 47 tant factor in minimizing the overall speed is the speed of choice for any will vary from the standard lapse rate Altimetry System Error (ASE) in the navigation system and has become a of .65º C / 100 meters; this rate will aircraft. We should remember that required input. The GPS navigator vary up or down depending on loca- because this is a dynamic correction and Flight Management systems use tion and the local weather. These three that it must be disabled, for example, true airspeed to compute winds aloft measurements provide the follow- when complying with altimeter certi- thus enabling the crew to fly at more ing computed air data output signals fication. Pitot Source Error Correction fuel-efficient altitudes and also for which supply primary and secondary (PSEC) may also be compensated for solving equations relating to ground instruments: Pressure Altitude, Baro- in a similar manner on some air- coordinates. Corrected Altitude, Vertical Speed, frames. Mach Number, Total Air Temperature, Common Terms Calibrated Airspeed, True Airspeed, Measuring Airspeed Pt Total Pressure: the sum of Digitized Pressure Altitude (Gillham), As with measuring altitude, mea- local atmospheric pressures plus Altitude Hold, Airspeed Hold, Mach suring airspeed is basically straight- dynamic pressures. Hold, and Flight Control Gain forward and deals with measuring the Algebraically then, total pressure Scheduling. impact pressure. Indicated airspeed equals the sum of static pressure (Ps) (IAS) is merely a reference to pitot plus impact pressure (Qc) System Corrections minus static pressure. There are how- Static Source Error Correction ever, several other factors to be con- Pt = Ps + Qc (SSEC) plays an important role in cerned with. In dealing with impact the overall accuracy of the air data pressure (Qc) we must also be aware Ps Static Pressure: the abso- system by making corrections to pres- of the compressibility that occurs lute pressure of still air surrounding a sure altitude. The purpose of SSEC is around the airframe and pitot tube. body; the atmospheric pressure at the to produce a minimum residual static The compressibility of the air mass as flight level of the aircraft. system error. All aircraft have some the aircraft moves through the air also imperfections in the static system as creates a drag coefficient. As a result Qc Impact Pressure: a cal- it is plumbed through the airframe of this, the air temperature must be culated value, it is the difference to include defects at the static ports known to solve for heat at a specific between total pressure and static caused by aerodynamic effects of the pressure. NASA has a technical note, pressure; it is the pressure created by aircraft in flight. This correction is D-822 that shows the relationship of the forward motion of the aircraft. important in any high performance Impact Pressure (Qc) for values of and jet aircraft. In order to correct for Calibrated Airspeed and serves as a TAT Total Air Temperature: these imperfections certain calibra- good reference. The aircraftʼs airspeed the temperature of an airflow mea- tions need to be made within the air indicators are calibrated at standard sured as the airflow is brought to rest data computer. sea-level conditions and there the without removal or addition of heat; The Airplane Flight Manual measured airspeed is true airspeed impact temperature. Performance Section for a specific (TAS). The net pressure and den- airplane will have in it a table or sity at all other altitudes will deviate OAT Outside Air Temperature: chart showing the actual corrections and therefore, the air data computer the temperature of the static outside in altimetry for that airframe. This must calculate a correction for them to temperature without the effects of information developed by the airframe maintain an accurate true airspeed at airspeed. manufacturer can be pre-programmed these altitudes. in memory by the air data manufac- True airspeed is the actual airspeed Conclusions turer and selected with program straps of the aircraft through the air or Mach We have certainly seen in the last that identify the particular airframe. number times the speed of sound. decade the need for more informa- Program modules are another format As the speed of sound is a function tion to be available to the crew in used to implement this correction. of static air temperature (SAT), here the cockpit. The growth of GPS, The current useful weight and angle again air temperature is required. If flight management systems, and the of attack may also influence the SSEC the air mass at altitude is relatively newer cockpit display technologies as the fluid dynamics around the static stationary, then true airspeed repre- has demanded a growth in previously
48 AVIONICS NEWS • JANUARY 2005 existing systems as well. The ADC80 series computers, for example, can be brought into RVSM compliance through SB32 and updating the SSEC by STC. This enormous growth dic- tates also an integration of data far beyond what most of us envisioned 20 years ago. The air data computer per- forms critically important functions that enhance the safety, reliability and performance of the aircraft. ❑
AVIONICS NEWS • JANUARY 2005 49