AboutMarc Ebelini Expertise Basis aircraft and engine maintenance. Avionics and other electronic questions related to computers and radio communications.
Experience FAA licensed comercial pilot, A&P mechanic, former authorized inspector, ground instructor certified in aircraft, powerplant and radio navigation. FCC commercial license.
Expert: Marc Ebelini Date: 2/17/2008 Subject: transponder (cessna 152)
Question QUESTION: does the solid-state blind encoder transponder works same as the static system works for the altimeter. Its a vacuum but what is solid-state?
does the transponder use the same static source that is used for altimeter.
ANSWER: The static system is required to read the outside air pressure so that the pressure altitude is accurate. Reading the air pressure in the cabin can be influenced by cooling ducts and or heater input. The static port is so located as to read the outside pressure as accurate as possible. Initial calibration for the port is done by the aircraft manufacture during flight test. It is important in keeping the opening area clean but should the hole get plugged by wax or other matter never clean it with a metal device that can distort the opening. The tubing that connects this opening goes to your static reading instruments whether it be Altimeter, blind altimeter (encoder) or calibrated airspeed indicator. This pressure which is of course negative with reference the the pressure of the surface for which you are flying over is to determine your pressure altitude. Always remember this is not the exact distance above the ground. Outside air temperature and barometric setting effects this reading. This information is then converted by the aneroid in your altimeter to the relative altitude you are at. The method to then convert this to the responding gray code varies. In altimeters it is normally mechanical such as a encoding disk that turns in unison with the dial pointers. Later blind encoder used other pressure sensitive devices that produce a gray code output to send to your transponder in mode "C" so that the center can read your altitude. The binary gray code was written so that there is only one bit change for each 100 foot increment. The gray must change only bit at a time to assure accuracy. More that one bit change at a time would indicate a malfunction.
In order for your aircraft to be IFR certified your static port needs to be checked for any leakage at the same time as the altimeter. Test boxes are also used to check altimeter readings transmitted by the transponder to test the encoders accuracy. There are gray code charts should the test box not decode the binary word. This helps troubleshooting any wiring failure. Hope this all makes sense.
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QUESTION: thank you for replying.
but i wanted to know the working principle of what you said"Later blind encoder used other pressure sensitive devices" i mean solid state blind encoder.
i wanted to know whether it uses the same static port used for altimeter so that i'll get to know if static port is blocked both altimeter and the transponder will be affected and as i don't see any other external port on the fuselage if any.
ANSWER: "The tubing that connects this opening goes to your static reading instruments whether it be Altimeter, blind altimeter (encoder) or calibrated airspeed indicator." This sentence I previously wrote answered your question. There is usually only one static system in an aircraft. Solid state has nothing to do with the static port except translate pressure received into a gray code. Static port test are conduced by sealing the opening to a tube from a vacuum pump device. Caution must be taken to induce the vacuum very slowly as to not damage the altimeter, this is very important. Clamping the vacuum source at a given altitude then determines if there is any leakage in the system. Since I don't know laws of your area the amount of leakage allowed you will need to investigate.
You appear to be confused with solid state. Your solid state encoder is a device that translates a given pressure to a gray scale as I previously explained. Placing the device on a bench with power will produce a binary output relative the the ambient pressure at your location. As a vacuum is induced into the static opening this binary number changes as the vacuum increases. In this case the solid state device produces a code that is equivalent to altitude. Other solid state devices are made to produce a binary code that may represent the actual vacuum in what ever form was required.
Follow the train of events. A pressure goes to the encoder that then converts the pressure to a binary word, this word is then connected to the transponder by a wire which then transmitted when requested (mode "C")to the control center. Blind encoders are normally used in place of an encoding altimeter. If your altimeter works but you mode "C" does not You must suspect the blind encoder. It failed, no power to it, plugged opening (this would result in getting one constant reading), open cable to the transponder or mode "C" not working. Hope I rang the bell this time.
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QUESTION: That was just great, and appreciate for having patience with me .
just trying to figure out a clear picture.
can you please tell me about 'encoding altimeter' does it has a seperate chamber to measure the pressure change like for the altimeter. And as it sound from its name 'encoding altimeter' it must be converting the mechanical reading into electrical signal and is integrated into the aircraft altimeter. Means both the device take reading from one device (i.e aneroid wafer etc).
why is it that the transponder or the controller will always be correct in checking/saying the altitude.
Answer Actually I answered all your questions but apparently you are still confused. But first I must answer the last sentence in your question.
The transponder that is only the messenger in this chain of events and will not always be giving the correct response. Why? Because the it is based on the standard barometric pressure. The center must correct the reply to respond to that reference. If the pilot does not have the local barometric pressure set in his altimeter then his reading will not agree with the centers. Remember the transponder has nothing the do with the altitude it only reports the information supplied by the encoders, Blind or other wise.
Now the solid state device requires a transducer to convert the pressure reading into an electrical format the solid state device can convert to this Gray code that it sends to the transponder. As I mentioned in my first response static pressure goes to any instrument that requires a static (outside pressure)reading. The tubing can go to your altimeter, blind encoder or calibrated airspeed should you have one. Some blind encoders may be altimeters without a dial using the encoding disk method to transfer a the binary code to the transponder. Other blind encoder use several different systems as transducers to convert the pressure to electrical information the semiconductor can use to perform it's conversion process. The state of the art has made these chips so sensitive to pressure change that they can detect changes of down to 10 foot increments. This data is available in RS 232 serial to drive electronic displays to show this information on Video screens in large aircraft. Are you still confused?
