One often misunderstood, overlooked and sometimes cursed bit of electronic gear is the humble encoder. With pseudonyms like digitizer, encoding altimeter or encoder and always hidden somewhere from sight, its difficult for many pilots to comprehend this fellows place in the avionics pecking order.
When its working, you dont know it. When it fails, its symptoms quietly manifest themselves in other systems with irritating results. An ailing encoder can cause your otherwise successful flight to deteriorate into a nightmare of hostile controllers and even force you to remain grounded at a busy airport until you get it fixed. Thats pretty powerful for a box that costs less than $200.
What The Thing Does
The information gathered by the encoding altimeter, pressure altitude, has grown in importance in recent years, an importance far greater than the accuracy and reliability of other gear that costs far more than a couple C-notes.
What is an altitude encoder, and how do we ensure a long and happy flying career. The purpose of an encoding altimeter or encoder is to convert atmospheric pressure to a data format compatible with a ATCRBS transponder, which is what ATC radar tickles or interrogates to paint a discrete four-number code on the controllers radar display.
That format is a parallel output on 10 discrete output lines, frequently termed a gray code. Any altitude between -1000 feet and 408,600 feet can be described in 4096 specific codes. (Obviously, not all codes are used, since the typical altimeter stops between 35,000 and 50,000 feet.) That 4096 number is, not coincidentally, the same number of individual codes available in a Mode-A transponder.
The resolution of a standard encoder is 100 feet. The transition between one altitude and another occurs around the 50-foot mark. That is, in a perfectly calibrated system, an airplane would squawk 1500 feet while flying between 1450 and 1550 feet. Altimeters are far from perfect and errors creep in. Well get to that later, in calibration.
One frequent area of confusion is the difference between an encoding altimeter, an encoder, or an altitude digitizer. Digitizer is an anachronistic term, applied to the system in the days before the whole world knew digital meant microprocessor or computer controlled. Here, digitizer just means it converts analog pressure information to a parallel code.
The difference between an encoding altimeter and an altitude encoder is a matter of location. An encoder usually means that the electronics are in a remote-mounted unit, while an encoding altimeter is a combination unit. The encoding altimeter has a normal barometric altimeter and shares its case (and static pressure) with an encoding module.
There may be a direct connection between the altimeter hands and the encoder module. Because of the increased complexity of the mechanical construction, the combination typically costs more than a plain altimeter and an encoder. It is easier to install, if only because the plumbing is reduced. Also, you dont have to find a firewall or other remote location to mount the unit, which can be a consideration when running both static pneumatics and wires.
A separate encoder can still be dropped into many airplanes in a day, including required static check. Furthermore, if the thing fails, the airplane can still be flown and repairs are cheaper. In general, the population of encoding altimeters is falling.
The modern encoder is a simple unit. A pressure transducer is ported to static pressure. The electronic output of the transducer is converted to pressure altitude by some circuitry and converted to the proper code for the transponder. A nine-wire connection contains all of the altitude data for 35,000 feet, the D4 wire is connected to go up to 50,000 feet.
As we all know, the primary reason for Mode-C information is air traffic control. By transmitting your altitude to ATC, your vertical separation is assured. What many people dont know is that the transponder only provides pressure altitude, referenced to 29.9213 in. Hg. The ATC computers automatically apply the local barometric pressure to your reply when you are lower than FL180. The important thing to remember is that a mis set baro correction will lead you astray, but not your encoder.
Which brings up the next point. ATC has used Mode-C to keep airplanes apart for decades and now TCAS-equipped airplanes also use those bits of information to paint you as a threat.
Contemplate for just a moment that the lives of a couple hundred souls in a $30 million airplane are depending on your $159 encoder that was stuffed in before lunch by your local spark chaser. Okay, so theyre not depending on it, but still, that information is vital to the entire ATC system. You can understand that the Mode-C is very important and why ATC gets testy when the encoder in your C-150 begins to flicker between 3500 feet and FL330.
The encoder must be connected to the same static source as the altimeter. Although we have seen units vented to the cabin, it isnt legal if the airplane has a static system. Frequently, the connection is a simple T-fitting, with a hose leading off to the encoder.
Some common problems crop up in this area, including an encoder thats the lowest point in the system (can you say condensation?) and hoses that are split, broken or deteriorated with age. Another consideration is that a static leak check is required whenever the static system is opened. For instance, if the encoder is sent in for repair, the system must be checked after the unit is removed, if youre going to cap the static and fly the airplane. It must be checked again after the unit is reinstalled.
All encoders are TSO-approved, as are all transponders. Whats even nicer, they all talk to each other. Theres no need to buy a Bendix/King encoder to work with a Bendix/King transponder (actually, the Bendix/King instrument stuff is made by somebody else anyway). You can pick and choose the best price, because performance is guaranteed. Since the transponder/encoder system is so vital to a happy relationship with ATC, wouldnt a dual system make sense? Yes indeed. There are systems with two transponders and one encoder, systems with two transponders and two encoders, and rarely, a single transponder fed by two encoders.
There are some critical items to know in these situations. A single encoder driving dual transponders is common. Since encoders by nature are hidden, you may think you have a true dual system, but, in reality, a single point failure in the encoder can render all your Mode-C capability inoperable.
You must never have two transponders transponding at the same time. In a dual system, one is always forced to standby or switched off. Most transponders, and many encoders have a warm-up period. Unless your second transponder is correctly wired for hot standby, you could be waiting 45 to 90 seconds for the transponder to come on. The warm-up on a solid state encoder can take five minutes, during which time your altitude will be in error by hundreds of feet.
Having a ready source of pressure altitude around can prove handy and more than a few other systems have taken advantage of the encoder output. One issue must be clear, though. Anything else hanging off the encoder must not interfere with the transponder Mode-C operation.
Because one missing bit can create a wrong altitude readout, you must be sure that these accessories are properly installed and designed with isolation to prevent them from compromising the altitude readout.
One of the best uses for encoder information is in autopilot altitude preselect. Most altitude hold systems use a simple transducer method of maintaining a pressure altitude. You tell the autopilot to stay here, and it does.
If, however, you want to tell the autopilot to go to a specific altitude, you need to give it some way to tell one altitude from another. Hello encoder altitude output. The caveat is you must compensate these systems with the local barometric pressure by setting a knob.
Another consideration is encoder accuracy and maintenance problems. The altitude capture could be off if the encoder is wrong. Or it could be way off if the encoder was missing a bit. You could experience dramatic pitch reversals if the altitude preselect suddenly found that it was above the selected altitude instead of below, when the system lost a 4000 foot bit pattern.
The GPS system can, theoretically at least, compute a position in three dimensions. However, due to a geometric difficulty receiving GPS signals from below (the planet is in the way), the position solution is not as good as possible for altitude. Okay so why not use pressure altitude information? Good idea!
By giving the GPS some clues to the vertical distance from the surface, it can easily determine if the GPS position solution is in the cosmic ballpark. Although the GPS may not use the pressure altitude in its position determination, it will use the information to verify the information it receives and calculates. This is a big part of the RAIM (Receiver Autonomous Integrity Monitoring) scheme required of all IFR-approved GPS.
One important item for the GPS installation is the availability of those 10 extra pins. Some GPS receivers dont have the spare inputs and rely instead on another interface box. When the avionics shop spends your money for an altitude serializer, you will be buying a unit that takes in the parallel gray code and spits out a serial data stream on two wires for the GPS. Some encoders, notably the Transcal SSD120 or Shadin Falcon (around $850), include both outputs.
Every altitude device, whether a sensitive altimeter or an encoder, must be checked every 24 months for accuracy and integrity. This is where 80 percent of the encoder problems are discovered. (The other 20 percent are inbound to a major airport, when you have a meeting in 20 minutes, and ATC refuses to let you in.)
During the biennial check, the altimeter should be checked for accuracy, as is the encoder. They should be checked for correspondence (the difference in the readout) and leakage. Up to 100 FPM is allowed. More leads to a nightmare of plumbing inspection.
Specifically, youre allowed to have 125 feet of error in the encoder (at altitudes of less than about 10,000 feet), or, and this is important, a 75-foot variation between the encoder output and your altimeter. Why is this important? Because an excessive split between encoder and altimeter can cause alleged deviations from our assigned altitude.
What happens if the encoder is out of tolerance? It depends on the make and model. Some, like a Narco AR950, have many adjustments that are easily accessed to shave off a few feet of error. Others have a low-end and a high-end adjustment that are balanced for optimum performance.
Mechanically intensive units, such as a Transcal D-120-P2, have a mechanical adjustment that has to be made at ground pressure because the case is vented to static pressure. You hope it will stay within tolerance at the service ceiling.
Some avionics facilities will remove the altimeter and the encoder and have an instrument shop check both at the same time and make the proper correspondence adjustments. This is fine when adjustments are necessary, but adds cost and the danger of handling damage.
Failure Modes and Repairs
We have discussed calibration errors but whats the worst case scenario? Missing a C bit. This results in a few hundred feet of error and getting busted for altitude deviation. Busted bits can be caused by a defective encoder, a broken wire between the encoder and transponder, or even a bad transponder.
Its nearly impossible to tell where the problem may be without a technicians intervention. Then, the best course of action would be to bench test the transponder. For low-cost encoders, you can opt to send the unit in for service. In a few days, you can have it fixed for little cash. For slightly more dinero, exchange the encoder. More expensive instruments can also be exchanged and thats usually the best option. However, you can get them repaired at an instrument shop.
One very handy gadget for checking your encoder is the AirSport altitude alerter. (See Aviation Consumer, July 1998.) This device reads out your encoded altitude for a variety of uses.
Some transponders, notably the Bendix/King KT 79, KT 70, KT 71 and Terra digital units also read the encoded altitude. These dont have any baro correction, so youll have to set your altimeter for 29.92 to get the correct correspondence.
Who makes these gizmos? They are a niche market and there are some very good buys around. The technology is not terribly complicated, although getting the logarithmic pressure differential to altitude right from sea level to 35,000 feet isnt always as easy as it looks.
One of the first companies to concentrate in encoders is ACK. Their A-30 is a good unit for less than $200. Ameri-King is a dedicated small company known for encoders, ELTs and GPS annunciators. Theyre available for about the same price as an ACK and offer same-day repair service and a longer warranty.
One of the two major avionics companies to make their own encoder, Narco has been an established provider of the AR-850 to aircraft manufacturers. Their 20,000-foot model is around $235 and the 30,000-foot version is $333. Shadin manufactured the IIMorrow encoder and now does the selling and making of the $850 Falcon for upper end applications requiring serialized encoding.
Terra is-or was-the other full line avionics company with an encoder. Their AT3000 sells for less than $250. However, with the recent announcement by Trimble that its getting out of the avionics business, were not sure what will happen to the old Terra line, which Trimble bought three years ago.
Trans-Cal Industries (TCI) specializes in instruments, and has a long reputation for excellent aneroid (mechanical) encoders. Finding their D120-P2-T in an airplane always made me feel a bit better about having a successful biennial encoder check. Trans-Cals SSD120 is a solid-state version, available with ranges up to 80,000 feet, important for the new breed of civilian spacecraft.
The altitude encoder is a commodity. With no user features and a standard interface, similar warranties and reliability, price and availability are the deciding points. Brand loyalty? Buy one type and if it fails two years after the warranty runs out, try the other guy.
Thats with one limitation: Be sure that the plugs are the same. All of these things use a small DB-15 connector and not all are wired the same, even within the same company. The labor cost to rewire is not worth the trouble for a $200 box.
Also With This Artucle
Click here to view “Mode-C Legalities.”
by Gary Picou
Gary Picou is Aviation Consumers avionics editor.