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Practical AvionicsTM
No. 8	Index to All Subjects
In This Issue: Weather Radar: Hazardous to your Health? Autopilots: When Your Autopilot Rolls---or Rocks Datalink: What's in it for You? Audio: When a Microphone is a Speaker Does Radar Harm You? "I'm a DC-9 Captain and I recently flew with a former Navy pilot who could not wait to turn off the radar as soon as it was no longer needed. He told me that radar energy constantly leaked through the forward bulkhead and even wrapped around the fuselage. He said it posed a constant threat to safety. I disagreed---who is right? Also, will radar act as a deterrent against birds?" This question keeps surfacing so let's examine some facts. The FAA has published an Advisory Circular (AC-68B) which states that weather radar should never be operated in a hangar unless the energy is directed into an absorption shield. And radar shouldn't be operated during fueling operations. Finally, the Advisory gives safe distances between the radar and humans while the airplane is on the ground. For example: a 22-inch radar antenna with a peak power of 40,000 watts (average power of 24 watts) should be a minimum of 14.2 feet away. There is nothing in the Advisory, however, about operations in the air, but a discussion with several authorities developed these points. They suggest that being harmed by radar in flight has no basis in fact. A good example is what happened with the British Heron, a four-engine transport that was so small,. the radar antenna and landing gear couldn't both fit in the nose of the aircraft. Obviously, the nose gear couldn't be relocated, so the antenna was mounted in a pod over the pilots' heads. To check for dangerous radiation, the radar manufacturer (Bendix) measured the fields in the cockpit and determined they were too low to be harmful. The usual location for the radar antenna in multi-engine aircraft today is in the nose where several factors protect the pilot. First is the radar radiation pattern. Whether the antenna is a parabolic reflector or the newer planar array, it is designed to launch every possible watt in a swept area ahead of the aircraft. A small amount of energy escapes off the main beam as "sidelobes" but energy scattered to the rear (known as spillover) is blocked by the reflecting surface (the "dish") of the antenna. Even if some energy escapes to the rear, it would strike the large bulkhead between nose and cockpit, and be effectively shielded. The frequencies of weather radar (in the S and X band) are so high they have almost no penetrating power.. That is also the major benefit of radar; the waves hit raindrops and reflect back, rather than penetrate. The idea that radar can "wrap around the fuselage" is a strange one. Unless a radar wave travels in straight lines---both ways---it would move targets far off the centerline (or boresight) of the antenna, then show them in the wrong place. Yet another factor is that radar manufacturers devised a technique about 25 years ago that greatly reduces the transmitter power emitted by a radar. Back then, radars had crude frequency generators which could emit energy only over a wide range of frequencies. This meant the radar receiver had to be broad enough (wide bandwidth) to collect the returning echoes. As a result, the old receivers low amplification, which must be made up by high transmitter power. Today, the radar transmitter can produce very narrow signals which, in turn, allows the receiver to be narrow in bandwidth and high in gain. Thus, radar signals are now much lower in level. Does radar help prevent bird strikes? It is known that birds are influenced by electromagnetic energy. The U.S. Army, which once used homing pigeons for carrying messages, performed the following experiment. They attached magnets to the birds' heads and dropped them from an airplane. The birds became completely disoriented and simply spiraled down. However, experiments in Canada and Japan to clear runways of birds with radar waves (also electromagnetic) had no effect at all. -Archie Trammell has trained pilots of every major airline, FAA, NASA and corporates on proper operation of weather radar. To see details of his home study video, click: Wx Radar Course -The most popular engineering book on the subject is: Introduction to Airborne Radar Rolling Autopilot I recently acquired a customer, writes an avionics shop owner, with a two-year-old autopilot that would gradually roll to the right after being set on course. Some earlier repairs had failed to solve the problem, so I suggested taking it back to the factory. There they found a chafing wire, and also bench-tested the gyros. They stated that a new autopilot was installed. A test flight showed no problem, but after arriving home, the pilot now experienced a left-rolling tendency. How critical, the technician asks, is aircraft rigging and cable tension? The pilot also mentioned that the attitude gyro is a bit lazy to erect and tends to show an unlevel attitude. The autopilot is greatly affected by aircraft rigging, tension in flight control cables and tension in the autopilot bridle cables. In this case, the bridle tension on the roll servo may be out of adjustment. A lazy attitude indicator that does not fully erect certainly can cause problems because the airplane will chase it, but never achieve a wings level condition. After the gyro is fixed and cables adjusted, a full ground set-up is essential to be sure all (electronic) adjustments properly center the controls, and gains in the autopilot amplifiers are correct. This requires a test set for that autopilot to accomplish every step of the procedure. Finally, check all the brackets which hold pulleys for the control cables. A loose one can cause this kind of trouble. Datalink: What's In It For You? The worst accident in aviation history was not caused by a crash, thunderstorm or controlled flight into terrain . It was not due to mechanical failure, spatial disorientation or contaminated fuel. The single accident with the greatest fatalities was caused by....something we will get to in a moment. The next generation of air traffic control is just beginning and a major component is datalink. Instead of talking into a microphone or punching buttons, most pilot communications with the ground will be automatically accomplished through digital signals. They are faster, more efficient and far less prone to misunderstanding. It couldn't have happened at a better time because the upward spiral of air traffic threatens to clobber voice communications. Like the digital coding for the text you are now reading, hundreds of words are transmitted in less than a second. Datalink---computer-generated messages---will also bring many new services into the cockpit; including clearances, weather, traffic displays and other information. The FAA recently began upgrading its computers with CPDLC---Controller to Pilot Datalink. Airlines expect to reduce major delays; light aircraft will enjoy new services with relatively inexpensive equipment. An exciting prospect is the uplinking of National Weather Service radar displays to small aircraft that cannot carry airborne radar, including a look-ahead feature to view real-time weather hundreds of miles ahead. Datalink promises to reduce a major problem today----runway incursions---aircraft landing on each other or colliding on runways or taxiways, often caused by poor communications. Datalink will also provide a CDTI---Cockpit Display of Traffic Information---giving pilots a visual picture of nearby aircraft. It might have prevented that worst accident in aviation history, when two B-747's collided on a foggy runway in the Canary Islands. The cause was determined to be blocked communications between the tower and aircraft. Two pilots had keyed their radios at the same time, blocking spoken clearances. As datalink is phased in over the coming years, it should solve problems of being "stepped on," verbal misunderstandings between pilots and controllers, and the confusion of taxiing at night on poorly marked taxi- and runways. Microphone or Speaker A pilot walked into an avionics shop and said, "My speaker doesn't work." The technician examined the airplane and determined that the speaker was working perfectly. By the time the technician went out to the ramp, flipped some switches and returned, about a half-hour had elapsed. The shop charged a minumum of $60 for this service. The pilot scratched his head. He pointed to his microphone and said, "I think that's the problem." The technician said, "That's not a speaker, that's a microphone." The pilot replied, "But I speak into it." Yes, this episode really happened. Who was wrong? The pilot can't be blamed; he spoke into a speaker. He shouldn't be expected to know it's an "electret ceramic audio transducer." The technician reacted with his own knowledge and examined the cabin roof for the loudspeaker. Maybe he should have questioned the pilot more closely. As we will see in future issues of Practical Avionics, the most expensive part of an avionics repair can result from poor communication between pilot and technician Publisher/Editor - Len Buckwalter Your comments and suggestions are welcome: Copyright 2000 Avionics Communications Inc. Return to top		New Publication The most up-to-date avionics text available. PRINCIPLES OF AVIONICS

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