Warning: WX system will transmit microwave energy

WX system will transmit microwave energy By Jim Sparks The prime mission of airborne weather radar is the detection, processing, and display of weather phenomena with the objective of generating advance warning of potentially threatening...

stand alone radar control Radome with static strips and protective cap
Top: Standalone radar control panel located below multifunction display (MFD). Bottom: Radome with static strips and protective cap.

Safety concerns
Radiated energy is only one concern when it comes to working around radar. The various electronic equipment may include sealed components containing such things as beryllium oxide, acid, lithium radioactive matter, and even mercury. It is always advisable to take precautions to ensure protection against inhalation or contact. Electrostatic discharge (ESD) is always a concern around any electronic device and awareness here can prevent possible catastrophic or latent equipment failures.

Many systems still in use contain a Magnetron; which is the main power output device. As the name implies this unit uses magnetic principles and in fact it is so critical that in some installations if the magnetron is installed improperly it may affect the aircraft compass systems. Also a magnetron is subject to degradation as a result of aging and it is the recommendation of several manufacturers that routine testing including power output, frequency, and sensitivity be accomplished at least once a year.

Handling, storage, and shipment of a magnetron also requires special attention. Interaction with any ferrous materials can be detrimental. Collins, for example, recommends a minimum 4-inch space between the magnetic components and any ferrous matter. Even storage on metal shelving can be risky. It is desirable to ensure proper separation using wood or plastics but always be aware of the ESD potential. Much of this equipment is also susceptible to damage caused by mechanical shocks. In some cases dropping a receiver transmitter 3/4 of an inch can have devastating effects.

Flight deck displays
Flight deck displays take on many forms. Some employ the cathode ray tube principle while others use phosphorescence. Many EFIS systems even use liquid crystal displays (LCD). The type display may impact proper interpretation. Displays using phosphorescence are more prone to interference from sunlight and usually include some type of glare shield.

Selector panels in the flight deck control weather radar systems. In many commercial aircraft both pilots have independent control panels and depending on the installation system operation may be aircraft specific. A "mode" selector is the means of setting the different functions of the device. "Off" is not an acronym for "on full force," but instead deactivates the system. A "test" feature is often included and while the system including the transmitter receives power the emission of microwave energy is inhibited. This function can allow visual inspection of the antenna to verify proper mechanical operation without concern for a radiation hazard. "STBY" is a feature that will allow the system to be powered but once again transmission in inhibited. Aircraft with dual control panels may allow radar operation if only one panel is in STBY while the other is selected to some operating condition. It is always advisable when planning maintenance to make sure both controls when applicable are appropriately set.

Some systems have a target alert (TGT) mode and when selected the pilots may not have control on the range or the antenna degree of tilt. The ability of the system to automatically control the antenna is only possible by enabling the radar to determine the aircraft height above ground level. This is accomplished by allowing the radar beam to scan the ground every third sweep. This dipping of the beam should not affect the weather display. With TGT selected the system will evaluate a specific window in front of the aircraft in some cases up to 200 nautical miles. In the event of significant weather the flight deck display will provide the crew with a target alert as well as highlight the danger zone.

Selecting the "MAP" mode places the system in a terrain-mapping situation. This will allow prominent geographic features to be identified. This is most effective in the shorter ranges with the antenna tilted down.

Weather factors and performance
The "WX" or "weather" enables the radar to perform its primary function. Now the reflected energy is analyzed and based on rate and type of precipitation the flight deck display will depict as directed by the RT unit.

"Gain" is another feature where improper use can invalidate weather information. This adjustment will lengthen the transmitted pulse width plus impact the sensitivity of the receiver in newer systems. In fact with color displays, improper selection can alter the display by as much as one color level. Many systems have a "gain calibrate" feature. The pilot should always actuate this when one of the negative settings has been used.

A "tilt" function will allow for pitch adjustments of the antenna. This will allow the flight crew to map the ground or even determine the tops of storm cells.

"Range" control decides how far the radar will look in front of the aircraft. The greater the distance the arc of the transmitted signal is concentrated, close proximity storms may go undetected as it moves into the aircraft path from either side. In some systems with the radar operating with a range of 300 nautical miles, an area of 37 miles may be blanked on either side of the aircraft.

Wave-Guides have been used as a device to carry the microwave energy between the receiver transmitter and the antenna. This hollow tube is designed with a specific shape and should be kept free of distortion and damage. Contamination is another concern. In the event moisture should get into the tube it can disturb the signal and have an impact similar to the aircraft having an encounter with weather. To prevent this, pressurizing the Wave-Guide is often accomplished and in other cases a desiccant is used to draw moisture from the device.

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