Learjet Summer 2001

• Weather Radar Tilt Management
• Starter Brush Inspections
• Test Light Usage Cont.

Weather Radar Tilt Management

— Jim Wheaton

Storm season is upon us! The technology of radar is a great tool to avoid damaging weather, but it is important to remember its limitations. Radar registers only “wet” objects such as rain, wet hail and wet snow. This excludes “dry” hail, ice crystals and dry snow which are most common at high altitudes. Recently, Duncan Aviation technicians have repaired several aircraft which were flown into damaging “dry” precipitation that was invisible to properly functioning radar systems and alert pilots.

To minimize this possibility, it is imperative that flight crews train, practice and master the use of their radar’s manual tilt function. Some pilots rely on the auto-tilt feature to track cells, but this feature is designed to compensate for aircraft altitude changes only. Instead, the flight crew should manually adjust tilt as the aircraft moves in relation to the storm cell and focus on the most reflective precipitation, which is found in the bottom 2/3 of storm cells. Proper tilt management allows radar to see precipitation at lower altitudes and alert the crew to the potential of damaging precipitation at higher altitudes. According to multiple manufacturers’ technical reps, poor tilt management is the leading cause of radar performance complaints.

Starter Brush Inspections

— Chris Gress

During periodic brush inspection of Bendix Starter M/N 71B1-23A/P/N 6608268-6, prior to pulling the brush for inspection, remove the screw that holds the brush lead in place, noting the location of each brush.

Duncan Aviation technicians have seen many of these starters where the brushes were chipped or cracked due to the spring impact on the brush or forceful removal of the brush for inspection. The shunts don’t allow much room for the brush to come out of the holder. This is also true when re-installing the brush. Mechanics should use a hook-like tool to firmly hold the brush spring back so that the spring does not slip, thus impacting and damaging the brush. During reinstallation be sure to properly torque the screw. A loose screw touching the end bell cover will short the unit and cause considerable damage.

When inspecting the brushes, look for commutator pitting & look at the condition of the brush, such as burning, pitting, arcing and the hardness. Also check the fan to be sure it is not loose, blow the carbon dust out of the unit and inspect the drive jaw for abnormal wear conditions.

Duncan Aviation recommends a brush & commutator recondition every 800 hours, which will prolong the starter’s life & reliability at a lower cost.

Test Light Usage Cont.

— Bill Schroeder

Another use for the test light is to clean contacts in some circuits. Examples:

1. The Generator light does not come on when the generator is off. We have found that the likely problem is the contacts on a relay in the generator control panel are dirty, causing enough resistance to prevent sufficient current to turn the light on. By connecting a test light and power supply through the circuit for the light in the generator control panel, we can zap the contacts and the light will be back to normal.
2. The thrust-reverser (TR) throttle switch contacts are susceptible to silicone contamination. The symptom for this: no TR reaction when “piggy backs are pulled,” and the engines come up to reverse idle. To clean the silicone from switch contacts, operators must connect a power supply and test light in series with the switches and then operate the switches several times. The reverses will then operate normally.

NOTE: You may ask, “Where does the silicone come from that contaminates the throttle switches?” We believe it comes from some products used to clean the interior.

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