The Duncan Download Blog: Business Aviation Advice & Observations

Contributed by Curt Campbell, Avionics Tech Rep with permission from Carter Hart of Airframe Service

Isolation diode shorts in the stab actuator can disengage the autopilot.

Last week I received a call from Carter Hart of Airframe Service, who was working with an autopilot problem. The autopilot would engage properly, but would immediately disengage as soon as the pitch servo tried to drive the elevator.

The autopilot computer had been sent to the factory but was returned with no fault found. Through several telephone conversations, Carter and I concentrated on the autopilot pitch trim operation and troubleshot the aircraft wiring, and measured voltages and verified logic conditions in the aircraft. This process led to the discovery of a shorted diode internal to the horizontal stabilizer actuator.

Checking with our accessory technicians, it was determined that we could repair the actuator the same day it was received. The stabilizer actuator was pulled by Carter on Thursday and arrived at Duncan Aviation on Friday. The unit was inspected, repaired and returned to service that same day then shipped back to Carter for delivery on Saturday. Carter reports the aircraft is now flying fine.

Duncan Aviation autopilot technicians have seen the isolation diodes in the stab actuator fail before, and we are trained to assist in diagnosing this problem without having to pull the stab from the aircraft. We can provide one-day repairs on all Learjet stab actuators if we are given prior notice of the unit coming in.

Contact Curt Campbell or an Accessory Technical Representative for troubleshooting assistance.

Written by Larry Troyer, Avionics Tech Rep

The FZ-500 Flight Director computer that is installed in many business aircraft and King Airs provides the mode annunciator drive out to the mode selector annunciators. If some of the mode annunciators do not test or light up the problem may be the FZ-500 computer. However, it could also be the lamp in the mode selector switch.

Great care should always be taken when replacing the lamps in the mode selector. Due to the construction of the switch sockets, it is possible to short the bulb supply voltage to the control terminal that comes from the FZ-500. This can spike and take out a very expensive part in the FZ-500 computer.

Contributed by Jeffery A Witt, Team Leader, Pneumatics

If your passengers or crew are beginning to complain about uncomfortable cabin temperatures, it is time to take a look at your aircraft's air conditioning system. The following is one of the most common solutions.

Stop the squawks. Change your socks!

The coalescer assembly is responsible for separating water from the air being introduced into the cabin. Unfortunately over time the coalescer fabric collects dirt and dust and the efficiency of the assembly is significantly reduced. Occasionally ice will build up on the safety valve contained inside the assembly and almost always causes the air to bypass the coalescer sock allowing moist air to enter the cabin, increasing the humidity, which in turn increases the likelihood of corrosion and the probability that passengers and crew will begin to feel the uncomfortable effects.

It is very simple to replace the coalescer socks and bags to increase the airflow to the cabin. Duncan Aviation is able to replace coalescer bags and socks on a send in basis on Falcon 900 & 2000, Beechjet 400 and Challenger 600 & 601.

Written by Dan Magnus, Avionics Tech Rep

Honeywell's GNS-XLS is a Flight Management System that provides control for an aircraft's navigation sensors, computer-based flight planning, fuel management and frequency management, among many other necessary capabilities. When this unit begins to malfunction, it is critical to get it back on line quickly.

I recently fielded a phone call from a DOM asking for troubleshooting advice when the screen on the GNS-XLS went white. Here are the steps I used to determine the cause and provide the solution.

The best way to determine if a unit is faulty is to install another and see if the squawk disappears. I sent out a loaner to arrive the next day. After the unit was installed, the screen remained white, telling me it wasn't the unit causing the squawk.

Next, I had him unhook the antenna. This immediately brought the screen back to its normal state. We replaced the antenna and the system is back on-line.

Squawk solved.  

If you need help troubleshooting or repairing your avionics, contact Avionics Tech Support.

A Learjet 35 pilot reported intermittent audio clacker in his headset, even though the stab actuator was not trimming. The audio clacker is designed to alert the pilot when the horizontal stabilizer actuator is trimming.

At Duncan Aviation, we connected the stab actuator to a Learjet test stand for function testing. The results indicated that everything was working properly. Knowing we couldn't give the unit back with a No Fault Found, we investigated and tested further.

With the trim-in-motion potentiometer connected to an "analog" Simpson meter, we ran the stab actuator the full length of its travel. We discovered if we stopped the stab actuator in a specific spot and lightly tapped on the potentiometer, the analog meter would read an erratic resistance (the needle jumped around). A change in resistance triggers the audio clacker in the pilot's head set.

After replacing the trim-in-motion potentiometer and retesting the whole unit, no other faults appeared. On the next trip, the pilot reported the audio clacker only went off when the horizontal stabilizer actuator was really trimming.

Written by Chris Gress, Component Solutions Manager

The Switching Valve Part Number 3213788-1-1 is in the process of being modified to a 3213788-3. The difference is the new P/N is an improved thermal compensation device. It is currently being installed on production aircraft. HBC will soon release information allowing P/N 3213788-3 to be installed on the Hawker 750, 800 series, 850 and 950 variants.

Because there haven't been any documents released to support field level modification or repair and overhaul, this modification must only be performed by HBC and P/N 3213788-3 must be returned to HBC for repair and overhaul.

Written by Jerry Cable, Duncan Aviation Accessories Tech Rep

Written by Larry Troyer, Avionics Tech Rep

I had a Challenger 600 customer call this morning with this squawk:

It was quickly determined something in the aircraft took out both computers, but what?

I recalled a similar situation with this computer where the aircraft had a short to ground on the FD VALID logic output pin that was taking it out and causing the same symptoms. After instructing the technician to check this wire in the aircraft, he found it was indeed shorted to ground. If left unfixed, this would again take out the flight director computer.

This is the second instance where I have seen this scenario occur.

This pertains to only aircraft with Honeywell SPZ-500 ( ) systems installed, which does cover a good share of corporate, twin turboprops and corporate jets of the 1980s and 1990s.