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Instrument Post Light and Dimmer Circuit Installation in an Alon Aircoupe

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Disclaimer: The following are miscellaneous notes about the installation of Grimes instrument post lights and an Ameri-King solid state dimmer circuit in an Alon Aircoupe. These notes are for informational purposes only and are not to be used for modifications to any aircraft. These notes may contain errors and/or omissions. The author is not a licensed aircraft mechanic or inspector. The author is not responsible for any errors or omissions, and is not responsible for any subsequent use of this information.

Approval Method: The equipment was installed after approval of an FAA Form 337. To support the submittal of the Form 337, an electrical load analysis was completed, the Ameri-King Installation Manual and copies of the TSO paperwork were obtained, sketches of the fabricated mounting bracket and bracket attachment method were prepared, wiring diagrams were drawn, and a proposed weight and balance sheet was prepared. The paperwork was submitted by the owner's A&P/IA mechanic, and FAA approval was received approximately 3 weeks later. All work was completed by the owner under the direct supervision of an A&P/IA. Following installation, a logbook entry was made which referenced the Form 337 and the revised weight and balance form.

Equipment Description: Twenty-one Grimes post lights were purchased used on EBay for approximately $5.00 each. They had been removed from a Beech Baron for an upgrade. The post lights were inspected, cleaned, repainted and tested prior to installation. New 18 gauge wires were soldered to each post light connector. This heavier-than-required 18 gauge wire was used for all wiring except for the multi-conductor cable, not for current carrying capability, but for physical strength.

Some of the post lights have long threaded shafts, and some have short threaded shafts. The post lights with long threaded shafts were installed on instruments with thicker instrument mounting flanges such as the altimeter and turn and bank indicator. The assemblies with short threaded shafts were installed on instruments with thinner mounting flanges such as the suction gauge and oil temperature and pressure gauges.

Grimes Instrument Post Lights as Removed From a Beechcraft Baron

The Ameri-King AK-551-LD-S solid state dimmer circuit was purchased from Aircraft Spruce for approximately $65.00. The solid state dimmer circuit allows a small potentiometer to be used to control the brightness of the connected lights. Other models of the AK-551 provide for different input and output voltages, and can provide up to four output circuits which can be dimmed individually. The primary reason that a solid state dimmer circuit was chosen to dim the panel lights was to eliminate the need to install a potentiometer or rheostat with high current carrying capability on the instrument panel. The potentiometer connected to the solid state dimmer is very small, requires less instrument panel space and does not get hot.

The AK-551 kit comes with the solid state dimmer circuit mounted on a rather large (5.5 in long x 4.25 in wide x 4.3 in high) heat sink, a 1K ohm 1/4 Watt potentiometer with a black plastic knob, a factory made multi-conductor cable with a mating plug attached, and an instruction manual. The factory supplied cable is quite short and had to be spliced to another section of multi-conductor cable for use in the Aircoupe. The Aircraft Spruce catalog indicates that the potentiometer has an on/off switch built in, but the potentiometer supplied with the kit did not have an on/off switch. When the potentiometer is turned down the lights go out but power is not removed from the AK-551 until the Panel Lights circuit breaker switch is turned off.

MIL-spec wiring and typical electrical connectors and AN/MS hardware were used for the installation. A panel mount type fuse holder was installed near the dimmer circuit to protect the dimmer output wiring.

Ameri-King AK-551-LD-S Solid State Dimmer Circuit

Mechanical Installation: The post light assemblies were installed in existing instrument mounting screw holes. These holes had to be drilled out to enlarge them so that the threaded post light studs would fit in them. In order to drill the holes, the instrument mounting screws were removed and the instruments pushed forward, away from the instrument panel. The instruments were temporarily supported with ty-wraps or rags as required. The instruments were not disconnected from their electrical or pneumatic connections. One new hole was drilled for a post light that illuminates the switches on the lower left hand side of the instrument panel. A shop vacuum cleaner was used to collect the drill filings as the holes were drilled. The aircraft battery was disconnected prior to beginning any work, and the aircraft electrical switch assembly was covered with a rag prior to drilling any holes above the switch assembly.

Post lights could not be installed in the mounting holes for the directional gyro or the artificial horizon. The mounting holes in these instruments have internal nuts, so post lights can not be installed in these holes. Post lights could have been installed outside of the mounting screws on these instruments, however the gyros in this aircraft will be replaced in the near future, and the replacement gyros will be purchased with internal lighting.

Post Lights Installed in Instrument Panel of Alon Aircoupe

Special small dimension nuts were used on the post lights. These nuts were required because the clearance between the flats on the nuts and the instrument case on most instruments is very small. A thin wall deep socket was used to tighten most of the nuts on the post lights. In some cases it was necessary to use a tiny ignition wrench to tighten the nuts due to clearance problems. Each post light was installed with a ring type lug and ground wire and a lock washer under the nut. Because the threaded posts on the post light assemblies are not threaded all the way to the body of the post light, it was necessary to use spacers or washers under the nuts in some cases to prevent the nuts from running up on the unthreaded portion of the threaded shaft. Different instruments required different spacer and washer combinations.

Note: The threaded shafts on the post lights are very fragile because the threads are cut almost all the way to the inside diameter of the hollow shaft. The nuts were tightened only about 1/8 turn beyond finger tight. Tightening the nut too much on one post light resulted in shearing the threaded part of the shaft, ruining the light assembly.

The potentiometer was mounted in a drilled hole in the U-channel that extends across the bottom of the instrument panel in the Alon Aircoupe. Clearance for the wires soldered onto the potentiometer was considered when selecting a mounting location. Heat dissipation and free air space around the potentiometer was not a factor in selecting a location since the solid state electronics and heat sink of the AK-551 are used to dissipate the excess energy in the dimmer circuit.

 Dimmer Circuit Potentiometer Installation

The only viable location that could be found to mount the AK-551 solid state dimmer circuit and attached heat sink was on the aluminum structure of the extended baggage compartment, aft of the rear wall of the baggage compartment. There was not sufficient room behind the instrument panel or beneath the seats to mount the AK-551.

A "U" shaped mounting bracket for the AK-551 was fabricated from 0.032 inch thick 2024-T3 aluminum. The bracket has a hole with a grommet in it to allow the multi-conductor cable to pass through. The bracket also has a hole to mount a fuse holder for the output channel fusing. After forming, fitting and drilling the mounting bracket, the bracket was lightly sanded with 320 grit aluminum oxide sandpaper, washed with lacquer thinner, and painted with flat gray epoxy primer. Four plate nuts were then installed on the bracket to facilitate the future removal of the AK-551 from the bracket. The platenuts were mounted with countersunk flat head rivets.

The bracket was attached to the existing Former F and the baggage compartment structure using eight 8-32 stainless steel screws, flat washers and fiberlock nuts. An 0.032 inch thick shim was used behind one edge of the bracket as a spacer to compensate for an overlap in the metal that the bracket was mounted on.

All drilled holes were deburred, and all edges of the fabricated aluminum parts were smoothed in accordance with good aircraft sheet metal practices. After all drilling was completed, all metal shavings were removed from the aircraft with a shop vacuum.

Fabricated Aluminum Mounting Bracket Looking Forward

Ameri-King AK-551-LD-S Installed Showing Cable and Fuse Holder

Electrical Installation: All of the new individual conductors that were installed were 18 gauge stranded MIL-spec wire. Connections on individual conductors were made with crimp-on ring type lugs and crimp-on butt splices. The multi-conductor cable was extended using solder connections and heat-shrink tubing. A length of shielded industrial instrumentation and control cable with the same specification number as the pre-made factory cable was used to extend the factory cable to the instrument panel area. The splice in the multi-conductor cable is located under the aft baggage compartment floor. The cable extends from the AK-551, through the grommet in the mounting bracket, and follows the factory original main wiring harness forward to a location forward of the lower left hand corner of the instrument panel. New Adel clamps were installed to secure the cable as required. At the lower left hand corner of the instrument panel, the multi-conductor cable was stripped back and the individual conductors were routed to points forward of the instrument panel as required.

Dimmer Circuit Cable Routing Under Baggage Compartment Floor

The AK-551 is powered from the existing Panel Lights circuit breaker switch. This breaker switch is rated at 5 Amps, which is an appropriate rating to protect the 20 gauge power wire to the AK-551. This power wire is contained within the 20 gauge multi-conductor cable, as are all of the post light system conductors that run from front to rear.

The output power from the AK-551 to the post lights is fused by a 5 amp AGC-5 fuse mounted in a fuse holder. This fuse holder is mounted on the bracket for the AK-551 and is only accessible by removing the aft baggage compartment wall. This location is not very convenient, and is not accessible in flight, however in order to protect the AK-551 output wire, the fuse must be located as close to the power source (the AK-551) as possible. An alternative might have been to mount the fuse holder on the aft baggage compartment wall, but that location would have complicated the removal of that wall for maintenance. Because this fuse only powers the instrument and avionics lighting, it was deemed that the fuse location behind the baggage compartment wall was acceptable. The instruments and avionics can be illuminated from the original overhead flood lights, so loss of the post lights and internal avionics lights are not a safety of flight item.

A new ground stud was installed near the factory original ground stud in the aluminum fuselage former just aft of the fuselage fuel tank. The new ground stud was used to tie all of the ground wires from the post lights and the ground wire from the potentiometer to a single ground point. The AK-551 itself (case ground) is grounded via the mounting screws to the aluminum baggage compartment structure.

Ground wires and power wires to the post lights were connected forward of the instrument panel in multiple branches using butt splices. It was necessary to do some planning prior to installing this wiring in order to have no more than two 18 gauge wires installed in each end of each pink butt splice, and no more than three 18 gauge wires installed in each end of each blue butt splice. All of the new wires behind the instrument panel were carefully ty-wrapped to ensure that they were properly secured and that they would not interfere with anything behind the panel.

The lighting dimmer wires for the Nav/Com, transponder, clock, compass, CHT, EGT, carburetor air temp and voltmeter were also connected to the circuit powering the post lights so that all lights are adjusted by the potentiometer.

Results: Installation of this equipment and the associated wiring added 2.25 pounds to the empty weight of the aircraft, and moved the empty CG aft by 0.07 inches. Maximum current draw by the AK-551 (with the described lights connected) was calculated to be 2.85 amps.

The post lights illuminate the instruments well. In particular, the oil pressure and oil temperature gauges in the lower left hand side of the instrument panel are much easier to see at night than they were with only the overhead flood lights turned on. The artificial horizon and the directional gyro are not yet lighted and this limits the usefulness of the post lights at the current time because with the overhead flood lights turned off, these instruments are not readable at night.

The resistance range on the potentiometer does not seem to be correct for this installation, because the potentiometer must be turned up about half way before the light from the post lights is visible. Only the upper half of the potentiometer range provides a variable amount of light.

The post light caps as purchased on EBay have red lenses in them. The red lenses seem to decrease the amount of light available on the instrument faces. Blue tinted or clear lenses would probably provide more light intensity, however the lighting is adequate with the red lenses installed. The compass has a built-in white light, and for any given potentiometer setting, the compass seems to be much more illuminated than the other instruments with the red post light lenses. At some time in the future, the caps with red lenses may be replaced with caps with blue or clear lenses.

Overall, this upgrade turned out very well and the owner is pleased with the results.