December 2014-
Q: Hi Steve,
I’m just about to replace a damaged engine mount on my PA-22-150.
Has anyone got a torque figure for the firewall attachment bolt (AN6-35 & AN6-24)? I would be grateful for any help.
—Info Seeker
A: Hi Seeker,
Unless the PA-22-150 maintenance manual specifically addresses that application (I don’t have a copy of it to know for certain), your go-to source for aircraft bolt torques is the table in Advisory Circular 43.13–1B, the mechanics’ bible.
The title of AC 43.13–1B is “Aircraft Inspection, Repair and Alterations.” The torque values in the table are for oil-free cadmium plated threads.
The table gives a torque range (in inch pounds) for bolts primarily loaded in shear and a maximum allowable tightening torque limit. The torque in a shear application for the AN6 bolt while using a MS20365 or AN310 nut is 160 to 190 inch pounds (13.3 to 15.8 foot pounds). The maximum torque for that bolt/nut combination is 390 inch pounds (32.5 foot pounds).
But, while this is a general guide, it doesn’t quite do it in this case because these bolts are loaded in both shear and tension—not only is the weight of the engine loading the bolt in shear (across the shank of the bolt) when the engine is developing power, the bolt is loaded in tension (along the length of the bolt).
I did further checking, and in the PA-28 service manual (-140 up through -235) the torque specified for the AN6 bolt and MS20365 nut in the mount-to-airframe application is 230 to 240 inch pounds (19.1 to 20 foot pounds). You’ll be okay using the 230 to 240 value for those bolts.
Happy flying.
Q: Hi Steve,
I have a Piper Aztec E and recently I’ve begun having starting problem on my left engine. When I select the left engine start position the engine spins over pretty well, but it doesn’t fire off immediately like it used to. The right engine turns a couple of blades then fires right up and runs smoothly.
Sometimes the left engine will start just as I quit pushing the start button. I’ve been told by one of the guys at the airport restaurant that it’s something called the ignition vibrator. I’ve heard a lot of talk about impulse couplings but can’t recall hearing anyone talk about an ignition vibrator.
Can you shed some light on this and tell me how to determine if it’s the problem?
—No-Fire Ned
A: Dear Ned,
Ignition vibrators are electromagnetic devices that generate and deliver a continuous stream of what’s termed “boosted output” low tension electric current to the primary winding in the coil of left magneto when the start button or switch is selected. During starting the right magneto is grounded and doesn’t fire.
During engine rotation, sets of points—called the advance and the retard sets—are closed, preventing the boosted output from connecting to the coil until the piston(s) are very near the top dead center (TDC) position on the compression stroke.
As the piston approaches TDC, the two point sets open and the boosted output is connected to the primary windings of the coil. This produces a continuous high tension spark flow out of the magneto coil. This flood of sparks is routed to the correct spark plug by the distributor section of the magneto.
Once the engine starts and the start button is released, the ignition vibrator is no longer energized and the right magneto is no longer grounded. The ignition vibrator is done until the next start cycle.
Bendix ignition systems coined the term “shower of sparks” to describe the action of its ignition vibrator. In contrast to a magneto with an impulse coupling that only generates one hot spark during the start sequence when the piston is near TDC on the compression stroke, an ignition vibrator supplies an almost continuous shower of sparks for approximately 20 degrees of crankshaft rotation.
There are a couple things that will cause poor vibrator performance. The first is low battery voltage. Since vibrators depend on electromagnetic force to generate the boosted output, there must be sufficient current flow through the coils inside the vibrator.
The second thing is a breakdown of the internal condenser which causes pitting in the point sets.
A four-step method of testing ignition vibrator performance is:
1. Disconnect the starter (at the starter or starter solenoid);
2. Remove all spark plug ignition leads;
3. Position the number-one piston at 10 degrees before top dead center;
4. Position one of the number-one cylinder spark plug wires near a ground (the cylinder fins, etc.) and watch for a hot spark through about 20 degrees of crankshaft rotation while the start switch in the cockpit is pushed to the start position and the prop is slowly pulled in the normal direction of rotation.
New ignition vibrator prices start at approximately $400 and can be purchased from any of the big parts supply houses, or yours can be rebuilt for around $225 by companies such as Aircraft Magneto Service in Washington.
Happy flying.
Q: Hi Steve,
I am very happy with my 1964 Cherokee. Now I want to install shoulder harnesses. What do you recommend?
—Safety Sally
A: Dear Sally,
Good move. One of the very first things I did to my Comanche was to install a set of shoulder harnesses for the two front seats.
There are two companies that sell FAA approved shoulder harness type seatbelt kits.
Alpha Aviation in Minnesota sells three-point kits in which the upper end of the diagonal belt is either connected to an inertia reel attached to the fuselage or bolted directly to the fuselage.
Each kit from Alpha Aviation includes belts for the two front seats and includes all new lap and diagonal strap belts. The kits are available in black, gray or fawn, and other colors are available as special orders. (There’s no extra charge for non-stock colors—it just takes a little longer for Alpha to ship the order.) Alpha estimates eight to 10 hours to install the set.
B.A.S., Inc. in Washington offers a shoulder harness installation that is a four-strap, over-both-shoulders kit. Each shoulder harness belt assembly reels in and out of an inertia reel that is attached to the fuselage above and behind the front seats.
B.A.S. kits are stocked in black, but a wide range of colors are available by special order. B.A.S. kits also include new belts and all new hardware.
The B.A.S. inertia reel kits are slightly more expensive ($1,330) than the Alpha inertia reel kits ($1,039); Alpha’s no-reel kits at $839 are the most affordable. (Prices are accurate as of Oct. 25, 2014 but may be subject to change. —Ed.)
Happy flying.
Know your FAR/AIM and check
with your mechanic before starting
any work.
Steve Ells has been an A&P/IA for 43 years and is a commercial pilot with instrument and multi-engine ratings. Ells also loves utility and bush-style airplanes and operations. He’s a former tech rep and editor for Cessna Pilots Association and served as associate editor for AOPA Pilot until 2008. Ells is the owner of Ells Aviation (EllsAviation.com) and the proud owner of a 1960 Piper Comanche. He lives in Paso Robles, Calif. with his wife Audrey. Send questions and comments to editor@www.piperflyer.com.
RESOURCES >>>>>
AC 43.13–1B
www.piperflyer.com/AC43131B
Aircraft Magneto Service, LLC
aircraftmagnetoservice.net
Alpha Aviation Inc.
alphaaviation.com
B.A.S., Inc.
basinc-aeromod.com
