Be impulsive - Bendix Magnetos & Airworthiness

December 2023

This ShopTalk will explore the aircraft maintenance field for small CFR Part 91 aircraft, and airworthiness requirements for these certified aircraft. We will delve into the interaction between Service Bulletins (SBs) and Airworthiness Directives (ADs) concerning certain Bendix (TCM) magnetos. Where practical, links to on-line documents (highlighted) have been provided.

One question to specifically focus on, “Who is responsible for the airworthiness of CFR Part 91 certified aircraft and specifically AD (Airworthiness Directive) 2005-12-06 impulse coupling inspection, per TCM (Teledyne Continental Motors) SB645?”

First, let’s define the word airworthiness. Wait, there is no definition in the FARs. So what is the definition?

Your CAR 3 and/or FAR 23 certified aircraft is based on the drawings that make that aircraft an FAA-certified aircraft. There are hundreds if not thousands of different drawings that define every assembly of a complete aircraft.

Here is a good example: Mooney aircraft have a shock-mounted pilot panel. This shock-mounted panel has multiple drawings that depict the locations, amount, and type of shock mounts installed by the Mooney factory during assembly when the aircraft is manufactured.

Now let's say that an avionics manufacturer wants their products to be installed in that shock-mounted Mooney panel but states to not shock mount their equipment. To do that, you must either have an STC that clearly states it's OK to remove the shock mounts or have a 337 field approval by your local FAA FSDO that says it's OK to remove the shock mounts or have a Mooney drawing that shows an option to not have the shock mounts installed. This document must be specific to your aircraft model and serial number.

No one can alter a certified aircraft without one of the above documents, period. To do an alteration without the above approved documents violates the type design or type certificate of that FAA-certified aircraft. Ergo, the aircraft is no longer airworthy. The owner and/or pilot is responsible for the airworthiness of that aircraft per FAR 91.7(a). The FAA expects the pilot to discontinue a flight if the aircraft is unairworthy: FAR 91.7(b).

Many aircraft owners/ pilots rely on their mechanic or certified repair station to maintain the airworthiness of their aircraft, but the reality is it is the responsibility of the owner/pilot. IA mechanics and repair stations are just advisors.

When performing an annual inspection, the IA mechanic and repair station have certain rules they must follow:

FAR 91.417

(i) The total time in service of the airframe, each engine, each propeller, and each rotor.

(ii) The current status of life-limited parts of each airframe, engine, propeller, rotor, and appliance.

(iii) The time since the last overhaul of all items installed on the aircraft that are required to be overhauled on a specified time basis.

(iv) The current inspection status of the aircraft, including the time since the last inspection required by the inspection program under which the aircraft and its appliances are maintained.

(v) The current status of applicable airworthiness directives (AD) and safety directives including, for each, the method of compliance, the AD or safety directive number, and revision date. If the AD or safety directive involves recurring action, the time and date when the next action is required.

(vi) Copies of the forms prescribed by § 43.9(d) of this chapter for each major alteration to the airframe and currently installed engines, rotors, propellers, and appliances.

These are the basic requirements the mechanic must comply with and a person who violates these rules can be held accountable by the FAA. However, it is still your responsibility as the owner/pilot to make sure these items are complied with FAR 91.7(a).

So what, as the owner/pilot, are you responsible for at an annual on a CFR 91-operated aircraft?

  1. Your plane must have a current annual before each flight.
  2. All airworthiness directives must be complied with and shown as such in the airframe log book at each annual.
  3. The information required by FAR 91.417 must be entered in the appropriate log book.
  4. Your annual must be completed per FAR 43 appendix D at a minimum and recorded in the appropriate log book signed by the IA-rated mechanic/repair station returning the aircraft to service.

Under CFR Part 91, you are not required to do Service Letters, Service Bulletins, Instructions for Continued Airworthiness, or recommended lifetime replacement items. However, for signoff of an annual inspection at an FAA Certified Repair Station, SBS, ICAs, and lifetime replacement items all must be complied with.

After over 40 years as an IA mechanic, I can see that most owners and a lot of IA mechanics do not understand these basic airworthiness items and the basic requirements under FAR 91 for FAA-certified aircraft.

I believe that this basic knowledge is not being passed down to the few new IA mechanics who are entering this occupation.

A good example of this is AD 2005-12-06 and the corresponding SB by TCM (Teledyne Continental Motors) about inspecting the impulse coupling on Bendix/TCM magnetos every 500 hours.

Some history: Bendix installed impulse couplings on some of its magnetos to retard spark timing and help with engine starting. Originally these used a riveted flyweight retention scheme, which was subject to wear and failure.

Teledyne Continental Motors (TCM) bought Bendix Electrical Components Division in 1986. In early 1992, TCM changed the impulse coupling flyweight retention means from riveted to snap ring. TCM also changed the flyweight shape to improve its wear characteristics and marked the new flyweight design with a letter S on both sides of each flyweight nose, which allowed snap ring couplings to be identified while still assembled to the magneto.

In 1978, FAA Airworthiness Directive 78-09-07 R3 was issued, which established mandatory inspection in accordance with Bendix Service Bulletin 599 at intervals of 1,000 hours. Over the next five years, this AD was revised twice and the inspection interval was reduced to 500 hours.

On 18 Jul 1996, FAA AD 96-12-07 was issued, which superseded AD 78-09-07, retained the inspection interval for riveted impulse couplings at 500 hours, and instituted a 500-hour inspection requirement for snap-ring couplings. This AD also required that the inspections be done by following TCM Mandatory Service Bulletin MSB645, which had superseded Bendix SB599D.

On 19 Jul 2005, FAA AD 2005-12-06 became the current AD, which superseded AD 96-12-07 for Bendix/TCM magnetos installed on Lycoming O-540 type (AEIO-540, HIO-540, IO-540, O-540, and TIO-540) engines. AD 2005-12-06 required a 100-hour inspection interval for riveted couplings and retained the 500-hour interval for snap-ring couplings.

When TCM bought out all of the Bendix line of magnetos except the dual-mag D-2000 and D-3000 series, TCM was required to issue a new service bulletin (SB645) showing how to do this inspection and which magnetos were applicable.

This SB645 by TCM was based on the original Bendix SB, only it left out the dual-mag series as TCM wanted nothing to do with that. As one can see, there is nothing new under the sun regarding the Bendix impulse couplings. As a matter of fact, all the Bendix part numbers for these S20 and S1200 series magnetos were retained.

Remember, ADs are mandatory but SBs are not. However, when an AD refers to a specific SB as how to do the AD or determine if the AD applies then that SB becomes mandatory. Now AD 2005-12-06 is very poorly written and one can easily assume it is a Lycoming AD but it references TCM SB645 and this is where I am seeing the mistake mechanics are making. They (the mechanics) need to look at the magneto part number and compare it to TCM SB645 to make sure it applies. When one completes this required task they will discover that this AD applies to most TCM magnetos with an impulse coupling installed. However, AD 2005-12-06 specifically applies to various Lycoming O-540 type engines. By superseding previous ADs, performing SB645 is no longer required for other engines! That's dangerous – regardless of which engine these magnetos are on, SB645 inspection schedules should be followed. FAA Special Airworthiness Information Bulletin No. NE-02-32 recommends that you, owners and operators of Teledyne Continental Motors (TCM), Lycoming and Franklin series reciprocating engines equipped with TCM (formerly Bendix) S20, S1200, D-2000 and D-3000 series magnetos with impulse coupled starting devices installed, inspect the impulse coupling.

Whew! That was pretty complex, so let me summarize for you. Referencing SB645, do you have a listed magneto? If you do, let’s continue. Are you operating a Lycoming O-540 type? If yes, AD 2005-12-06 is in effect and you must perform SB645. If you are not operating said engine, AD 2005-12-06 is not in effect and SB645 is not mandatory (but follow it anyway, do the 500-hour inspection). Now you can see how SBs tie into ADs, becoming mandatory.

Let's take a moment to discuss the new IA mechanic that is unaware of the history I have just laid out on the Bendix impulse coupling. It is not hard to understand why I have seen three of these instances missed recently by IA and repair station mechanics at annual inspections. Inexperience leads to mistakes and they can lead to accidents, you as the owner/ pilot are responsible for the airworthiness of your aircraft and the pilot is an easy target for the FAA to blame, FAR 91.7.

What about the D-2000/D-3000 dual mags? They still fall under the original Bendix SB and TCM SB645 for the impulse coupling and also must be inspected every 500 hours. Now there are two types of coupling cams but most of the 100-hour inspection cams have been long since replaced and the old style cam has not been available for decades. It is very doubtful you will have one of these styles, so your cam will most likely be due inspection every 500 hours.

How do we at KNR handle this issue? We send the magneto out for a proper 500-hour service and all ADs are complied with at that time. We use Aircraft Accessories of Oklahoma as they are a Certified Repair Shop that can bench test a magneto and make sure it performs to the Bendix/TCM specs.

Can your local mechanic comply with this AD? Yes, but of course, the AD does not mandate the 500-hour service (it's not just points and capacitor change) which will ensure you have a reliable ignition system making your engine more reliable and efficient. See the end of this article for an example of a typical 500-hour inspection.

While writing this article, I thought of some questions every IA mechanic should ask:
  • When doing an annual on an engine not listed in this AD, should we still complete an impulse coupling inspection every 500 hours like we have done since AD78-09-07R3 was issued 45 years ago? Likewise, should we do the impulse coupling inspection on Bendix D-2000 and D-3000 magnetos every 500 hours?
  • When doing an annual on a engine not listed in this AD should we just ignore the history of the impulse coupling and let that magneto run to failure as allowed under CFR part 91 certified aircraft? What if the failure is the impulse coupling and the engine digests the parts and fails in-flight?
  • As an AI mechanic doing your annual inspection, do I comply with the minimum FAA requirements? Or do I follow history and provide you a safe and reliable airplane?
  • Have we all been wasting our time and customer’s money for the last 45 years?

Some aircraft maintenance can be excessive but magnetos do not fall into this category. One can waste a lot of money doing items that will not help the reliability of your aircraft but some items should not be overlooked, magnetos are one of those items.

This game of aircraft maintenance is all about reliability and that is what will lead to safe and enjoyable flights with no maintenance issues.

Until the next ShopTalk, enjoy flying your airplane. If you have a question about this article or any other of the over 50 articles on our website,, feel free to e-mail me, or call our aircraft repair shop (307) 789-6866.



Note: For magnetos specified in SB645, follow TCM/Bendix SB645 procedures.

Cleaning, inspecting and servicing the distributor block.
Cleaning, inspecting and testing the coil(s)
Cleaning and inspecting the rotor, then re-magnetizing it.
Replacing the oil seal.
Cleaning and inspecting the drive gears (these are plastic)
Cleaning and inspecting and recondition the rotor attached to the drive gear.
Cleaning and inspecting the case(s)
Cleaning and inspecting or replacing the bearings
Inspecting all internal wires for any damage.
Replacing the points and felt pads.
Inspecting and testing or replacing the capacitors.
Inspecting the impulse coupling cam and spring for wear. (Owners, ask for this.)

After reassembling, the magneto is run at various RPMs on a magneto test-bench with varying spark gaps for each cylinder to verify the intensity/output and timing of the magneto.

If the magneto passes the bench test, the magneto shop will issue an 8130-03 airworthiness approved tag.


Bendix Service Bulletin No. 599A.
Bendix Service Bulletin No. 599D.