The engine that powers your airplane is a subtle evolution of a design from the 1950s and is largely unchanged from that original design. There have been material & quality control process improvements, and modern machining which allows for greater consistency in mass-produced parts, but it is still essentially the same engine. One area that has seen a significant improvement is the electrical system with the development of modern lightweight alternators. There is a perceived concern among operators about the alternator drive system (the gear that drives the alternator) that has grown in recent years due to an increase in reported failures. The old heavyweight alternators were functional but rarely made it to TBO. It was common to pull a heavyweight alternator off for overhaul or replacement several times in a TBO cycle. This replacement allowed the mechanic to inspect the alternator & drive hub assembly, and replace any worn parts. With modern alternators getting to (or close to) engine TBO, your mechanic may not be inspecting the alternator drive system as frequently as the maintenance manual specifies.

The front-mounted alternator used in most 300 and 400 series Twin Cessnas has a drive hub assembly that is driven by a face gear mounted near the front of the crankshaft. The complete alternator & drive hub assembly is made up of:

1. Alternator

2. Drive Hub (or Drive Coupling)

The spring-type drive hub was superseded in 1996 by the elastomeric style (pictured)

3. Thrust Washer (bi-metal washer)

4. Castellated Nut

5. Cotter Key

6. Woodruff Key

The drive hub is often incorrectly referred to as a clutch. It is designed to dampen power pulses and “slip when abnormal torque is required to rotate the shaft” (Ref. M-0). The drive hub is made up of four main components:

• Gear

• Cup & tower (one machined piece)

• Bronze bushing

• Elastomeric element (rubber)

As you can see from Figure 3, the drive hub assembly sits inside the engine. If any of these components deteriorate, metal from the gears and/or rubber from the elastomeric drive hub can drop into the oil supply. This can lead to engine damage and/or unexpected loss of engine power. We’ll touch more on the potential for damage later.

The face gear turns at the same RPM as your crankshaft and the drive hub gear ratio is 3:1 meaning the alternator is screaming along in excess of 7,500 RPM when you’re at max RPM on the engine.

The drive hub is mounted on the alternator shaft. It is retained by a bi-metal thrust washer along with a properly torqued castellated nut and cotter key. If torque is lost on the castellated nut, “Loss of drive coupling retention may result in engine damage and/or unexpected loss of engine power.” (Ref. ASB)

Common causes of this type of damage includes:

• Incorrect installation

• Failure to remove the shipping washer on new alternators

•Improper torque of the castellated nut

• Reuse of the thrust washer

• Improper installation of the thrust washer

• Incorrect cotter key installation

• Loss of torque due to wear of the thrust washer

• Etc.

______________________

Now that we have the basics out of the way, let’s dig deeper. What do you, as a pilot/owner need to know and do to keep the alternator drive system healthy?

14 CFR 91.7 states, “No person may operate a civil aircraft unless it is in an airworthy condition. The pilot in command of a civil aircraft is responsible for determining whether that aircraft is in condition for safe flight. The pilot in command shall discontinue the flight when unairworthy mechanical, electrical, or structural conditions occur.”

Owners & pilots do rely on a licensed A&P mechanic to maintain the airworthiness of their airplane. That having been said, the pilot in command still has final responsibility every time the airplane takes off. Your mechanic must have the appropriate engine and airframe manuals to service your airplane.

When it comes to the alternator drive system, all of Continental’s technical information is contained in Continental Motor’s Standard Practice Maintenance Manual (M-0). Continental created this manual to contain years, if not decades, of Service Bulletins and other maintenance documents. It is over 500 pages and is an essential reference for any mechanic (or curious owner) working on Continental engines. In M-0, Continental specifies an alternator inspection during the 500-hour engine inspection. The instructions for the inspection are in section 6-4.22 in the current revision of M-0 (Revision 1, Change 2, August 2021 at the time of writing). The manual directs the mechanic to remove the alternator and perform the “Alternator Drive Coupling Inspection” in Section 6-4.22.1 on all gear-driven alternators and then inspect the alternator itself according to the relevant OEM alternator manual.

The “Alternator Drive Coupling Inspection” entails a visual inspection of each gear tooth on both the face gear and the drive hub gear, a visual inspection of the elastomeric element, and an “Alternator Drive Coupling Slippage Inspection” with the drive coupling installed on the shaft according to Section 10-4.1.4. This section has instructions for performing the drive coupling slippage testing. The tools used for this test are an “Alternator Drive Hub Spanner Wrench” (Table 2-1 Special Tools suggests Part Number 9001-IA from Ideal Aviation) and an “Alternator Drive Hub Torque Tool” (Figure 2-5 in the manual has instructions for fabricating this tool).

After passing inspection, M-0 directs the mechanic to reinstall the alternator according to section 10-4.1.5.

If the drive hub needs to be installed on a replacement alternator, Section 10-4.1.3 has specific instructions.

Critical points are:

• Remove the shipping washer from the alternator

• Never reuse the thrust washer

• Install the new thrust washer with the bearing surface (copper color) facing the alternator

• When torquing the castellated nut, if the cotter key slots do not align, increase the torque up to the maximum spec. NEVER loosen the nut to line up the slots. If you cannot achieve alignment, replace the nut.

• Perform the slippage test specified in section 10-4.1.4

Hartzell Engine Technologies (HET) also has a recommended inspection for the alternator drive system that is applicable to their line of gear-driven alternators. It can be found in HET “Alert Service Bulletin (ASB) No. 081 Rev. A” (available at hartzell.aero) published on 09/19/19. It recommends performing an inspection on the castellated nut within the next fifty hours of operation to verify the model of nut as well as checking that there is not a gap in-between the nut and thrust washer using a feeler gauge. This is a one-time inspection and should be recorded in your logbook. The reason for this ASB is that the castellated nut may not torque flush with the thrust washer and “could compromise the integrity of the drive coupling retention system.” Furthermore, the ASB states: “Loss of drive coupling retention may result in engine damage and/or unexpected loss of engine power.”

Attention should be paid to alternator out (or low voltage) lights. Follow the checklist and instructions in the Pilot’s Operating Handbook. If the checklist does not resolve the issue the ASB states:

“IMMEDIATE: If a loss of alternator output or erratic output indication occurs during flight, land as

soon as practicable. ...”

If the problem is electrical, your electronics will only last as long as the battery (Ref. POH). If the problem is mechanical, you may be facing engine damage and/or an unexpected loss of engine power.

In conclusion, modern alternator systems are reliable and work well when correctly installed, inspected, and maintained. According to Continental, your mechanic must remove the alternator and inspect the gear drive system every 500 hours in compliance with the directions laid out in Continental’s Standard Practices Maintenance Manual (M-0). In addition, your mechanic should comply with all other published recommended maintenance intervals for your airframe and engine.

Fly safe everyone!

References

• Hartzell Engine Technologies

Advisory Service Bulletin

ASB 081 Rev. A

(Available on hartzell.aero)

(Referenced as ASB for citations)

• Continental Standard Practice

Maintenance Manual M-0

(Revision 1, Change 2 August 2021)

(Referenced as M-0 for citations)

• Relevant Pilot’s Operating Handbook

(Referenced as POH for citations)

Any mechanic working on a Continental engine should have a current copy (printed or digital) of the Continental M-0 manual. If you want a copy of the manual, you can get it from Continental’s website. Go to continentalmotors.aero to create an account to subscribe to the manual.