Airplane Owner Maintenance

A Maintenance Oriented Podcast For Airplane Owners, Pilots, and Mechanics

Year: 2020 (Page 4 of 4)

134 – An Unlikely Cause for a Hot Mag on a Cirrus

February 2, 2020 / / 4 Comments

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“There’s no RPM drop when I select left mag, and if I turn the ignition switch off, the engine continues to run.”

That’s what the owner of this Cirrus reported when he arrived at our airport the other day.

One of our technicians got to work to troubleshoot the problem. Here’s what he found:

  • The magneto P-lead wiring checked good.
  • The ignition switch checked good.
  • It appeared the problem was with the magneto.

So let’s pause and ask, “Which magneto, left or right?”

Good question, because this stuff can sometimes be confusing.

When the pilot got no RPM drop when selecting left mag, it likely means the ability to ground (or turn off) the right mag has been lost. So even though we are selecting left mag, it’s the right mag that actually has the problem.

And sure enough, that’s exactly the case here… upon removing the point cover from the right mag (which also contains a capacitor with a stud for the P-lead attachment,) we determined with an ohm meter check that the capacitor in the P-lead circuit had failed in the open position, making it impossible to ground (or turn off) the right magneto.

The point cover along with the capacitor, mounts to the magneto as seen on the left side of the following picture:

The new capacitor has been ordered and the Cirrus should be flying again real soon. You can hear the full story in today’s podcast episode.

The other P-lead story in this episode is from an A36 Bonanza I’m inspecting. This is a very different situation… this one had some chafing P-leads in the engine compartment that were a problem-in-the-making. Take a look:

After pulling the wires down a bit, the worn areas became really obvious:

These wires have 4 layers:

  • An inside center wire (the actual P-lead.)
  • A layer of insulation on the center wire.
  • A layer of braided shielding to prevent radio noise.
  • An outside layer of insulation.

Thankfully, the inside wires (P-leads) were still protected… if those inside wires had shorted to ground, the magnetos could have failed to operate. If the P-leads are shorted to ground by touching the airframe, it’s the same effect as turning the ignition switch off.

Here’s one last picture of the P-leads with some context of how they are attached to the magnetos:

So… any time you’re working in the engine compartment, whether for an oil change, or anything else, it’s a good idea to look around for any signs of chafing… sometimes you might be able to avoid a future in-flight problem by catching the issue early.

133 – The Firetrucks Rolled When a Navajo’s Landing Gear Did Not Extend

January 26, 2020 / / 0 Comments

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I was sitting in my car that day, having lunch with my friend Matt, when we noticed the airport fire trucks were rolling out toward the runway. Although they exercise these trucks regularly, this time seemed different.

And sure enough, a Piper Navajo made a low pass and it was obvious the landing gear was not extended. I decided to go inside and try to find out what was going on.

After some conversation with the unicom people, I offered to talk with the pilot. They gladly agreed.

Thankfully, we had maintained several Navajos in the past at Classic Aviation, so I was reasonably familiar with the landing gear system. I spoke with the pilot on the radio, and confirmed he was able to extend the gear with the emergency extension hand pump, and the gear selector handle had returned to neutral, with all three gear down lights on.

This seemed promising, and we anticipated an uneventful landing. A low pass confirmed the gear now appeared to be extended.

A successful landing was a relief to everyone.

After landing, the nose baggage compartment was opened, and it became obvious the hydraulic fluid in the power pack was very low… apparently so low the engine driven pump could not pump the gear down.

Here’s where a good understanding of the system is helpful.

There is a “standpipe” in the hydraulic power pack that has a unique function. If the fluid level gets too low, the standpipe reserves enough remaining fluid for the emergency hand pump to pump the gear down. It’s a great safety feature in this airplane, and others with hydraulic landing gear.

We filled the power pack with MIL-5606 aviation hydraulic fluid, and recommended the pilot get the system checked for leaks.

Listen to the audio for the complete information on this one, but the main lesson is “Be familiar with your retractable landing gear system, especially the emergency procedures.”

Here are a several suggestions to make this happen:

  • Practice your emergency landing gear extension procedure, in flight, with a competent and knowledgeable instructor or pilot, at least once a year.
  • Visit your maintenance shop during the annual inspection and operate the emergency gear system with the airplane on jacks.
  • Watch the gear operate on jacks, from outside the airplane, so you can have a visual picture of what is actually happening when you retract and extend the landing gear.
  • Memorize your landing gear emergency procedures, and keep the manual/checklist in a convenient and accessible location in your airplane.
  • Finally, make sure your landing gear system is well maintained and properly adjusted, with full hydraulic fluid (if applicable) to ensure safe and reliable operation.

Other podcasts mentioned in this episode:

Aviation Careers Podcast episode 255

Stuck Mic AvCast episode 229

132 – Real Inflight Magneto Failures & Troubleshooting

January 17, 2020 / / 0 Comments

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Note: scroll to the bottom of this post for a special announcement.

Have you ever experienced an inflight magneto failure? If not, you might some day… but if you take action on the recommendation in today’s episode, you can dramatically reduce the likelihood of that happening.

The left mag on this Cessna 172RG failed in flight, and after opening the mag, it became obvious what happened.

The finger electrode that is part of the distributor gear inside the magneto, had gotten loose from its mounting position, and was able to rotate freely 180 degrees, while still occasionally resting in its proper position, which made it somewhat intermittent. Take a look:

The finger electrode should stay firmly in it’s mounting point as in the following picture:


The LAA, or Light Aircraft Association, put out an excellent, one page Airworthiness Alert February 21, 2019, about this very issue. Here’s a link for that document:

http://www.lightaircraftassociation.co.uk/engineering/Slick%20Magneto.pdf

That document also has a link to the Slick SB1-15A that also addresses the issue. Here’s that link:

http://www.lightaircraftassociation.co.uk/engineering/SLICK.pdf

The bulletin recommends changing the copper electrode in certain Slick magnetos to a monel electrode, which is much stronger. This is definitely a worthwhile service bulletin to comply with for the affected Slick mags.

Now back to the Cessna 172RG… it also needed a new ignition switch kit. The Gerdes ignition switch is subject to AD 93-05-06 which requires inspection and lubrication each 2000 hours, although I sometimes wonder if this interval is frequent enough. Take a look at the carbon tracking and wear inside the ignition switch:

This condition can cause the switch to malfunction… it’s easily remedied by installing an A-3650-2 Ignition Switch Kit, which includes replacement of this contact plate, as well as the 3 contact cups in the switch housing. Notice the difference between the old cups and the new cups… any contacts that have the silver plating worn off, must be replaced… most of the switches I’ve inspected, have needed a new kit.

Replacing the switch plate is fairly simple, but slightly tedious, by transferring one wire at a time, from the old switch plate, to the new one:

The second story we talked about in today’s episode is about an inflight magneto failure in an Aerostar. The pilot did some excellent inflight troubleshooting, and landed safely after determining the right engine left mag had failed, and was able to fly to a safe destination with the right engine selected on right mag only.

So after today’s episode, I have several tips about magneto troubleshooting:

  • When you select one mag, if the engine immediately runs rough, it’s most likely a spark plug issue.
  • When you select one mag, if it’s a more smooth, but large RPM drop, it may be a magneto problem.
  • When you select one mag, if the engine dies immediately, it might be a mag, or a shorted P-lead, or the ignition switch, (the switch being the most unlikely, in my opinion.)
  • Causes of ignition system troubles, in the order of likelihood:
    • Spark plug issues.
    • Magneto issues.
    • P-lead wiring.
    • Ignition lead, or corrosion in magneto attachment.
    • Ignition switch.

In fact, I seriously wonder if both issues talked about in today’s episode could have been avoided. I don’t have information on the 172RG’s magneto, but the Aerostar’s failed magneto had about 1000 hours since overhaul.

Find a reputable magneto shop, and keep your mags in tip-top condition.

My favorite magneto shop to recommend is Aircraft Ignition Services LLC, operated by Kevin and Leah Herrington in Honey Grove, Texas. They’ve been doing excellent work for us at Classic Aviation LLC. Check out their website:

http://aircraftignitionservices.com/

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Announcement: Carl Valeri is the host of the Aviation Careers Podcast and the Stuck Mic AvCast. He recently interviewed me for an episode of each of these shows (ACP255 and SMAC229), and it was so inspiring to talk with him. You can find these here:

http://www.aviationcareerspodcast.com/

http://stuckmicavcast.com/

I highly recommend the podcasts, career coaching, scholarships guide, and other resources Carl provides. Thanks Carl!

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