C172S checklist item... 12V adapter off for takeoff

[TangoWhiskey]

I purchased the PIM for the C172S and T182T NavIII, and for the 1978 C182RG that I rent.

In reading over the C172S NavIII PIM, there's an item I never noticed before, and I've been flying C172's for years. They call for the 12V accessory plugs to be turned OFF for takeoff; there's a switch for it on the panel.

First question I have is WHY? I like to know the WHY behind checklist items. I can't imagine that the 12V adapters are off to reduce power drain during takeoff... anything you have plugged into that adapter is going to be a low-draw item, current-wise.

Second question is, how would this affect use of a portable GPS? I don't have one (portable GPS), so I'm curious if you pull the power off the plug, will it switch to internal batteries with no interruption or reboot?

 

[jesse]

First question I have is WHY? I like to know the WHY behind checklist items. I can't imagine that the 12V adapters are off to reduce power drain during takeoff... anything you have plugged into that adapter is going to be a low-draw item, current-wise.

To increase lift.

Second question is, how would this affect use of a portable GPS? I don't have one (portable GPS), so I'm curious if you pull the power off the plug, will it switch to internal batteries with no interruption or reboot?

Most of them will switch to battery and say something like "External Power Lost -- Powering off in 30 seconds". If you don't press a button--it turns off.

 

[Let'sgoflying!]

Could it have something to do with the 30yr? old AD that required the ext power receptacles to be disabled or have a dedicated CB (I think that's how it read anyway). Now you can have em but you can't use em?!

 

[TangoWhiskey]

To increase lift.

I have a feeling this could turn into another treadmill thread!

 

[gismo]

Unless that tidbit is in the limitations section, it's advisory only and I'd pass on that advice.

 

[HPNPilot1200]

Hmm, never noticed that. The CheckMate C172S Nav III checklist I have does not call for the 12v plug to be off, though I normally leave the switch in the off position if nothing is plugged in.

Check the Electrical Schematic in Section 7 of the AFM and see where the 12v power is tied to. I only have a C172S Nav II AFM with me, but the 12v power on the Nav II is on the Electrical Bus 1, tied to the glareshield, overhead, and courtesy lights circuit breaker. Perhaps the wiring is a bit different for the Nav III models.

 

[TangoWhiskey]

Hmm, never noticed that. The CheckMate C172S Nav III checklist I have does not call for the 12v plug to be off, though I normally leave the switch in the off position if nothing is plugged in.

Page 4-18 of the Normal Procedures section has the following for "BEFORE TAKEOFF", starting with item 26:

• Autopilot - OFF (if installed)
• CABIN PWR 12V Switch - OFF (if installed)
• Wing Flaps - UP-10° (10° preferred)
• Cabin Windows - CLOSED and LOCKED

Check the Electrical Schematic in Section 7 of the AFM and see where the 12v power is tied to. I only have a C172S Nav II AFM with me, but the 12v power on the Nav II is on the Electrical Bus 1, tied to the glareshield, overhead, and courtesy lights circuit breaker. Perhaps the wiring is a bit different for the Nav III models.

Section 7, page 7-70, has a section dedicated to the 12V Power Outlet. It reads:

A power converter located on the cabin side of the firewall, forward of the right instrument panel, reduces the aircraft's 28VDC power to 12VDC. This converter provides up to 10 amps of power to portable devices such as notebook computers and audio players. The power output connector (POWER OUTLET 12V-10A) is located on the center pedestal (refer to Figure 7-2).

For airplanes 172S10103 and on, a switch located on the switch panel labeled "CABIN PWR 12V" controls the operation of the power outlets.

Page 7-46 shows the overhead lights and 12V cabin power both coming off the "Electrical Bus #1" CABIN LTS/PWR breaker... the other items hanging off that same bus are:

Fuel pump (not running during takeoff);
Beacon light;
Landing light;
Flaps;
a feed to Avionics bus #1, guarded by the Avionics switch;
a feed to the Esssential Bus, guarded by a diode;
a guide to the cross feed bus tying EB1 and EB2 together, also protected by a diode.

 

[Ken Ibold]

I'm guessing someone was rolling down the runway trying to program a handheld that was plugged into one, lost control and went into the weeds. And this is how the lawyers are trying to prevent getting sued for THAT again.

 

[HPNPilot1200]

Weird, seems things are wired almost exactly the same. The only reasons I can think of are to conserve 10 the extra amps used by the 12v power outlet for other more important items on the electrical bus 1 (beacon, landing light, flaps, avionics bus 1 feed, etc) or to prevent the use of electrical items that can be plugged into the outlet itself during takeoff.

Cessna must have found some reason to install a panel switch to control the electrical power going to the outlet if they only started putting them on aircraft serial number 172S10103 and on. I'm not quite sure what the exact reason is though.

 

[TangoWhiskey]

Weird, seems things are wired almost exactly the same. The only reasons I can think of are to conserve 10 the extra amps used by the 12v power outlet for other more important items on the electrical bus 1 (beacon, landing light, flaps, avionics bus 1 feed, etc) or to prevent the use of electrical items that can be plugged into the outlet itself during takeoff.

Cessna must have found some reason to install a panel switch to control the electrical power going to the outlet if they only started putting them on aircraft serial number 172S10103 and on. I'm not quite sure what the exact reason is though.

Interestingly, the 12V plug is NOT a cigarette lighter style plug... it's an airline-power style (ARINC 628) except that it outputs 12V instead of 15V as on an airliner. You need an adapter like this one to use any accessory that has a cigarette-style 12V plug. The Cessna manual even lists a compatible Radio Shack part number! At least they don't try to sell you an 'FAA-certified' version for $200!

Also, page 2-17 (Section "Operating Limitations") (so it is mandatory, not advisory) says:

The 12 Volt Power system (POWER OUTLET 12V - 10A) is not certified for supplying power to flight-critical communications or navigation devices.

Use of the 12 Volt Power System is prohibited during takeoff and landing.

Use of the 12 Volt Power System is prohibited under IFR unless the operator of the airplane has determined that the use of the 12 VDC power supply and connected portable electronic device(s) will not cause interference with the navigation or communication systems of the airplane.

The BEFORE LANDING checklist on page 4-22 likewise includes "CABIN PWR 12V Switch - OFF (if installed)" in it's items.

 

[skyflyer8]

I'm guessing someone was rolling down the runway trying to program a handheld that was plugged into one, lost control and went into the weeds. And this is how the lawyers are trying to prevent getting sued for THAT again.

Oooh, oooh! **Raises hand** I've got such an example in my lesson plan binder under the "cockpit resource management" section!

Pilot tries to adjust GPS / attach it to leg, apparently during takeoff roll, cable gets caught in seat lever, seat slides, plane loses control and hits stuff. Pilot aborts takeoff, taxies back, and takes off again.

Accident report: http://www.ntsb.gov/ntsb/brief.asp?ev_id=20011218X02405&key=1

 

[TangoWhiskey]

Accident report: http://www.ntsb.gov/ntsb/brief.asp?ev_id=20011218X02405&key=1

I can't BELIEVE there are pilots out there that would take a runway diversion into the weeds, feel a "few bumps" and thumps, and then just taxi back and takeoff without an inspection... sounds like he continued to his destination even though aircraft performance seemed less than he was used to experiencing.

Man, shut down the engine, GET OUT and check the airplane--he was MISSING PART OF HIS PROP and had a BUCKLED fuselage!!!

 

[Let'sgoflying!]

I can't BELIEVE there are pilots out there that would take a runway diversion into the weeds, feel a "few bumps" and thumps, and then just taxi back and takeoff without an inspection...

or worse yet, I knew a guy who did get out, examined the seriously bent lift strut, then hopped in and took off! Piper cub, fatal accident

 

[cwyckham]

Interestingly, the 12V plug is NOT a cigarette lighter style plug... it's an airline-power style (ARINC 628) except that it outputs 12V instead of 15V as on an airliner. You need an adapter like this one to use any accessory that has a cigarette-style 12V plug. The Cessna manual even lists a compatible Radio Shack part number! At least they don't try to sell you an 'FAA-certified' version for $200!

Also, page 2-17 (Section "Operating Limitations") (so it is mandatory, not advisory) says:



The BEFORE LANDING checklist on page 4-22 likewise includes "CABIN PWR 12V Switch - OFF (if installed)" in it's items.

Sounds like they're either worried about electromagnetic interference from whatever's plugged into the outlet, or from the transformer itself. Since they don't have to certify whatever you plug in, it seems a bit much to put it in the Operating LImitations section. Perhaps the transformer doesn't conform, so they were forced to placard it for critical operations such as IFR and landing/takeoff in order to certify.

 

[TeenDoc]

The Cessna manual even lists a compatible Radio Shack part number! At least they don't try to sell you an 'FAA-certified' version for $200!

Radio Shack doesn't carry them anymore. When I found out four years ago, I went to an old Radio Shack store in Patterson, NJ, found 5 of them and bought them, just in case. I've only used one in 4 years and only to charge my cell phone during long trips.

 

[MauleSkinner]

Maybe they're trying to eliminate "unimportant" equipment for takeoff, so that an failures that occur are things that you more likely want to abort for. Troubleshooting electrical smoke isn't that big a deal during cruise flight, but doing it during takeoff is a bit more work-intensive.

Maybe (pure speculation on my part, btw) the same guys who write checklists and limitations for the Cessna jets, trying to keep you from having to make a high-speed abort for no good reason.

Fly safe!

David

 

[TangoWhiskey]

Troubleshooting electrical smoke isn't that big a deal during cruise flight, but doing it during takeoff is a bit more work-intensive.

That makes the most sense to me of anything I've heard so far...

I've written to Cessna Support to find out their answer to this question, and also to find out how soon "after takeoff" the 12V system may be enabled/powered up, and how soon "before landing" the system must be disabled/powered down, as the POH is silent on this matter.

I'll report back with what I find. Makes sense to go to the source for the answer, anything else is just conjecture on our part... fun conjecture (part of the learning process) but still guesses nonetheless.

 

[ScottM]

I can't BELIEVE there are pilots out there that would take a runway diversion into the weeds, feel a "few bumps" and thumps, and then just taxi back and takeoff without an inspection... sounds like he continued to his destination even though aircraft performance seemed less than he was used to experiencing.

Man, shut down the engine, GET OUT and check the airplane--he was MISSING PART OF HIS PROP and had a BUCKLED fuselage!!!

A guy at 3CK landed hard, prop strike when he came down on the nose, then took off again without shutting down and did several more touch and goes. Claimed that he did not notice and weird vibrations at all even when he was shown the prop was bent at the tips. As Bruce would say, SIGH!