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At the club I fly at, there is an older Cessna 172 that has a manual "Johnson Bar" that is used to put the flaps down.

In the newer 152, and I believe the rest of the planes (I have yet to fly them), the flaps are controlled via a electronic (or hydraulic?) lever.

It appears to me that the manual flaps are more reliable, more maintainable, and a hell of a lot more fun in my opinion. Additionally, the manually flaps don't require a lot of strength to operate IMO. Does anyone know why the automatic design is favored over the manual counterpart? Same question applies to car transmissions...

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mechanical needs control rods/cables running through the frame – ratchet freak Apr 16 '14 at 22:12
This guide has some interesting details. – Farhan Apr 16 '14 at 23:02
Not electronic. Electric. It is just a switch and an electric motor (maybe some limit switches, too). Electronics include electrical circuits that involve active electrical components such as vacuum tubes, transistors, diodes and integrated circuits, and associated passive interconnection technologies. – Skip Miller May 29 '14 at 1:42
I think whether one prefers manual flaps or electrical is very much a matter of personal preference. Personally, I was very happy to move from manual flaps to electrical. And I'm unhappy to have to put up with Johnson bar flaps in the Cessna Skycatcher I'm checking out in. The bar is really in the way for me. I have to rest my forearm on the raised bar, and that's very uncomfortable. – Terry Sep 27 '14 at 20:37
Today, things like manual flaps with a giant bar on the floor are fun and interesting retro designs. Back in the 1960s powered anything was modern and progressive. The manual bar would have been as exciting as manual steering / transmission on your 1965 Buick Riviera. You can't promote a retro feature from the originating era. – paul Sep 28 '14 at 23:19
up vote 15 down vote accepted

Nobody can definitively answer this for you except Cessna's 1965/1966 engineering team (the year they made the change), but there are two reasons I can think of:
Because switches are cooler than Johnson bars; or Because everyone else is doing electric flaps.

Much like with manual transmissions, some people just don't like the extra work of manual flaps, and for those folks electric "flip a switch and don't worry about it" flaps are a selling point.

In terms of reliability it's a trade-off (as are all engineering decisions), so let's look at a few of the factors:

Mechanical Flaps

  • +Flaps still work when the battery is dead.
  • +The pilot can control the extension/retraction rate to some extent.
  • +Usually cross-linked with a bar (so you can't have a "split-flap" condition)
  • +Simple to rig, adjust, and maintain.
  • -Actuation requires more pilot skill
    You need to develop the muscle memory to grab the handle without looking.
    You need to learn to govern the extension/retraction rate smoothly.
  • -The failure mode is usually "Flaps Up"
    You could lose your flaps on short final if the cable or lock mechanism fails.

Electric Flaps

  • +"Easier to Operate" (you don't need to reach down to the floor)
  • +Extension/Retraction rate is constant (governed by the motor)
  • +Fewer moving cables running through the fuselage (wires replace them)
  • +Typical failure mode (e.g. dead battery) is "Flaps stuck where you left them"
    Less chance of losing your flaps on short final.
  • +Depending on the design they may save some weight over a mechanical system.
  • -You lose control of the flaps if the battery dies (no-flap landings are more likely)
  • -You have one or more motors to maintain/replace if they fail
  • -Depending on the design it's possible to have a "split-flap" condition

Cessna's engineers and marketing folks looked at those factors (and probably many others) and decided that electric flaps "made sense". On the other side of the GA fleet, Piper's engineers looked at the same factors and decided to keep the Johnson bar flaps (which are still found in the PA-28 series today).

From a manufacturing standpoint it makes sense for all of the aircraft a manufacturer produces to use the samecontrol mechanisms -- either all the planes use mechanical flaps or all the planes use electric flaps because it simplifies production and allows everything to run through on one assembly line rather than stopping at the flaps and diverting aircraft to different teams.

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I guess I'm just bitter that they didn't go with "let's make it an additional feature" – OneChillDude Apr 17 '14 at 0:51
The older I get, the less I like gimmicky things like electric flaps and the more I like good simple mechanical linkages. Failure rates are comparable, but diagnostics and repairs are so much easier on mechanical... – Brian Knoblauch Oct 17 '14 at 18:38
Just a side note, Johnson bar systems can encounter the split flap issue and it happened to one of the Warriors I fly in (not while I was in it though) – Dave May 24 '15 at 3:55
@Dave Yeah, in the PA-28 you can have a pushrod (or more likely cotter pin) failure that has the potential to really ruin your day (one flap is no longer linked to the torque tube). Likewise the cause of split-flap conditions in light GA planes' electric flap systems seems to be flap rollers coming off the track (as opposed to any intrinsic problem with the electric aspect of the system) - You could have an electric system that moves a torque tube and be in the same situation as Johnson-bar/pushrod flaps as far as split-flap conditions go. – voretaq7 May 26 '15 at 21:09

I did fly with a woman once who could not get the flaps down to 40 degrees. I guess it was a combination of seat in forward position and the bar way back and also it gets a bit heavy at the end. As I remember the bar was about her shoulder and you don't have much strength so far back. That is how I remember it anyway, it happened in the early '70's.

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I don't really see how this anecdote, however interesting, really answers the question as asked. Are you saying that this type of problem was a major driver behind Cessna adopting electrically operated flaps in the 172? If so, it would be better if you can edit your answer to make that more clear. – Michael Kjörling Aug 5 '15 at 8:28

One can imagine a relatively simple and robust linkage between the lever and flaps on a low wing e.g. Piper Cherokee. Maybe not so straightforward for a high wing like a C-172, where there is likely already a complex cable routing for the ailerons.

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Simple, yes. Robust.... well as robust as any other cable linkage in the control system I suppose :) (Getting the Johnson bar to work in a high-wing is just a matter of some different pulleys to change the angle - a little more complex, but no more than say the ailerons.) – voretaq7 May 26 '15 at 21:11
I don't claim to know with any certainty, but pretty sure I once saw a cutaway illustration of the Piper (low wing) system showing pushrods, levers, and a torque tube - no cables. – Anthony X May 27 '15 at 0:14

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