Mixture control bottoms-out against the firewall?

I have a maintenance question.

I read somewhere that the mixture control should not bottom-out against
the firewall when full rich, as the pilot would not be able to tell if
the full-rich position was achieved, or if the knob just hit the firewall
stop.

However, I'm wondering if this is just a rule of thumb, or if there is
an actual FAR which dictates it, and which FAR that is?

Thanks,

--
Mark Hansen, PP-ASEL, Instrument Airplane
Cal Aggie Flying Farmers
Sacramento, CA

Mark Hansen wrote:
I have a maintenance question.

I read somewhere that the mixture control should not bottom-out against
the firewall when full rich, as the pilot would not be able to tell if
the full-rich position was achieved, or if the knob just hit the firewall
stop.

However, I'm wondering if this is just a rule of thumb, or if there is
an actual FAR which dictates it, and which FAR that is?

I'm not aware of any such FAR. I would think if the FARs said
anything about it, the place would be:

------------------------------
Sec. 23.1147 Mixture controls.

(a) If there are mixture controls, each engine must have a
separate control, and each mixture control must have guards or must be
shaped or arranged to prevent confusion by feel with other controls.

(1) The controls must be grouped and arranged to allow--
(i) Separate control of each engine; and
(ii) Simultaneous control of all engines.
(2) The controls must require a separate and distinct operation to
move the control toward lean or shut-off position.
(b) For reciprocating single-engine airplanes, each manual engine
mixture control must be designed so that, if the control separates at
the engine fuel metering device, the airplane is capable of continued
safe flight and landing.
------------------------------

There could, of course, be something more specific in the maintenance
procedures for your airplane or engine.

On 02/14/06 07:45, Roy Smith wrote:
Mark Hansen wrote:
I have a maintenance question.

I read somewhere that the mixture control should not bottom-out against
the firewall when full rich, as the pilot would not be able to tell if
the full-rich position was achieved, or if the knob just hit the firewall
stop.

However, I'm wondering if this is just a rule of thumb, or if there is
an actual FAR which dictates it, and which FAR that is?

I'm not aware of any such FAR. I would think if the FARs said
anything about it, the place would be:

------------------------------
Sec. 23.1147 Mixture controls.

(a) If there are mixture controls, each engine must have a
separate control, and each mixture control must have guards or must be
shaped or arranged to prevent confusion by feel with other controls.

(1) The controls must be grouped and arranged to allow--
(i) Separate control of each engine; and
(ii) Simultaneous control of all engines.
(2) The controls must require a separate and distinct operation to
move the control toward lean or shut-off position.
(b) For reciprocating single-engine airplanes, each manual engine
mixture control must be designed so that, if the control separates at
the engine fuel metering device, the airplane is capable of continued
safe flight and landing.
------------------------------

Yes, that seems like where it should be. I guess it's not a regulation,
then.


There could, of course, be something more specific in the maintenance
procedures for your airplane or engine.

I think I read this in 'Aircraft Engine Operating Guide' by Kas Thomas.
I'll have to dig that out and re-read it.

Thanks for your help.

--
Mark Hansen, PP-ASEL, Instrument Airplane
Cal Aggie Flying Farmers
Sacramento, CA

Roy Smith wrote:


I'm not aware of any such FAR. I would think if the FARs said
anything about it, the place would be:

Even that FAR doesn't apply to a much of the GA fleet.
It doesn't to mine. You couldn't tell the mixture from
the cabin heat control as it came off the factory line.

Cherokee Six?
Ron Natalie wrote:
Roy Smith wrote:


I'm not aware of any such FAR. I would think if the FARs said
anything about it, the place would be:

Even that FAR doesn't apply to a much of the GA fleet.
It doesn't to mine. You couldn't tell the mixture from
the cabin heat control as it came off the factory line.

Robet Coffey wrote:
Cherokee Six?

Navion, the mixtuure, the cabin heat, the hydraulic bypass control
carb heat and probably a few others I'm not remembering were the same
aluminum knob. The throttle was a big doorknob sized thing. I've now
got modern throttle/mixture/prop controls.

Oddly the Navion did have a flap-shaped flap control and a wheel-shaped
gear control (unlike the contemporary bonanza which just had the same
shape toggles for both).

"Mark Hansen" wrote in message
...
I have a maintenance question.

I read somewhere that the mixture control should not bottom-out against
the firewall when full rich, as the pilot would not be able to tell if
the full-rich position was achieved, or if the knob just hit the firewall
stop.

However, I'm wondering if this is just a rule of thumb, or if there is
an actual FAR which dictates it, and which FAR that is?

While it's probably not a regulation, most service manuals specify that both
throttle and mixture knobs have a certain amount of "springback" instead of
the knob contacting the panel; the Piper Cherokee manual, for instance,
specifies 1/16" to 1/8" springback.

On 02/14/06 10:13, John Kunkel wrote:
"Mark Hansen" wrote in message
...
I have a maintenance question.

I read somewhere that the mixture control should not bottom-out against
the firewall when full rich, as the pilot would not be able to tell if
the full-rich position was achieved, or if the knob just hit the firewall
stop.

However, I'm wondering if this is just a rule of thumb, or if there is
an actual FAR which dictates it, and which FAR that is?

While it's probably not a regulation, most service manuals specify that both
throttle and mixture knobs have a certain amount of "springback" instead of
the knob contacting the panel; the Piper Cherokee manual, for instance,
specifies 1/16" to 1/8" springback.



Is it possible to determine whether or not this is specified in the
service manual for the particular Cessna aircraft I'm flying? It's
a 172N model. Are the service manuals available online?

I guess what I'm looking for is something that I can show to the maintenance
people at the club, as they said the mixture control is working correctly
when I complained that it should not hit the firewall.

Thanks,

--
Mark Hansen, PP-ASEL, Instrument Airplane
Cal Aggie Flying Farmers
Sacramento, CA

Mark Hansen wrote:


Is it possible to determine whether or not this is specified in the
service manual for the particular Cessna aircraft I'm flying? It's
a 172N model. Are the service manuals available online?

I guess what I'm looking for is something that I can show to the
maintenance
people at the club, as they said the mixture control is working correctly
when I complained that it should not hit the firewall.

Thanks,


Mark,

I don't know about online, but this most certainly *is* spelled out in
the maintenance manual, under "Engine Controls: Rigging". (It's chapter
11 in my 1969 182 service manual). Besides clearly stating that all
engine controls be set up with a 1/8" cushion, this is one of the most
basic, rudimentary procedures that first year A&P students are taught.
I suspect the mechanics you are talking to are trying to shine on the
job of replacing and re-rigging a worn out mixture cable.

Happy Flying!
Scott Skylane
N92054

Is it possible to determine whether or not this is specified in the
service manual for the particular Cessna aircraft I'm flying? It's
a 172N model. Are the service manuals available online?

Cessna gives NOTHING away. Paper copies of their manuals will
start at around US$180, and if there's any temporary revisions they
charge $12 or $15 for two or three pages.
The 172 manuals I have here (L and M) call for some "bounce."
It's standard aircraft practice to ensure that full travel is obtained
at the engine device, whether it's throttle, mixture, carb heat or
whatever. Even flight controls should hit the stops at the control
surface's mechanism before the cockpit end hits anything. Control
systems flex and stretch, especially under load.
Many companies will publish manuals or other service
information on the 'net to make sure it's available, and others want
every buck they can squeeze from it. Cessna requires a rather expensive
subscription ($360/yr) for service info (updates, service bulletins and
service letters, etc.) while American Champion maintains a website to
do the same thing. Lycoming's website publishes a service manual
currency page, but McCauley and Cessna do it only through subscription.
All of them are owned by Textron, yet there's no consistency.

Dan