Question - Engine Starting Checklist in Cessna 172R or 172S

eetrojan

eetrojan

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Maybe I’m overthinking this, but I’m curious if this subtle difference in the order of doing things matters.

The “starting engine” checklist in the POH for a fuel injected Cessna 172R includes steps 6-8 when the engine is cold (highlighted in yellow):

172R_POH_Starting_Engine_Checklist.jpg


Basically, the three steps 6-8 for when the engine is cold are:
  • Pump ON
  • Mixture RICH for 3-5 seconds, then to IDLE CUTOFF
  • Pump OFF

On the other hand, the owner’s custom checklist in a local 172R rental says that when the engine is cold, the steps are:
  • Mixture RICH
  • Pump ON 3 SEC (then OFF)
  • Mixture LEAN

In summary, the official POH says move the mixture in and out while the pump is running, and then turn the pump OFF, but the checklist in the rental says turn the pump on and off while the mixture is in, and then pull the mixture out.

Does it matter? Or, is it just six of one, half a dozen of another?

I suspect it doesn’t matter at all, but figured I'd toss it out there. Thoughts?
 
For FI engines, mixture rich, fuel pump on till you see fuel flow, crack the throttle and crank, once stated fuel boost off, lean for ground ops.

Hot engine starting, same deal but without the boost pumps.

If it won't start, prime with the pumps like before, but when cranking have the mixture at ICO and full throttle, right when it starts, throttle idle and mixture rich.

Worked for me
 
If this thread is about personal magic recipes for engine starting, this thread will go pages. ;) But the OP is just asking about the difference between mixture rich before aux pump on or aux pump on before mixture rich. The answer is that it doesn't really matter. No fuel is getting pumped into the cylinders until the mixture is opened, so turning the pump on first is simply pressurizing the system ahead of the mixture valve until the mixture is pushed in. The mixture is the gate - you can do it either way. To me, it makes more sense to push the mixture in before aux pump priming.
 
Last edited:
I agree that it is just two ways of stating the same thing.

David
 
The first is POH, second is to reduce time that the aux fuel pump is on. Both ways work.
 
I fly 2 different 172's and both have their individual querks when it comes to starting up...

One likes full mixture before aux fuel pump prime, and the other likes the mixture full lean until the engine fires
 
Xscaliber said:
One likes full mixture before aux fuel pump prime, and the other likes the mixture full lean until the engine fires
Click to expand...

You may be confusing the OP - those two steps are not mutually exclusive, one just happens before the other.
 
For me starting the damn plane is often the most stressful part of the flight. The more people you have watching you, the less likely it is to start the first time. Hot starts at a high altitude airport are my favorite! :mad2:
 
narchee said:
For me starting the damn plane is often the most stressful part of the flight. The more people you have watching you, the less likely it is to start the first time.
Click to expand...
I work at a flight school and a renter plus his friends got stranded at an airport because he couldn't start the engine so I drove and picked them up. It was about an hour drive. Once I got there, I tried starting the plane and it fired right up! It must have been embarassing for the pilot.
 
For cold
Fuel pump on
Mixture rich for 3-5 secs
Lean
Start
Rich to get the engine running

Works for every 172r I've flown.


Sent from my iPhone using Tapatalk
 
With the R/S models I've flown (about 50 hrs worth), I've never had one fail to start cold by using the POH-version of pushing the mixture in, turning the aux pump on for 3-5 seconds (verify fuel flow) and turning the aux pump off and pulling the mixture back out. Granted I doubt I ever cold started in less than 20-degree temps, so the procedure may need to be altered for more fuel once you drop to single-digit temps.
 
nimdabew said:
The first is POH, second is to reduce time that the aux fuel pump is on. Both ways work.
Click to expand...

I'm going with this answer. Probably just a shortcut to avoid the common error (when following the checklist as a step-by-step "do" list) of turning the fuel pump on, waiting a month and a half to read the next line, and then moving the mixture control.
 
Thanks all. Makes sense now.

I like RoscoeT's idea (post 3) of thinking of them being required, in series, such that the second one turned on (mixture RICH, or pump ON) is the "gate." As to why some might prefer the non-POH order, I appreciate the explanation that the "custom" checklist may be based on the theory that it puts less time on the pump.
 
James331 said:
For FI engines, mixture rich, fuel pump on till you see fuel flow, crack the throttle and crank, once stated fuel boost off, lean for ground ops.

Hot engine starting, same deal but without the boost pumps.

If it won't start, prime with the pumps like before, but when cranking have the mixture at ICO and full throttle, right when it starts, throttle idle and mixture rich.

Worked for me
Click to expand...
Glad it works for you.

But before issuing blanket advice, recognize that it is NOT the procedure for all FI engines.

If you follow your suggested procedure in the B55 Baron, there is a good chance you'll flood the engine and it won't start (you don't start the Baron with the boost pump on).
 
jordane93 said:
I work at a flight school and a renter plus his friends got stranded at an airport because he couldn't start the engine so I drove and picked them up. It was about an hour drive. Once I got there, I tried starting the plane and it fired right up! It must have been embarassing for the pilot.
Click to expand...

It sat for an hour and the flooded engine had cured itself.
 
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