8KCAB Why start with mixture lean?

[jkh]

So, in a previous thread I asked about "the best way to start a 2014 8KCAB" since I had one checklist that showed priming with the fuel pump for 3-4 seconds with mixture rich (of course) and then starting with mixture lean, then jumping to push the mixture in after start. The other checklist shows the same steps except that you start with mixture rich and obviously don't change it.

So I thought I might try an experiment with my 2014 cold, and it starts fine either way. I literally can't tell the difference, other than needing to jump rapidly onto the mixture knob after start with "procedure A" vs "procedure B". For a warm start, it's always been mixture rich with maybe 2 seconds on the fuel pump to prime it, which also works just fine.

Obvious question: How did this whole "start with mixture lean" for cold start procedure get into some of the 8KCAB checklists? Was it to prevent some scenario, like an engine fire on start, observed in some very rare number of cases or is there some strong mechanical argument to be made for doing things this way? Thanks!

 

[Bartman]

Isn't the fuel pump left on for a cold start but turned off for a warm start?

 

[jkh]

The fuel pump is just used to prime the engine - it's not necessary to operate with the fuel pump on in either a warm or cold start.

 

[Bob Turner]

After 21 years of successful starts - only twice has it let me down, and one of those I just let it sit for a half hour and started it - I still get apprehensive when trying to go home with a hot start.

Cold - it always starts. Crack throttle, fuel pump prime three-four seconds, mixture lean, pump off, starts first blade.

Hot - mixture aft, pump on. “Blip” on mixture - half a second, not fully rich, back to cutoff. Pump off. Throttle wide open. Seems to start on tenth blade.

Looking for a better way. Rather it be the third blade.

 

[Bartman]

The fuel pump is just used to prime the engine - it's not necessary to operate with the fuel pump on in either a warm or cold start.

when I had my Super D I would prime and leave the pump on for cold starts making a full lean mixture necessary. If you are starting with the pump off then it seems that the mixture being left full rich would be fine but then start up is only using the primer load until rpm's are high enough to make fuel pressure with the mechanical pump. it would be interesting to watch fuel pressure as the engine starts to see where the mechanical pump is kicking in.

@Bob Turner maybe try longer shot of prime when hot to more fully clear vapor from the lines and wet the system?

 

[Decathlon 065]

From what I read every plane is a little different. I have learned the hard embarrassing way and over time developed a 10 point check list for the cold start that must be executed with exact procession otherwise it’s off to the races. Hot start at the fuel pump or in front of the FBO… jeez talk about humbling. Give it one good go…. If nothing I’ll act like I’ve become distracted with something else to save face… let it sit for a few then go with the 10 point check list.

 

[Bob Turner]

Ten points? Does that include things like turn the master on?

 

[Decathlon 065]

Ten points? Does that include things like turn the master on?

 

[Bartman]

Ten points? Does that include things like turn the master on?

Bob, don't let your familiarity with the planes you fly allow you to lose sight of the fact that most all of your students will go on to fly maybe ten to twenty hours a year, if that. For them, having a checklist that lists the proper sequence of steps is vital. Turning things on and off isn't so much the point as doing it all in the proper sequence and teaching them discipline to always use and respect checklists is your job and might save a life one day. Seems drastic but it's true, lots of examples out there in the crash reports.

I started to discern in my post between recreational pilots and future professionals but checklist discipline is even more important for professional pilots where they might do the same checklists over and over each day and will be tempted to do things from memory and not back up their flows with a paper or electronic checklist. Then you get a gate agent rushing the crew or taxi instructions get dicked up and the next thing you know the flaps haven't been set for takeoff and they're crashing on the airport entrance road.

I get it that we fly simple aircraft but checklist discipline is an absolute, develop a list for your plane that you'll use. For the experimental I bought, there are odd things about it that a career of Lycomings didn't prepare me for so I have a formal checklist that I plan to use. And it's expensive to fix if I dick it up!

 

[Bob Turner]

I agree. I am rated in five rather large jets, and am familiar with checklist discipline.

What I see routinely as a CFI is incredibly long checklists that are whizzed through for ground ops and ignored in flight.
You want a checklist designed to make you want to use it, carefully, with the knowledge that you are covering those things that are necessary for safe flight.

A checklist that says Master- ON, mags- ON, Starter- DEPRESS is begging to be ignored. If you miss one or more of those things you are not in any danger at all - you simply put the airplane back in the hangar, and then try to remember how to start your Mercedes.

Opinion, of course. At my airline, if you could not remember to turn the lights on for takeoff, you took off in the dark. On the Airbus, we had three items after start, and one item during taxi. Worked fine.

 

[Tangogawd]

Ah yes, we call that "checklist fatigue" at work.

 

[darrell whitley]

Yup we did a flow and backed it up with a checklist like most operators. After the Burbank incident we cleaned up the checklist and only did a few calls after taxi began. Important stuff only. They wanted both guys to be heads up. Especially when we approached the runway for takeoff.