What's a hot start?

[mattaxelrod]

Well, I think I can figure out that it's starting the plane back up when the engine is already hot. What are the differences, and what should I aware of?

 

[bbchien]

Well, I think I can figure out that it's starting the plane back up when the engine is already hot. What are the differences, and what should I aware of?

In turbine talk this is a condition when unburned fuel pools in the ingnition and spreads to the other sections. Very bad. Shortens the life of the turbine.

In pistons, it merely means that the fuel will likely boil when suction pumped by the hot engine pump into the manifold (or cyls if fuel injected). The result is, no fuel delivered. Of course here too you can get fuel pooled in the manifold and the big backfire, in which case you lean it off and keep cranking it it.

There are some generic start procedures by engine but the best bet is the POH.

 

[Henning]

Well, I think I can figure out that it's starting the plane back up when the engine is already hot. What are the differences, and what should I aware of?

That indeed is the reference to a piston recip. When applied to a turbine (not turbo) engine it refers to a quarter of a million dollars or more. What it is is an internal overtemp situation that typically ends up with the replacement of the burner cans, nozzels, turbine blades, casing, stators and rotors in the aft section of the engine, basically, nearly everything downstream of the flame front. This is generally caused by insufficient airflow through the turbine before the pilot comes in with the fuel. The base causes are low battery, plane sitting with wind blowing up the exaust, just plain wasn't paying attention. You do have a second chance to save the situation though, if you EGT or TIT rising too fast in relation to RPM, you still have the chance to pull fuel before the needle passes the redline. At that point we get to another dual use accronym: HSI, Horizontile Situation Indicator, or as in this case Hot Section Inspection, which basically does what it says. Allowing a Hot Start to happen will typically lose you your job.

As to what's the difference in a piston engine, not much really. You still need the basics, fuel, air, spark and timing. In a cold engine, it requires a denser fuel air mix because the vaporization rate at cold temps is not as good, and you need to make sure you get enough fuel molecules to the spark plugs to get a fire, when the engine is warm, the vaporization rate is high so not as much fuel is needed to fire evenly. That's why we prime cold engines, to put some fuel in a cyl and allow the fuel to vaporize and spread evenly throught the cyl. With a hot engine, we don't really need the extra fuel, but with a fuel injected engine, we run into a problem here. The spider for the FI and metal feed and distribution lines from the fuel servo to the nozzels at the get hot and as it just sits there with no new fuel flowing, the fuel just evaporates from the lines. Now, the trick is to bleed the lines without putting too much fuel in the cylinders and getting the plugs wet thereby fouling the plugs.

My method for starting any hot FI Recip.

Throttle Idle
Mixture Rich
Boost pump until fuel flow starts to rise,
Open throttle 1/4-1/3
Mixture 1/2 (or where it would be properly leaned for the conditions)
Mags Hot
Starter Engaged

 

[mattaxelrod]

OK, I was primarily interested in the term as it may have applied to a piston single--specifically a Cherokee.

Matt

 

[Henning]

OK, I was primarily interested in the term as it may have applied to a piston single--specifically a Cherokee.

Matt

Yeah, I caught that when I went and reread, I edited, so you may want to go back and read the second paragraph. Sorry bout that.

 

[Henning]

OK, I was primarily interested in the term as it may have applied to a piston single--specifically a Cherokee.

Matt

Oh, BTW, for a carb engine in your Cherokee, Don't prime, mix rich, throttle closed, start cranking, go to full throttle in a space of 1 second and come back to 1/3 or idle as the engine catches. Never pump the throttle on a non turning, downdraft carbed engine. The fuel from the accellerator pump (if so equipped) will just run down in the airbox. Pretty much a fire hazard. BTW, if you have a "fwooop" and a fire, full throttle, mix almost off and keep cranking. Safest thing for them flames is to be sucked through the engine where they belong.

 

[EdFred]

For a hot start on my carb'ed Cherokee:

Mixture full lean / cutoff
Fuel Pump On
Throttle open
Mags Hot -> Start (all one switch)
While cranking start to increase the mixture
After it fires and begins to run on its own power:
Starter disengage
Throttle --> Idle and simultaneously:
Mixture --> Rich

 

[mattaxelrod]

well, the last 2 responses pretty much said opposite things with regard to the mixture. I guess everyone has their own way to do things.

(BTW, I have a pa28-160, if that matters to the discussion.)

But I appreciate all the responses.

 

[Henning]

For a hot start on my carb'ed Cherokee:

Mixture full lean / cutoff
Fuel Pump On
Throttle open
Mags Hot -> Start (all one switch)
While cranking start to increase the mixture
After it fires and begins to run on its own power:
Starter disengage
Throttle --> Idle and simultaneously:
Mixture --> Rich

There really is no reason to go to start at ICO mixture with a carb. Vaccuum from venturi effect draws the fuel in. As for fuel pump on, except to fill a fuel bowl that might be down just a bit, it will have no effect. Until the engine is turning, it will not get fuel without using the primer, so you may as well start with the mixture set somewhere where the engine will run. The proceedure you posted is good for starting a flooded engine. Which BTW is a technique some people use for hot start, they just flood the engine and then use flooded engine proceedure. This is ridiculous in my book, it's harder on the starter and a greater fire potential. You should be able to start a properly tuned engine in 3-4 blades whether hot or cold.

 

[Henning]

well, the last 2 responses pretty much said opposite things with regard to the mixture. I guess everyone has their own way to do things.

(BTW, I have a pa28-160, if that matters to the discussion.)

But I appreciate all the responses.

Doesn't matter what HP, its a updraft carb, that's what matters. Just think about how a carbureted engine gets its fuel. It has to develope an airflow through the venturi of the carb, therefore, you can't flood the engine with the mixture, because the engine will not pull more fuel than it can use, so, since it's hot and ready to run on normal mixture, you may as well start with the mixture there. Now as you start cranking, advancing the throttle pumps fuel into the airstream directly by mechanical effort of the accellerator pump, this will control any extra fuel required to start, typically the engine will light off within 2 blade, before you get the throttle full in. With carbs nearly any proceedure will work because the fuel flow is automatically regulated with the airflow, so you can't screw up too bad without pumping away at the primer or throttle.
It's when you come into Fuel Injected engines that it makes a big difference. The Bendix system on most recip aircraft is a very simple constant flow system where the pilot can flow fuel into the engine without it turning and getting the spark plugs wet. This, and not flowing enough prestart fuel to clear the air out of the system are the two things that make "hotstarts" such a bugger on recip FI engines. You have to do it within a resonably thin thin set of parameters for the engine to fire off on a blade or two.

 

[CFIse]

well, the last 2 responses pretty much said opposite things with regard to the mixture. I guess everyone has their own way to do things.

It's black magic - lot's of people have their sure-fire absolutely always works on all kinds of engines for starting them hot, and for them it is, but when other people try to replicate it it never works - who knows why. That's why you can find almost as many ways to start a hot engine as you can find pilots.

Oh sure - people will tell you to follow the POH - but then everybody who has ever done that and been sitting on the ramp with a rundown battery and a long night ahead of them will tell you that never works and you need to sprinkle the blood of a diseased chicken over the back seats before touching the mixture control and then it ALWAYS works.

 

[mattaxelrod]

I was told that sprinkling the blood of a diseased chicken only works on Cherokee 140's.

 

[PoAdeleted5]

I was experimenting yesterday with hot starts on my Cherokee 180C. Mixture at full lean, mags on L, fuel pump on, turn the starter, stroke the throttle just a little and she was already firing, push the mixture rich and switch mags to both.

That's the procedure in the 180C owner's manual.

 

[poadeleted20]

An ounce of prevention is worth a pound of cure.

One way to prevent hot-start problems caused by overheated fuel lines is to open the cowling as soon as you shut down and let out all that heat. A friend of mine instrumented the engine compartment of his Grumman and found that the air temp in there shoots to 240 F after shutdown (no more air flow, but lots of heat being dumped out of the cylinder heads). Opening the cowl as much as possible (even just the oil door on those stinky all-screwed-down types) makes a big difference by preventing the problem from arising.

 

[Kaye]

It's black magic - lot's of people have their sure-fire absolutely always works on all kinds of engines for starting them hot, and for them it is, but when other people try to replicate it it never works - who knows why. That's why you can find almost as many ways to start a hot engine as you can find pilots.

Oh sure - people will tell you to follow the POH - but then everybody who has ever done that and been sitting on the ramp with a rundown battery and a long night ahead of them will tell you that never works and you need to sprinkle the blood of a diseased chicken over the back seats before touching the mixture control and then it ALWAYS works.

Amen

 

[Carol]

It's black magic - lot's of people have their sure-fire absolutely always works on all kinds of engines for starting them hot, and for them it is, but when other people try to replicate it it never works - who knows why. That's why you can find almost as many ways to start a hot engine as you can find pilots.

Oh sure - people will tell you to follow the POH - but then everybody who has ever done that and been sitting on the ramp with a rundown battery and a long night ahead of them will tell you that never works and you need to sprinkle the blood of a diseased chicken over the back seats before touching the mixture control and then it ALWAYS works.

HAHA. True

 

[drhunt]

Thanks for the tips and more importly, a better understanding Henning. Just a couple weeks ago, I was giving rides to grade school kids as part of and aviation day at the school. First group was great. Had a hot start on the second group and lost a lot of confidence points with the second group as they listened to me crank my Archer over and over trying to get it started...not a great way to introduce kids to flying...OTOH, probably pretty realistic, huh?