Merlin 66 Fuel Cut-off Lever

[Shortways]

The short version of my question:

Were the Merlin 66 and newer ever started with the Fuel Cut-off Lever closed until it fired up on its manifold priming fuel?

The background:

1. The Pilot's Notes for the Spitfire Mk. IX, XI and XVI with Merlin 61, 63, 66, 70 and 266 engines describes the engine start with the Fuel Cut-off opened and Boost- (or Wobble-) Pump off,
after priming the carburettor with either pump for 30sec.

2. The A.P.1590 RR Merlin Service Manual for the 66 upto 85 describes the engine start with the fuel line to the Cut-off Valve (including the carburettor) pressurised, Boost-Pump ON and
Fuel Cut-off Lever closed until the engine fires.

The first one is of course clearly a pilot's manual while the second is a mechanics manual, but I have seen more references about using the second method, which seems to give an advantage in starting the engine and getting it running smoothly quickly, without having to use the priming pump to keep it alive after initial firing. This because of the fact that there is pressurised fuel waiting to be released instantaneously into the supercharger and mixed in the existing airflow to the manifold.

Using procedure 2. there is of course the danger of switching on the Boost-Pump with the Cut-off open, which would flood the Supercharger and lead to its destruction at light up, or at least an engine fire, with fuel pouring out of the carburettor and onto the ground. But the same danger exists during the 30sec carburettor prime and ... using procedure 1. with a failing start, for whatever reason, would also make the (geared) engine fuel pump, pump fuel into the supercharger. Albeit considerably less than by the Boost-Pump.

 

[Shortways]

I just found an answer to my own question. I found another Pilot's Notes. This one is specifically for the F.IX and its text about "Starting the Merlin 66 and 77" exactly conforms with the second, Service Manual, procedure. It's too bad that there is no timestamp on these manuals. It would be very interesting to know when and why the changed the procedures, although anyone can take an educated guess abou the why of course.

On nice addition to the "After Warming Up" procedure, the Run-up, Magneto test etc., is that it mentions needing 3 men on the tail and 1 on the starboard wing tip when powering up to +4, +9 and even takeoff power for testing the engine and its services.

I wiuld love to see a video or picture of that!

 

[Shortways]

I just found an answer to my own question. I found another Pilot's Notes. This one is specifically for the F.IX and its text about "Starting the Merlin 66 and 77" exactly conforms with the second, Service Manual, procedure. It's too bad that there is no timestamp on these manuals. It would be very interesting to know when and why the changed the procedures, although anyone can take an educated guess abou the why of course.

On nice addition to the "After Warming Up" procedure, the Run-up, Magneto test etc., is that it mentions needing 3 men on the tail and 1 on the starboard wing tip when powering up to +4, +9 and even takeoff power for testing the engine and its services.

I wiuld love to see a video or picture of that!

... Aaand then you notice that the F.IX version is the "2nd edition" and the IX, XI, XVI one is the "3rd edition" , which in combination with the fact that the XI and XVI are later renditions of the Mk.IX makes the latter the most up to date procedure.

So...

Why did they change the procedure? Was there a change in the engine or did they just want to bring the procedure back in line with that of the Merlin 61/63 which had a Slow Running Cut-Off Ring that needed to be pulled for priming the carburettor and then released?
You obviously cannot hold that ring while pressing the starter and booster coil with your left hand, while keeping your right hand on the (manifold) priming pump to assist the start. So did they want to make the procedure more uniform?

 

[ThomasP]

Interesting questions.

Would it make a difference if the engines in question had a float-type or an injection-type carb?

 

[Engineman]

Without doing all the reading of the manuals, I suggest that the big factor is where the "carb" was actually single-point fuel injection and excess fuel could build-up in the supercharger Volute. Merlins with float carbs did not have enough airflow when starting to pull fuel from the float chambers and so the start is on the primer-fuel. Injected engines also need priming but some can build excess fuel in the volute with "CUT-OFF" not selected, leading to excessive fire risks.

Eng

 

[ThomasP]

I may be misremembering, but it seems like I read something in one or more manuals about one or another Merlin/V-1650 starting procedure requiring it to be done just so in order to prevent engine fires.

Sorry I cannot be more specific, but it was a while ago and I read way too many manuals and such.

 

[Engineman]

I may be misremembering, but it seems like I read something in one or more manuals about one or another Merlin/V-1650 starting procedure requiring it to be done just so in order to prevent engine fires.

Sorry I cannot be more specific, but it was a while ago and I read way too many manuals and such.

Yes, we were starting a Griffon Spit with Stromberg pressure carb, I think, in the 90's and the fuel selector had been put in auto-lean or Run (can't remember the specific options) by someone.
Anyway, the start was impressive, with 3 to 4 foot long orange flames from the exhausts with the engine on the starter but not picking-up. Pilot chose to keep turning on the starter rather than stop it, it took about a full minute to see the flames die down and the engine start! I think he closed the fuel to cut-off but there was a lot of fuel in the supercharger. Really thought I was going to use the big fire extinguisher that day!

Eng

 

[Shortways]

Interesting questions.

Would it make a difference if the engines in question had a float-type or an injection-type carb?

Hi guys, thanks for answering.

The Merlins from the 61 onwards had the Bendix Stromberg pressure injection carburettor with Automatic Mixture.
The 61 and 63 for the F.IX had the whole Mixture Lever Removed and had No Boost- or Wobble Pump.
For the 66 (LF Mk.IX) and 70 (HF Mk.IX) and onwards the lever was re-introduced as a Fuel Cut-off control and they also incorporated either or both a Wobble- or electric Boost Pump.

Both procedures below are based on priming the manifold as well as the carburettor. The engine really fires on the manifold prime fuel, as the fuel from the carb needs to pass the supercharger and the intercooler, which is pretty difficult when the engine is not yet at an rpm where it can draw mixture. The Pilot's Notes and Service Manual actually state to use the Priming Pump to keep the engine going during light-up.

The 2nd Edition Pilot's Notes (Spitfire F.IX - Merlin 61/63/66/70) has the procedure where the carburettor and fuel line to the Fuel Cut-off Valve are primed and pressurized by either pump with the Fuel Cut-off closed. The Cut-off is only opened when the engine fires. As you remembered correctly, there are a number of warnings to never open the Cut-off with the pump running and/or engine not running/starting. Indeed because of the risk of flooding the supercharger and subsequent fire or explosion.

My feeling ... and yes this is an "educated" GUESS, is that they had a number of these fire incidents as Engineman describes. Both mechanics and pilots were used to putting the Mixture Lever forward to full rich for starting the older engines and old habits die hard when under pressure of a scramble. On top of that, they probably had a bunch of aircraft with these older engines still flying around as well. So, maybe hasty mistakes?

The 3rd Edition Pilot's Notes (Spitfire IX, XI & XVI - Merlin 61/63/66/70/266) is indeed clearly more recent, seeing the later Mk. XI and XVI with the Packard 266 added; This reverted the procedure to just priming the carburettor and fuel line, but not leaving it pressurised. After stopping the pump, the Cut-off Valve was opened! The procedure is more focussed on using the pump for only 30sec, while the previous procedure just left it on (if it was electrical).