A description of this starting procedure is included in the military pilot's manuals for many aircraft. It also can be found in a plethora of airline operating manuals of the period. I should mention at the outset that I certainly wasn’t the one who decided upon this way of starting these round engines, it goes back much farther than my first experience with these machines. It was included in the USAF 51-42 "Aircraft Engineering for Pilots" manual in an effort to standardize starting techniques. While an occasional backfire – in earlier days – may have been considered somewhat humorous and a cause for rolling of the eyes among those nearby when a garden variety round engine was being started it was definitely not funny on the R-4360 "corncobs". I'll guarantee you it instantly called down the wrath of the maintenance officer and line chief upon the unfortunate one at the flight engineer's panel when it occurred. Since that time every occasion that I've watched – or more
correctly, heard – a backfire during starting I am instantly reminded of one of our pilots on the old piston Convair 340/440s. This captain was an outstanding example of the salesman's art, possessing – in spades – all the warmth and "bon-homme" fellowship endemic to that breed. However, known among the copilots as "tickle-tickle, boom-boom", he had never really quite mastered the primer starting technique contained in the title of this piece. And, in truth, I didn't attach a name to it even though it was in common usage by that time – in the early sixties. One caveat – this
discussion pertains to carbureted engines only, fuel injected engines would have to be a subject for another article. A few years ago an expanded checklist was needed when we started flying the newly restored, locally based B-25 "Miss Mitchell". In that expansion we wrote a fairly lengthy and detailed section discussing the whys and wherefores of the USAF starting procedure. The following (in italics) is a direct quote from that section.
At this point we need to stop and briefly discuss how this prescribed start differs from the one commonly used prior to and during the WW ll era. As aircraft radial engines developed from a single to two and then four rows of cylinders the resultant growth in complexity of the intake/exhaust systems made them much more vulnerable to backfires and other symptoms of improper starting procedures. The need for a standardized procedure that could be successfully used on all USAF round engines resulted in the development of what was referred to as a "controlled quantity" start in the training manuals of the period. It basically involves all starts beginning from a known mixture condition of no fuel and then progressively enriching the stoichiometric fuel/air ratio with the
primer until combustion occurs. In this way you should always know where you are, mixture-wise, eliminating the sometimes rich and sometimes lean (searching for a combustible mixture) random movements of throttle, primer and mixture with the associated backfires and problems. This procedure was subsequently adopted by other operators along with the airline industry and continues in use today.
The following description of a normal start includes the procedures to be utilized after all pre-start procedures have been completed in the BEFORE START checklist.
*During start, the pilot's right hand is used solely for the primer, energize and engage switches.
*The left hand is used for the throttle, then the magneto switch, then back to the throttle to control the starting RPM and then for the mixture (or as otherwise needed), i.e., a "free safety".
*Actuate the energize switch for a few seconds (actually, just enough time to note that the fuel pressure drops off and then recovers from electrical load), then the mesh switch also, noting free rotation of propeller indicating absence of hydraulic lock.
*After minimum of six blades, turn the magneto switch to BOTH.
*Actuate primer switch and hold (a warm engine may need only intermittent prime).
*When engine begins firing, transfer attention to tachometer and control RPM with throttle.
*Release energize and mesh switches, continue actuating primer.
*Stabilize the RPM at 800-1000 while running on the primer, primer switch may have to be intermittently toggled depending upon OAT / engine temperature / primer flow rate. The primer technique can only be developed through experience.
*After RPM stabilizes, advance mixture to RICH, then release primer switch after the RPM starts to sag from an over-rich mixture.
*After the first engine has been started, check four things: Oil pressure, hydraulic pressure, suction and then fuel pressure after turning OFF the boost pump. (When terminating shut this engine down first so that the other engine's vacuum and hydraulic pumps can be checked.)
*Repeat the above items and start the remaining engine.
*After starting the second engine there are only two things to check: Oil pressure, then fuel pressure after turning OFF the boost pump.
So, that's what we've included in the training of all new B-25 pilots. Using this procedure eliminates the snarfing and cracking which all too many people assume is characteristic of any B-25 start.
In talking with a lot of crews I've noticed one commonly encountered mis-perception that a backfire can occur from either a too lean or too rich mixture. This should be laid to rest for all time! A backfire occurs only from a too lean mixture. If it's too rich you might get some afterfire and the engine may tend to die but not a backfire. The most common cause of backfire is excessive throttle – a setting beyond that which would provide approximately 800-1000 RPM. The “before start” part of Miss Mitchell's checklist
calls for the throttles to be set at an approximation of this position.
With very low O.A.T.s you can pull the throttle back a little bit so as to provide less airflow. This has the effect of creating a slightly richer mixture for these conditions. With exceptionally cold O.A.T.s we used to try everything out on the line to enrichen the mixture on those North Central DC-3s, including dumping in alcohol from the auxiliary carburetor de-icing system. Anything to get something that would ignite. But, looking back, this was really a situation where the engine shouldn't have been started, it should have been pre-heated more before ever attempting a
start. Just one more example of the abuse those poor old 1820 Wrights absorbed and one that I don't believe would ever be duplicated with our present day pampered warbirds.
I've been advised of one technique being advocated in which the throttle is opened to a setting that would provide enough air to properly consume the amount of fuel from a continuously actuated primer. The problem here is that the throttle very likely will be advanced far enough to cause a backfire on a significant number of occasions. Also, it very likely may be up in the RPM range where the carburetor functions through the main metering circuit rather than through the idle mixture circuit, leading to a rapid acceleration and a bunch of RPMs immediately after start. This, of course, results in an instantaneous lack of adequate
lubrication to the moving parts along with other bad things.
Now, let's discuss some anomalies, at least those that I'm aware of. I'm sure that there are engines that lie outside of my experience in starting. The procedures in this article are derived from USAF and airline manuals written back when the big radials were king and that needs to be kept in mind here!
First off, single row radials like the 1820. I've always gotten great starts on the B-17 or the Wright DC-3 with just priming normally and then, when the engine fires, moving the mixture to RICH and getting off the primer slightly later. Keep your finger near the primer switch. Sometimes it'll want to die, if it does just give it a brief shot of prime each time the RPM starts to fall to keep it running for those first few seconds.
Next in my experience is the 1830 on the PBY where the thing has an antique type primer system with spider lines to the top cylinders only instead of the more modern type where the system discharges directly into the supercharger diffuser. This thing, in my mind, is a fire trap and should have been replaced l-o-n-g ago. But, some are still around and, in this case, the fuel distribution pattern just can't support a normal start RPM of 800-1000. In this case we've had better luck in using the mixture start in which the mixture is moved to RICH while cranking just long enough to note some fuel discharge from the blower drain, then placed back to IDLE CUT-OFF. Avoid a throttle position more than that which would correspond to 800-1000 RPM. After the engine fires return the mixture to RICH. (As a side note here, we've had a problem with achieving a full IDLE CUT-OFF on one of the PBY's engines. In this special case we had to wait until we wanted fuel before turning ON the fuel boost pump because of the internal carburetor leak.)
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