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Technical ForumA collection of both new and previously published articles addressing a variety of technical and operational matters related to Pups and/or Bulldogs. In the main, the contributions have come from experienced Club members and are offered for the sound advice they contain and, in some cases, to promote further discussion. The advice should not necessarily be taken as the view of the Club. The aircraft's Flight Manual is always the over-riding authority on operational matters and flying techniques, just as maintenance procedures and certification thereof must always be performed in accordance with the Air Navigation Order (or equivalent overseas legislation). The Type Design Organisation for both Pups and Bulldogs is de Havilland Support Limited, based at Duxford. All questions relating to continued airworthiness, fatigue data analysis, provision of manuals, drawings or other design data, and proposals for amendments to maintenance or operating procedures should be addressed via www.dhsupport.com.
Pup CablesA recent ‘Letter to Operators’ was sent out by the CAA advising
of the possibility of stress corrosion occurring in certain control cable
end-fittings. Curiously, it was not sent to operators, but only to
design, JAR145 and M3 maintenance organisations. However, our new Design
Authority, DHS, have thoughtfully provided access
to this LTO. You should be aware that there are 12 cable end-fittings
on the Pup which fall into the category at-risk (i.e. have part numbers
starting MS.21260-): there are four in the elevator trim system, four
in the aileron system and four in the rudder system (the IPC shows where
they live). Whilst there is as yet no specific inspection mandated, you
would be well advised to instruct your maintenance organisation to follow
the guidance in the LTO. The end-fittings most at risk are those over
15 years old since first fitted and where they have been exposed to humid
salt-water-contaminated atmospheric conditions. If your Pup has been regularly
parked outside within a few miles of the coast, then a particularly close
look is advised. As there is no finite lifing policy on these cables when
installed in the Pup, I suspect most of the fleet cables are well over
15 years old. Flying into the Yellow SectorThree incidents of fuel starvation is three too many; in two cases the engine stopped in flight and in the third the engine coughed! Why? Because pilots flew the aircraft into the Yellow Sector on the fuel gauge. This resulted in two landings in which, for all or part of the time, the engine stopped completely, one of which was a full dead stick landing. Thankfully, in all cases, the incidents ended safely. The common factor was that the pilots continued to fly their aircraft into the Yellow Sector on the fuel gauge for 20 minutes or more. Pilots are reminded that fuel gauges are not perfect indicators of actual fuel contained in the tanks; no aircraft fuel gauge is that accurate. Secondly, there is always a gallon or so of fuel in the system which remains unusable. Consequently, as the Yellow Sector for each tank is only 2 gallons (for both the Pup 100 and the 150) if the aircraft is flown into the Yellow Sector on both tanks simultaneously, after twenty minutes you will be down to about 1.7 gallons on the Pup 150 and only slightly more on the Pup 100. If the gauge decides to read accurately and you have, say, 1.5 gallons usable, you will be flying on air within a few minutes and Presto! an unscheduled landing will result.
Skysport have revised procedures as follows: increase the Yellow Sector
calibration to a new safe minimum fuel load of 3 gallons per fuel tank
on Pup 150s and Pup 100s (making a total of 6 gallons) and 4 gallons per
tank for the Bulldog (making a total of 8 gallons). Pilots should land
and refuel before allowing the aircraft to fly into this new safe minimum
fuel load. Winter TipsA few words on how to treat your Pup during the British winter. So, a bit on starting and warming-up applicable to all marks, and the problems of soft ground handling (mostly for Series 2 a/c.) We will start (appropriately) with: STARTING...On really cold days even the best well-maintained Pup can be difficult to start, especially if left outside or in an unheated hangar. Messrs Lycoming and/or Rolls Royce thoughtfully put their carburettors in a low, high pressure area where ram effect would aid performance when airborne, but gave little thought to the priming problems associated with low-slung carburettors in cold weather. Mixture RICH, Master ON, Fuel ON, Boost pump ON & Starter Master ON will fill the carb OK and arm the starting circuitry; but the “prime with 4 stokes of the throttle” on a wintry day only results in a few ccs of cold fuel dribbling back down into the cold air box. Throttle SET, Left mag ON (both mags for Series 1) and engage the starter will usually fire things up nicely on a pleasant summer day, but in winter a few blades pass slowly past your eyes, perhaps accompanied by a cough, a short run, then silence. More priming, more blades (slower if your battery is tired) another cough, more silence. We’ve all been there. Why? Well, the battery has less volts when cold, so the starter current is down, and the cold oil is more like treacle. These factors combine to make the engine turn quite slowly when cranked, so the pistons cannot induce the necessary flow of induction air to promptly suck sufficient fuel vapour to the cylinders. Moreover, the fuel is less volatile when cold, so the mixture sucked up the long inlet pipes from the carb to the cylinders is not rich enough to encourage or maintain combustion. So how do we improve matters? Lots of ways – some help a little and some help a lot, and not all are options to all owners, but they all help. A heated hangar would be an almost total solution (unless you spent 2 hours doing your pre-flight), but is impractical for most of us. However, almost any hangar is better than no hangar. Multigrade oil (e.g. Aeroshell 15W/50) is much thinner when cold than W80, so saving the battery from having a hernia on cold mornings and giving faster cranking during starting attempts (as well as much improved initial lubrication). Regular spark plug maintenance (including timely replacement), good mags and HT leads will ensure best ignition at all times, but more noticeably during starting. But getting an adequately rich mixture up into the cylinders is THE secret. When you prime the engine using the throttle you are relying on fuel being squirted by the accelerator pump into the carburettor venturi. The amount of fuel squirted is proportional to the rate as well as the extent of the throttle movement. So don’t move the throttle too slowly. The fuel is squirted upwards during priming, entering the induction system just below the throttle valve. The fuel immediately runs back down into the cold air box under the carb (and from there onto the nose wheel tyre if you take all day to do your pre-start checks). What we need to do is to suck the fuel up the induction pipes towards the cylinders during priming so that the surface of the induction pipes is wet with fuel. We then stand a good chance of having a sufficiently rich fuel/air mixture being sucked into the cylinders when we go for the start. So, Mixture RICH, Master ON, Fuel ON, Boost pump ON & Starter Master ON. With the Mags OFF, hit the starter and as each blade passes give one full fairly fast stroke of the throttle. Release the starter and then, without hesitation, give 3 more priming strokes, select Left Mag ON (BOTH in Series 1 a/c), throttle SET and select START. The engine will fire at the first blade and will continue to run. Release the starter, (Mags to BOTH in a Series 2), Starter Master OFF etc. etc. If it is a very cold day and the engine falters some seconds after the start, a few short firm strokes of the throttle will produce extra mixture enrichment from the accelerator pump. Over-priming a Pup with a warm engine on a hot day is always a possibility, and this makes pilots so wary that under-priming in the winter is the more common cause of poor starting.
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