- Pup Cables
- Flying into the Yellow Sector
- Winter Tips
- SB105
Technical Forum
A 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 Cables
A 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.
Ray Clegg
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Flying into the Yellow Sector
Three 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.
Roger Hayes
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Winter Tips
A 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.
WARMING UP…
With the engine now running smoothly, warm it to min 100 deg CHT and 30 deg Oil Temp as per the Flight Manual. This can take ages in winter if you leave the mixture set at RICH, but do yourself and the engine a favour. After start set 800 rpm for 30 seconds, increase smoothly to 1200 rpm and then lean slowly to best mixture (mixture knob about half an inch forward of where any roughness begins during the leaning process). The engine will warm up much quicker, using less time and fuel; the plugs will collect less lead and will be less likely to foul if you’re burning a little oil. Summer or Winter, I always lean the mixture to aid warming a cold engine, BUT, don’t forget to reset mixture to RICH before testing the engine and for take-off. (If you do forget, you will get a warning when carrying out the HOT air check at 1800 rpm, as the drop will be noticeably less than usual.)
SERIES 2 AIRCRAFT
ON SOFT GROUND… No problem to you chaps (and ladies) with lots of concrete to play on, but for we strip flyers soft ground in the winter is a real problem. The well-seasoned strip pilot will already be using the larger (600 x 5″) main wheel tyres as they give greater compliance to deal with the odd bump in the grass as well as spreading the aircraft weight over a considerably greater area. So they are a serious help on soft ground. But we seem to forget that the weight on the nose wheel is not inconsiderable on a Series 2 a/c, and this usually stands on the narrower 500 x 5 tyre. When we carry out engine power checks we notice the nose drops markedly, which clearly demonstrates the additional down force on the nose wheel at higher power settings. Small wonder then, on a soft field, that it is the nose wheel, not the mains, that sinks through the surface. Lots of up-elevator will undoubtedly help, but once the nose is sinking any extra power applied to pull you through the mud has the opposite effect – it pulls the nose down further. I know: I’ve been there; twice. However, Mr Beagle kindly gave us the solution in the form of spinning weights. That’s right; those lumps of lead we bolt to the ventral fin once every 3 years for the C of A renewal test flight. Forty pounds of lead on the rear of a Series 2 Pup makes the nose wheel load around 95 pounds lighter and this makes a huge difference. We have used this wheeze on ’NR for the past 5 years during the soggier winter days. It works a treat, and has often made the difference between going flying safely and not even bothering to try. The 40 lb penalty is not all bad news either as, being at the rear, it reduces trim drag quite a lot.
N.B. A warning note if you fit the spinning weights. Take care to park into wind and either chock both mains or park some distance from other aircraft. The reduced weight on the nose wheel means the a/c will weather-cock much more easily on the ground if the wind changes, and the rudder will be driven against its stops, perhaps causing some internal damage you cannot see! Again; I know. I returned to ’DU some years ago at Headcorn and found ’DU had swung through 70 degrees – fortunately without hitting the adjacent a/c. I was convinced someone had moved the a/c until I noticed the smooth sideways skid mark made by the nose wheel on the muddy surface and the change in the wind direction. I learnt (a little more) about flying that day.
Ray Clegg
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SB105 – without cutting corners
B121/105, the main spar doubler NDT inspection done by an Eddy Current technique, was introduced a couple of years ago by the Pup Design Authority, now BAe Systems at Prestwick. The CAA made the inspection mandatory by issuing an Airworthiness Directive (001-05-98), and most owners will have complied by now with the SB as it had to be first done within 50 hours of receipt or at 2000 airframe hours, whichever came later. To obtain access for the inspection, the SB called for slots to be cut in the fuselage skin and through each chine angle immediately forward of the main spar. Owners may remember Jim Ellis’ wonderful article in the Spring ’99 Beagle News describing the “fear and trepidation” with which he watched the slots being cut and waited “with bated breath” for the inspection results. On completion of the SB, the sections cut out of the chine angles (nearly-90° angled members to which the fuselage side and bottom skins are riveted) are not replaced by any repair scheme etc.
Most engineers and owners were not entirely happy with slots being cut right through what they perceived as structural bits of Pups, but BAe had considered the loads and stresses in the areas concerned and were content that all was safe. After all, the slot cutting and inspections had been OK for 14 years on the RAF Bulldog fleet before this SB was introduced to Pups. Nevertheless, inspection without having to cut slots (always with the attendant risk of damaging the main spar) was seen by many as an ideal, if difficult, objective.
Engineers looking after AZDG (and, later, AZDA) saw that removal of the main undercarriage legs would give adequate access to introduce a modern slim NDT probe beneath the main spar to sweep the suspect radius of the main spar doubler, so complying totally with the spirit of the SB, if not exactly the letter… This method would require the main gear to come off every 800 hours (that’s the SB frequency) but would leave the chine angles intact. Having found out that (at least) two Pups had been so inspected (just as our own Pup AXNR was coming up to its initial SB105 inspection) I decided to regularise this method to help all BPC (and other) Pup owners where the initial inspection had not yet been done. Phone calls to DG’s engineers and NDT specialist, receipt of copies of their certification, many phone calls and letters to and from the Design Authority at Prestwick, a proof-of-concept inspection on our Pup and a quick word with the CAA have led to BAe at Prestwick not only accepting this method, but they are re-writing the Service Bulletin to introduce this alternative method of compliance. The details of exactly how we did the inspection on AXNR have been passed to BAe to assist them in that re-write. The revised SB should hit your doormat any day now. All very well, those with the slots already cut will say; but what benefit is there for me? None is the short answer – except to see how the Club can and does work with the Design Authority to achieve improvements in Pup engineering matters (remember the extensions to handgrip life, and the dreaded SB102 saga).
For those with SB105 yet to do, I would seriously recommend this alternative method; not cutting structure has to be the preferred option. Are there any penalties with this method? Yes, one. Both main legs need to come off every 800 hours; this will cost money each time, but how often does your Pup achieve another 800 hours? I would suggest removing the legs every 800 hours can be viewed as a positive penalty (?) as it provides the perfect opportunity for close inspection of the main gear trunnion bearings (an otherwise never-removed item) and a thorough inspection and service of both shock absorbers and their attachments.
Ray Clegg
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