Photo File – The Heat Is On

By me
All photos me too, copyrighted

While I often rant here about Croatia’s summer weather – an eclectic mix of searing heat and violent thunderstorms – these past few months have seen thermometers run off their scales, with several airports reporting temperatures in excess of 40 degrees Centigrade… no small feat, since readings are taken two meters above grass and in shade. Out on the tarmac, 55+ was nothing out of the ordinary, making life out in the open particularly unpleasant.

Thankfully though, the heat and humidity had not deterred the brave men and women of general aviation from their passion, with the region’s airports chocked full of everything from cheap-and-cheerful ultralights to high-flying, million+ Euro turbine singles. So having already come to terms that my summer would consist of flying from one oven to another, I’d decided to make the best of it and see what’s on offer on the country’s GA aprons… 🙂

Whenever I’m in a rut for not having snapped a light aircraft in awhile, I can always count on Dubrovnik Airport (DBV/LDDU) to come to the rescue! Even though both GA aprons had on this day been overflowing with various private and business aircraft, G-UAVA was the one that had instantly caught my eye – if anything for being one of the very few Twin Comanches still flying in Europe. Born in the early 60s, the PA-30 was an extensive twin-engine conversion of the earlier PA-24 Comanche, a “heavy cruiser” that had been the top of Piper’s single-engine offering all the way until the late 70s and the appearance of the PA-46 Malibu. Even though it is far from the most elegant twin out there, the Twin Comanche nevertheless has several aces up its sleeve – the biggest of which is a design penned by the legendary Ed Swearingen, a freelance engineer known for his passionate love of speed and low fuel consumption. Most famous as the father of the Merlin bizprop and Metro feederliner, Swearingen had used all of his talent in designing the PA-30, creating a 300 km/h aircraft powered by engines of only 160 HP that together drank just 17 USG per hour. While they do not sound like something to write home about, these numbers are identical to what the SINGLE engine Cessna 210N could manage on its 310 HP – and all the more amazing given the extra drag and weight penalties of the second engine. G-UAVA itself had been manufactured in 1967, and can additionally boast so called “turbonormalized” engines, a special variant of the classic turbocharged setup running at lower manifold pressures and cylinder temperatures – thus increasing engine life and durability with very little loss in performance. Another interesting detail is the slope of the apron and runway; my camera’s internal balance had said that this shot is perfectly level! Built on an undulating plain that is the only suitable piece of flat land for dozens of miles around, the airport is well known for its “uneven nature”, which can cause problems if you’re not prepared for it.

After Dubrovnik had served up its best offer, Split (SPU/LDSP) had also decided to deliver! Cessna’s first post-war twin, the 310 had remained in continuous production for 26 years, and spawned such a number of versions that they ate up half the alphabet. The Q model pictured here was the type’s last snub-nosed variant, with the subsequent 310R – the last series to go into production – receiving an elongated and aesthetically far more pleasing job that had included a lot of additional storage capacity. A fine example of a classic 70s Cessna paint scheme – proudly advertising the fact that the engines sport a fuel injection system and not the common man’s carburetor – D-IBMM had been manufactured in 1973, and can still be seen happily flying all over middle and southeastern Europe.

A cute little canary coming in to make an already fun day of flying and photography at Split Airport (SPU/LDSP) all the better. A pretty rare bird, the Do-328JET is – as it says on the tin – a turbofan variant of the 33-seat Do-328 turboprop, a sleek and sexy design that can still today be seen flying with smaller regional operators in and around the Alps. Even though it had always been a well designed, robust and quality product, the Do-328JET had one fatal flaw: it was the brainchild of two small companies (Fairchild and Dornier) that went head-to-head with the likes of the much more established ERJ-135 and CRJ-100/200 in a market that does not easily forgive design missteps. Dornier’s wobbly financials had further deepened the hole being dug under the design, the result of which are only 83 examples of the type ever made. Today however, it is enjoying a small Renaissance as a business jet – as well as a speedy and capable utility aircraft for both civilian (such as ADAC) and military operators (including the USAF).

The second oldest airworthy Skyhawk in Croatia – manufactured in 1966 – observing proceedings at Split (SPU/LDSP) from its elevated position halfway up the airport’s famous hill. Located just a 100 or so meters from RWY 23, the hill tops out at just 10 meters above the airport elevation, and in addition to a GA hangar and fuel farm features an olive garden – as well as a small church that predates the airport by a couple hundred years… not a bad feature to have INSIDE the airport fence! BDM itself is similarly native, having flown in country ever since the early 70s and the first of the Yugoslav government’s aeronautical shopping sprees (intended to equip flying clubs and schools with modern Western machinery). As an H model – Reims-built no less – it still sports the Skyhawk’s original six-cylinder O-300 engine developing 145 HP, quite a more charismatic (if inefficient) package than the modern fuel-injected four-pop IO-360.

Enjoying a bit of sun and fresh sea air on Croatia’s highest – and most challenging – airport. Perched on a high plateau surrounded by hills near the top of the eponymous island, Brač Airport (BWK/LDSB) sports a cocktail of characteristics that requires you to be very much awake on landing, including a 1750 ft elevation, a 1.4% runway gradient (1.7% in places even), notorious rotors and turbulence on all approaches, summer temperatures well above 30 degrees Centigrade – and a tight 1600 by 30 meter runway that often gives bother to business turboprops and jets, let alone the occasional airliner. On this day however, the stars of the show were the lighties, here a typical “summer holiday mix” of aircraft from Hungary, Romania, Germany and Slovenia. Type-wise, there was a lot to choose from as well, with just this lineup boasting one of the more powerful Morane variants, the Mudry CAP-232 aerobatic single-seater… as well as six-cylinder Mooney and a mint Skyhawk that – despite being 35 years old – looks like it had just rolled of the production line.

Taking a quick stroll through Varaždin’s (LDVA) small corrosion corner. Already disused and mostly abandoned prior to having been flipped over in a storm in 2012, CDZ is one of Croatia’s oldest Skyhawks, manufactured way back in 1967. Unfortunately, despite quite a bit of history in its logbooks, this is as far as it will ever get, since repairing it would actually cost more than buying an airworthy late 70s/early 80s example. Indeed, the extensive buckling down the tail (evident on both sides) is a telltale sign of major structural failure in the underlying load-bearing frame, requiring the whole back end of the airplane to be replaced at the very least. Though it had, damage-wise, fared much better, the country’s sole PA-28-235 hiding in the background – and registered, rather ominously, 9A-DIE – is pretty much in the same boat. Completed in 1965, it too had not seen much air these past few years, and looks to be another candidate for a “Coke bottle conversion”…

Just when I thought I’d used up all of my luck for finding rare piston singles, I stumble upon this magnificent Sierra at little old Lučko. Fairly atypical by the standards of the company, the Sport/Musketeer/Sierra family was Beech’s attempt at replicating the success of Piper’s legendary PA-28 Cherokee series. Standing at the top of the lineup, the 24 Sierra was essentially a 200 HP Musketeer with retractable gear that had hoped to take on the extremely popular PA-28R Arrow. Sadly though, none of these models had managed to make a significant impact on the market, partly because they were made to Beech standards – and therefore more expensive – and partly because this segment of the market had never really been the company’s forte. But more on the 24 in a separate post!

Though we had already met before several years ago, it is nevertheless always nice to see this old trooper once again. One of the very few early 206s still flying in Europe, HA-CPA celebrates its 50th birthday this year, a fact that had not – in true utility Cessna tradition – prevented it from working hard well into old age. Many moons ago actually a resident of Croatia, CPA had on this day popped into Lučko for a state skydive championship, for which it was the sole official dropship. An interesting detail is the pronounced chin under the nose, a leftover from the early Cessna 210 on which the 206 is based that had housed the 210’s nose wheel when retracted (a bit more info available here).

In common with many Cessna models of the 60s, CPA’s flight deck is, by modern standards, a jumbled mess – but it nevertheless does have a certain odd charm. Interesting details are the flap position indicator (partially obscured by the right yoke) with color-coded fields representing maximum flap extension speed – and a Soviet EGT gauge below the CDI, apparently salvaged and reused from a light transport twin (possibly even the An-14).

Taking a short breather on Croatia’s sole truly private airstrip. Nestled in rolling terrain 20-odd kilometers south of Zagreb, Pisarovina Airfield counts among the more scenic places to land at in the area, ringed by dense woodland and the Vukomerić Hills to the north, vast arable fields and fisheries to the south – and airliners on approach to Zagreb Airport (ZAG/LDZA) above. Indeed, the airfield is within spitting distance from both ZAG’s control zone and the Pisarovina NDB – the focal point for all approaches to RWY 05 – making getting in and out quite a fun and refreshing experience. Though several aircraft – and even a flight school – are based here, on this day we were the only plane in town, which made us feel a bit… conspicuous…

A short & sweet additional feature to accompany the photo above: a “dash cam” video of the approach to and landing on Pisarovina’s RWY 04… a fair bit of thermal turbulence that day, but what can you do (also, the camera shake on landing is exaggerated – the runway is relatively smooth, but the camera was mounted on a suction mount on the windscreen, which is flexible and tends to wobble about with every bump).

Short Photo Report – Piper PA-30-160 Twin Comanche, N55AG

By me
All photos me too, copyrighted

Apart from arriving in questionable style – a 30 year old Skyhawk is not the most elegant of aircraft it must be said 😀 – an added benefit of flying to the coast during the tourist season is that you can always find some interesting aircraft when you get there. Being quite close to the central European mainland, Croatia’s five coastal international airports and three port-of-entry airfields offer a convenient way of reaching the Adriatic without much undue hassle, providing the locals with a steady and varied flow of interesting light (and occasionally heavy) aircraft :).

And while by the end of May the season had still not reached full steam – with the aftershocks of the recession still being felt across Europe – I was confident that, for my last Instrument Rating training flight, I’d be rewarded with something really nice :D. My destination for the day, Zadar’s Zemunik Airbase/Airport, has a history of interesting GA aircraft, its strategic location at the midpoint of the country’s coast providing easy access to a number of well known and frequently visited destinations – all the places that a man with a plane might visit :).

With that in mind, I had crossed my fingers and hoped for the best. Thankfully, my luck held out, and this is what I’d found… 🙂

My first ever Twin Comanche! 🙂 Developed from the PA-24 Comanche single - in it's -400 series for a long time one of the fastest piston singles ever produced - the PA-30 is powered by two Lycoming IO-320s, each developing 160 HP from four cylinders. In essence the same engines that power - among other things - the Skyhawk and Piper Warrior, coupled with constant speed props they give the PA-30 a cruise fuel consumption of just 16 GPH, making it one of the most economical aircraft in its class

Unlike similar single-to-twin conversions (such as the Beech TravelAir developed from the Bonanza), the Twin Comanche was developed out-of-house by Ed Swearingen, a man well known for his high-speed piston twin modifications. Designed to replace Piper's own Apache twin - whose big brother, the Aztec, can be barely seen in the background - the normally aspirated Twin Comanche can zip along at 172 knots and 20,000 ft, and with a full 120 gallons aboard continue on for more than a 1000 NM. A pretty solid set of numbers for a "weedy" 320 HP!

Alongside the normally aspirated models (which I believe this one is, couldn't find its data plaque), the PA-30 was also offered in turbocharged and turbonormalised versions (see bottom of post). Some models were also offered with 200 HP engines, while the later PA-39 Twin Comanche C/R (the 39 is no typo 🙂 ) received engines spinning in opposite directions (C/R = counter-rotating) to remove the "critical engine" effect during a single engine failure

Looking quite good in its simple, retro scheme. Though registered in the USA, it is possible that N55AG is permanently based in Europe (most probably the UK), but for various reasons kept on the US register

Time again for a little (engine) digression to make sense of all the turbo- terminology :D. Essentially, all piston engines have two key parameters that define their power output: the RPM and the manifold pressure, the pressure of air in the intake manifold (part of the intake system) – and consequently the cylinders. On a normally aspirated (atmospheric) engine at sea level, the maximum manifold pressure, achieved at full throttle, will never exceed about 28-30 inches mercury – or, more plainly, atmospheric pressure. As the aircraft climbs however, the air pressure drops and the manifold pressure drops with it. This results in a progressive decrease in power until the altitude at which the power produced is just sufficient to hold the aircraft in the air without sinking. This altitude is – in a nutshell – the fabled ceiling, above which the aircraft cannot climb no matter how much the pilot wants it to :). Depending on the displacement and HP of the engine – and the power requirements of its associated aircraft – for normally aspirated engines this altitude is between 10 and 20,000 ft.

If you had wanted to increase this altitude, the most practical way would simply be to either delay the manifold pressure drop so that it doesn’t start immediately after sea level but somewhere higher up, or widen the manifold pressure range so that you have more “pressure reserve” before you reach the point above which you cannot climb.

On modern engines, both of these are achieved by use of the turbocharger. A familiar component from automobile engines – especially Diesels – this is a high-speed compressor ramming air into the cylinders at high pressure, and is driven by a turbine (a glorified windmill 😀 ) spun by the engine’s exhaust gasses (hence the much-misused “turbo” prefix). In aviation applications, the turbocharger is always variable-speed, controlled by a component known as the waste gate, which controls the amount of exhaust gas ducted over the turbine, hence varying its speed. As the aircraft starts climbing from sea level, the waste gate progressively increases the speed of the compressor – thus increasing the amount of air rammed into the cylinders – to keep maximum manifold pressure attained at sea level regardless of the drop of atmospheric pressure and density.

If the climb continues when the compressor reaches its maximum speed, it can no longer compensate for the decreasing pressure, and the manifold pressure starts to drop (the compressor remains spinning at top speed). The altitude at which this occurs is called the critical altitude, and for most modern turbocharged pistons it is between 8,000 and 10,000 ft (though on pressurized aircraft, with their big high-volume compressors, this can be as high as 15-17,000 ft).

Because the turbocharger also widens the manifold pressure range, the pressure now has a longer way to fall before it reaches the point where the power produced is equal to the power needed (though, depending on type, turbocharged engines may have a higher minimum manifold pressure in order to produce enough exhaust gasses to keep the compressor spinning at max. speed). For example, the TSIO-360 engines on the Piper Seneca III have a maximum manifold pressure of 40 in Hg, roughly 10 more than atmospheric pressure. A more extreme example were the big radials of WW2 which could sustain up to 70 in Hg for short periods! A side effect of this is that the increased amount of air in the cylinders means that the amount of fuel has to be increased as well to keep the fuel-air mixture stable, which can significantly increase available power.

When both of these effects combine, the ceiling can increase to over 30,000 ft – though more often than not aircraft are limited to a lower altitude due to other design factors. For example, the Beech Duke has an absolute ceiling of a tad over 30,000 ft; but it is limited to 25,000 ft operationally because of limits of its pressurization system.

For all the turbocharger’s plus points there’s naturally a raft of minuses – the biggest being the aforementioned increase in the amount of fuel injected, meaning an increase in fuel consumption. This is exacerbated by the fact that while a normally aspirated engine gradually uses less fuel as it climbs (due to the dropping manifold pressure), the turbocharged engine does not and burns the same amount as at sea level. Only after passing the critical altitude – when the manifold pressure starts to drop – does the consumption start reducing with altitude “as it should” :).

Another problem is engine wear and tear. Most of today’s turbocharged piston engines – especially the lower displacement ones – are derived from normally aspirated models that operate at significantly lower pressures and temperatures. And while virtually all of these “mainstream” piston engines are designed and built with the possibility of turbocharging in mind, this is somewhat of a “jack of all trades, master of none” solution – if you make the engine turbo-proof, it’ll most probably be heavy and uncompetitive in the normally aspirated market; if you make it to sweep away the normally aspirated competition, it may be too light and brittle to withstand a lifetime of turbocharging. Another issue is cooling – the rarefied air at altitude is not so good at carrying away heat as the dense air near the surface. On a normally aspirated engine, the engine temps reduce with altitude, so the rarer air can still do its cooling job effectively. On a turbocharged engine, you have the same reduced cooling but – up until the critical altitude – just as much heat produced as at sea level.

Most engines have struck a good balance between the two, though the price to pay is a reduced Time Between Overhaul (TBO) and a slight reduction in service life due to the stress the whole engine block has to absorb (this is the reason why automotive Diesel engines – which work at much higher pressures than petrol engines – are made out of steel instead of aluminium). Some engines even have to have special operating procedures to enable them to meet their TBOs: for example, the Mooney Bravo – powered by the Turbocharged Lycoming Sabre (TLS) sporting a whopping great compressor – has to descend at a high throttle setting and airbrakes extended to avoid shock-cooling the engine!

And that’s all fine and dandy – but what if all you wanted was better altitude performance and to avoid all the hassles and problems stated above? What if you live in Switzerland and your airfield’s pattern altitude is 15,000 ft? 😀 Or, on a more serious note, in Croatia where you want to jump as high as practicable above the Velebit mountain range on a 35 C day, but generally don’t need the extra power?

One practical, working solution is the turbonormalised engine. Identical in design, operation and construction to the standard turbocharged engine, it nevertheless differs in one significant detail: while turbocharged engines increase the manifold pressure above normal atmospheric, turbonormalised ones do not, operating continually in the 28-30 in Hg range until the critical altitude. While this may sound less significant than it is made to be – and even appear to be a step back, given that there is no power increase – it does neutralize some of the standard engine’s operational problems, most notably increased wear and tear. With the engine now continually operating within normally aspirated pressure limits, the TBO penalty can be significantly reduced – but not completely removed, since cooling (as described above) is still an issue, but on a lower scale.

Because the engine is now operating in a lower pressure range, the direct fuel consumption is also reduced and at its greatest pretty much equals what the normally aspirated engine would burn at maximum throttle at sea level. But, as before, this fuel burn is kept to a higher altitude, so it’s not as rosy as it sounds, especially if – as in the situations described above – you just need the extra altitude performance to clear obstacles, but normally spend your time below the engine’s critical altitude at sea level consumption.

Despite this, the turbonormalised engine has met with some success, mostly on small displacement engines of light singles and twins – but, being somewhat of a niche product designed for a specific application, hasn’t become as widespread as the normal turbo (yet). To many people, just the ceiling increase and slightly better performance do not offset the added maintenance costs of the compressor and turbine, leading many manufacturers to simply select the “full package” classic turbo – or go turboprop – giving their customers more for their money…

Okay, this little and short digression has gotten a bit out of hand :D. Consider it as a bonus feature to the original photo report… 😀

NOTE: for more detailed information on turbonormalised engines – my text being somewhat abbreviated for simplicity’s sake – you can go to here