Monday, June 11, 2007
Underlying every aircraft design is a set of tradeoffs. Sometimes the disadvantages outweigh the good attributes and there are aircraft that I like in spite of a long list of disadvantages. And there are others that I should love, but I just find interesting. I'm often asked is "What's your favorite airplane?" A perfectly reasonable question since I have first hand experience with a bunch of different light aircraft. People who like clear-cut answers are often frustrated by my response: It depends ...
What I might find pleasing or objectionable is purely subjective and might not match another pilot's conclusions. One objective way to evaluate an aircraft is to clarify the sort of flying a pilot has in mind - what's the mission? Do they envision flying to costal destinations where they might have to frequently fly instrument approaches? They may want to regularly fly to a high-altitude airport with a short runway that is near a vacation home or they may need to commute between two metropolitan areas for work. Some people value an aircraft that is economical to maintain and fly while others of more robust financial means just want to be able to regularly haul a lot of people and luggage.
While I appreciate there is a tension between keeping flying affordable and performing necessary (often unexpected) maintenance, I have little desire to fly aircraft whose owners are trying to low-ball their operating costs by leaving things broken and making excuses for inoperative equipment. Apparently there is no shortage of pilots and instructors willing to fly these "buckets of bolts" and I wish them many happy landings. I've had my share of unwanted adventure, thank you very much. Aircraft are remarkably reliable, but a consistent history of good maintenance is something I've come to appreciate more and more. It should come as no surprise that I like instructing in newer aircraft because they are less likely to have been abused, they have modern equipment, have better designed components, and they don't look like they've been to hell and back.
Newer isn't always better and I think the new Diamond Twin Star may be a good example. I'll state from the start I've yet to fly a Twin Star and what follows are just my opinions and observations. Since I've not read a performance and cost comparison of the Twin Star with other light twin aircraft and I'm not beholden to any particular manufacturer, I'm in a unique position to offer some observations that others might not want to point out. I do have significant experience in DA20 and DA40 aircraft and I find them to be stable, well-constructed, and good-flying airplanes. I recently had a chance to see a Twin Star up close and I can attest it appears to be equally well-constructed and has many good design features. My concern with the Twin Star can be summed up simply as price-performance.
Many things about the Twin Star appeal to me. As I've mentioned in an earlier post, I'm a fan of diesel engines - I drive a VW TDI and to me, diesel engines are ... well, cool. Diesel fuel (and the aviation analog jet-A) just makes sense because, per unit volume or weight, these fuels pack more potential power: one gallon of gasoline contains 124,000 BTUs and one gallon of diesel fuel contains 139,000 BTW. Besides, sooner or later 100 LL aviation gasoline is going to, pardon the pun, go the way of the dinosaurs. Jet-A is here for the foreseeable future. And no, I don't think running an aircraft on bio-diesel is a good idea.
The Twin Star's turbo-charged Thielert diesel engines don't fit into the conventional model for aviation engines. For one, the engines don't have a TBO - they are time-rated. This has to do with the FAA's certification requirements, so when the engines reach 1000 hours, they are replaced. Diamond has arranged a replacement strategy that keeps the cost around $20,000US per engine. The goal is that the engine's reliability will be established over time and the FAA will agree to an increase in replacement time to 2,400 hours. The engines' fuel economy is simply spectacular with a total fuel burn for both engines of 11 to 12 gallons per hours. Compare that with 17 to 20 total gallons per hour for a Seminole or a Duchess, which have similar takeoff weights.
The Thielert engines are FADEC (Full-Authority Digital Engine Control) and are purported to be very easy to operate - there's just one lever, no mixture control (diesel engines always run lean, but that's a whole 'nother topic), no prop controls, and engine starts should theoretically be much simpler. Remarkably, I overheard a Twin Star pilot recently state that the checklists for the plane were "a mile long." And I thought FADEC was going to make everything simpler!
A Twin Star accident occurred in Germany when retracting the gear after takeoff with a low-charged battery caused a momentary voltage drop resulting in a FADEC failure that made both engines stop. In fairness to Diamond, the pilots in this accident took off after an external power start due to a low battery - something that is specifically forbidden in the flight manual. Still, this accident indicates that there may be some pitfalls to FADEC that might still need to be worked out.
The Twin Star is not particularly fast - somewhere in the range of 145 to 155 knots true airspeed. The takeoff performance is also nothing to write home about - 1350 foot ground run at sea level and standard conditions. Compare that with an 800 foot ground run for a Duchess at gross weight under the same conditions.
I've read that an engine shutdown and prop feather in the Twin Star is remarkably simple and quick, which is fortunate because the single-engine climb performance is a sobering 170 feet per minute at sea level under standard conditions. Compare that with 250 feet per minute for a Duchess at gross weight under the same conditions. The Twin Star can sustain a climb rate on one engine up to 6,000 feet, a single-engine service ceiling comparable to a Duchess and a bit more than a Seminole. A fully-equipped Twin Star has a useful load in the neighborhood of 1100 pounds, compared with 1470 pounds for a Duchess. The advantage for the Twin Star is that less of that weight will be consumed by fuel since the plane doesn't burn as much and has smaller fuel capacity. One huge advantage the Twin Star has is known icing certification thanks to an optional TKS system.
The Twin Star looks unconventional, even futuristic. Between the three-bladed props, the rakishly pointed nose, the winglets, and the angle of the T-tail, the plane has lots of sharp angles. The first thought that came to my mind when I saw the Twin Star in the flesh was "Where is my fly swatter?"
The Twin Star's interior shares many of the same design features as the Diamond singles - a fixed front seat with adjustable rudder pedals, two control sticks, and a canopy that provides a spectacular view. Note that the canopy can also create a cabin that is hotter than hades, which led to the Diamond Katana being nick-named the KaSauna. The Twin Star's canopy can thankfully be left open during ground operations, which one hopes will keep the temperature bearable on warm days. The control sticks are fine with me, but they aren't always user-friendly. In the DA40, I often tried to move my left knee out of the way so the pilot with whom I was flying could reach the flap switch only to inadvertently move the control stick on my side to the right, causing the plane to bank to the right. And riding along with a pilot who is making a challenging landing in changing, gusting winds can result in the front seat passenger getting a thigh massage that is probably illegal in many states.
The main block to the Twin Star, to my mind, is the cost: Renting a Twin Star in my area is in excess of $300US per hour after completing some pretty pricey, mandatory ground school. Granted the $300 is a wet rate, but pah-leeez! The Twin Star may be very frugal with fuel, but it ain't no bargain. Maybe the cost will become more affordable with time. For now, for pilots wishing to earn a multiengine rating, a well-maintained, conventional twin like a Seminole or Duchess is far more cost effective.
Would I like to fly a Twin Star? You betcha! For now, I'm buying an occasional lottery ticket in hopes that my ship will come in.