Monday, June 11, 2007

Playing Favorites

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.


Dave Starr said...

Thanks for an insightful post, John. Like many I have been fascinated by this aircraft and never had that much information on it. It doesn't have quite the 'shine" it originally had in my mind when you run the numbers, but if I owned one I sure wouldn't keep it in the hangar.

I am appalled by the "low bat" accident. It's very, very poor engineering to design a system with that narrow a fault margin. Almost all aircraft will at least run if they lose battery power and most electronic control systems I'm aware of have at least a "limp home" mode even if the main processor senses it is unable to operate properly.

What may actually have happened in the Germany incident is more simplistic ... it's common diesel design practice to use a very simple spring-loaded-closed/solenoid opened fuel shut off valve ... even many diesel farm tractors won't start/run without a battery because battery voltage is needed to open the fuel shutoff valve to get fuel to the simple mechanical injector pump.

I'm not sure what would happen to your VW TDI if the battery voltage goes low enough while the engine is running. I'm a confirmed diesel fan also, here in the Philippines I simply love my simplistic but effective 2.5 liter Mitsubishi. The filling station attendant, always wanting to practice his English on me, asked the other day if I had been buying my fuel elsewhere since it had been so long since he had seen me. I just hadn't been driving much the past couple weeks, but I do smile virtually every time I look at the fuel gauge.

I've probably watched avgas disappearing llonger than you have ... when I was a lineboy we had 80/87, 91/96 and 100/115 all available at a very small GA airport and the "rest" of aviation, of course, ran on copious quantities of 115/145. A diesel or other multi-fuel engine is sorely needed in the marketplace.

John said...

Hey David,

Didn't realize you had relocated to the Phillipines!

What I understand of the Twin Star accident in Germany is that when the pilot retracted the landing gear, it caused a low spike in the system voltage. The low voltage brought down both Engine Control Units, the engines quit, and the props feathered. Both alternators are supposed to be able to provide power to the ECUs with no battery power, but they failed to do so. I've read that an Airworthiness Directive is in the works ...

My VW TDI has an ECU, too. Automotive marketing just doesn't use the term FADEC, but the concepts are the same. I did have a Mass Air Flow sensor fail a few months back and the fuel injector pump did go into "limp" mode with the injector pump continuing to provide fuel to each cylinder in some crude way, I just couldn't climb a steep hill at more than 20 MPH. Without any battery juice, the worst case would be I be stranded at the side of the road, as opposed to crashing with my landing gear partially retracted.

Ron said...

The actual prohibition isn't against starting the airplane with external power, or against starting the airplane with a low battery. What the POH says is:

"Taking off for a Night VFR or IFR flight with an empty battery is not permitted. The use of an external power supply for engine starting with an empty airplane battery
is also not permitted if the subsequent flight is intended to be a Night VFR or IFR flight. In this case the airplane battery must first be charged."

The DA42 has all sorts of weird stuff on it. It runs a lot of fuel through the engine and returns most of it to the tank in order to heat the fuel. There's a limitation against starting the engines if the ambient temperature is below 23F. The engine instrumentation requires monitoring a lot more temperatures (gearbox temp, coolant temp, etc). The fuel system is rather complicated, especially with the optional aux tanks installed.

It's not that I dislike the DA42. Ever time the price of gas goes up, it looks better and better. But it's not the perfect airplane, that's for sure. On the other hand, it does have the advantage of being one of only a handful of piston twins in active production...

Ron said...

Actually, let me clarify that last comment. That's what the POH said -- and still says, AFAIK -- but after the dual engine failure in that DA42, Diamond did issue a service letter to clarify that one of the two engines must be started without the use of external power.

The thing is, their insistence on starting one engine without external power is not in the POH, it's only in the service letter. The POH claims that if you're flying Day VFR, you can start the whole plane on external power, indicating that Diamond was convinced that the ECU would not stop supplying fuel to the engine if the batteries were fully depleted, but the G1000 might fail. Perhaps this has something to do with the voltages required to operate those particular piece of equipment.

Either way, it seems clear that the confusion wasn't just with the guys flying that TwinStar, but with the factory and their POH.

Speaking of which, if you've flown the Eclipse and DiamondStar, you're probably familiar with how poorly Diamond's manuals are written. They're awful.

John said...


Thanks for your comments, but be aware that several sources report that an AD is going to issued to cover this issue. The rumors are that a back-up battery may be added for the ECU. I don't think the original manual covered this because the designers didn't think the failure that occurred could occur - the hubris of engineering!

An article that is to appear in a future issue of the German magazine Pilot und Flugzeug claims that the root cause has yet to be determined, but that the electrical load required for gear retraction is currently not isolated from the Engine Control Units.

The author also claims that factors other than a deeply discharged battery may also cause the ECUs, and subsequently the engines, to fail. Diamond is saying it's a problem in the Thielert design, but the Austrian aviation agency apparently doesn't buy that assessment.

It appears the best solution would be for the DA42 to be fitted with a dual bus design, but hey, I'm just an interested pilot ...

John said...

Oh, and I should add that routing heated fuel back into the tank is a common design for diesel engines.

My VW TDI has the same sort of arrangement where excess fuel from the injector pump is routed back through a thermostatic valve, through the fuel filter, and back into the tank.

Garrett said...

Return type fuel systems were common amongst all modern engine types, but have recently become uncommon in gasoline engine use in favor of returnless systems with PWM fuelpump control. I like return type systems; they let you solve some pretty big problems quite simply.

The Thielert ECU reset on gear retraction issue is ridiculous. That this wasn't anticipated is just insane. I've built/bought/tested/screwed with a lot of different engine controllers, and ALL of them behave poorly at very low voltage. That this design made zero allowance for that possibility is bad, but its even worse that the thing autofeathered the props.

That last bit is important: ECU resets are usually merely an annoyance and transient. They take milliseconds, and often are misinterpreted as an ignition miss. Feathering the prop so soon is just outrageously dumb. Without the prop feather, this would have been a non incident.

That said, its an awful idea to take off in an airplane with a glass cockpit and FADEC with dead batteries regardless of what the POH says or how clever the design is. You've just dramatically increased your risk of all sorts of failures, and you've dramatically reduced your options when those failures occur. I won't drive a car with a dead battery because I know firsthand how risky it is.

Dave Starr said...

@ John, yes I moved here full-time last November. My wife's family is here and I wanted to actually retire, versus work like a beaver in a retirement business which is what I was doing in the US. So far I am moderately successful in doing nothing much.

@ Garret ... I absolutely concur. It is bad juju to start any modern vehicle on external power if the reason is a flat battery. I realize there is often alot of pressure to get a flight in, people who don't want to wait, etc., but there is good reason to take the time to have the battery inspected and properly bench charged rather than relying on the alternator(s).

I did a double take at the POH restriction on starting the engines below 23F .. there are a _lot_ of place sin the world it gets colder than that, for sure ... but Philippines ain't one of them ;-)

Anonymous said...

The DA-42 is actually not THAT slow.
On a recent trip we had 163KTAS @ 18,000', ground speed of 175 at a powersetting of 73% with an OAT of -10C, thats standard +11.
On the return we had 170KTAS @ 17,000', powersetting 83%.
Three on board departed with full fuel.
Evidence is here, pic 61.jpg

According to the POH you are only allowed to start ONE engine on ground power, the other one needs to be started normal procedure.

Plastic Pilot said...

About the DA42 flat battery accident, I just would like to say that yes, it is a bad design, but it is also poor airmanship to start a flight in an all electric plane (engines and avionics) with a doubt about the battery status.

Apparently, the battery had been drained by the boarding lamp, close to the door. As this lamp is supposed to used for boarding / unboarding the plane, it is plugged directly on the main bus, so as to be turned on before turning the main switch to on.

The error from my point of view is not to have a timer turning this lamp off.

On some PA32 Saratoga, there are some reading lights behaving the same.