After the study of the DA42 V-speeds, it’s time to have a look at the TwinStar’s performance figures. The chapter 5 of the Aircraft Flight Manual contains all the performance graphs. Their presentation is quite standard and using them is really easy. They always start with the temperature scale, go up to an altitude line, then descend along a guideline to integrate the aircraft weight and finally descend or climb along guidelines for wind. If you know the DA40 ones, these are exactly the same. The principle is the same as what Piper uses for the Archer AFM, even is the presentation is slightly different.

One of the major modifications approved for the DA42 is related to an increase of Maximum Take-Off Weight (MTOW). If this modification, known as 42-088 is installed, the MTOW is increased from 1′700 t0 1′785 kgs. This might look small, but it makes possible to take one more adult passenger. More about this when I’ll post about weight and balance. Because this modification is optional, two families of performance graphs have been publised: one for weights between 1′300 and 1700 kgs and another one for weights from 1′700 to 1′785 kgs.

The chapter 5 starts with the usual warnings about the figures being valid only for a well maintained and properly operated aircraft in ISA conditions. Two other warnings caught my attention:

1. For a safe take-off the available runway length must be at least equal to the take-off distance over a 50 feet obstacle
2. No take-off should be attempted on grass longer than 25 centimeters

The first one is a bit restrictive but given how short the take-off distances are, it makes sense. It also helps to ensure that a re-landing is possible in case of engine failure immediately after rotation. The second one sounds more a lawyer thing, like the “contents may be hot” found on take-away coffee cups. Which pilot would attempt a take-off in grass higher than 25 centimeters ? Starting an engine with the propeller partly in grass ? “Take-off from the middle of a forest is not recommended”. Duh.

Take-off performance

My personal way to get an idea of a new aircraft’s performance is to compare to what I know the best: usual conditions in Geneva. This means 20°C at an altitude of 1′500 feet, no wind. Well, the no-wind is not exactly typical but it’s a better hypothesis than assuming a few knots of headwind. I also assume take-off at maximum take-off weight to remain conservative.

In these conditions the take-off distance (50ft obstacle) is 580 meters at MTOW 1′700 kgs. Here again the DA42 figure are quite close to those of almost all single engine aircraft. This figure makes possible to operate it from almost any runway in Europe, to the exception of some very short grass runways. The ground-roll distance in the same conditions is 380 meters. With the modification for increased maximum take-off weight to 1′785 kgs, the MTOW the take-off performance are degraded: 780 meters over the 50 feet obstacle and 480 meters ground roll. This is still sufficient for safe operation out of relatively short runways and respect the warning about having as much runway as the distance over the 50 feet obstacle.

Climb performance

The AFM contains four graphs related to climb performance: take-off and cruise climb with or without the maximum MTOW modification. The difference between take-off and cruise climb is the climb speed: 77 KIAS for take-off climb and 85 for cruise climb. The differences between take-off and cruise climb are less than 100 feet per minute, and the climb rates of the version with the higher MTOW are about 100 feet per minute lower than the standard version.

The magic figure about climb performance at MTOW seems to be 1′000 feet per minute. I took examples at various altitudes and temperatures, all at MTOW:

• MSL, 20°C : 1100 feet per minute
• 5′000 ft, 10°C: 1050 feet per minute
• 10′000 ft, 0°C: 920 feet per minute

Assuming 1′000 feet per minute and a speed of 85 KTAS, the climb gradient is 11%. The SIDs in Geneva are quite demanding, because of the surrounding high-ground, but they require only 7% up to 7′000 feet. The DA42 can easily beat that… at least when both engines are running. Flying these departures with a Saratoga or Bonanza was just fine, but a DA40 with three on board could not meet the requirement…

Single engine climb performance

Did you already hear the saying that “Light twins makes possible to control the glide after engine failure” ? The DA42 is no exception. At MTOW, the climb performance with one engine inoperative is reduced to the following values:

• MSL, 20°C : 200 feet per minute
• 5′000 ft, 10°C: 150 feet per minute
• 10′000 ft, 0°C: 75 feet per minute

Another way to approach the single engine climb performance is to note the temperature resulting in a zero rate of climb at different altitudes:

• 6′000 feet – 42°C – Very unlikely
• 8′000 feet – 30°C – Unlikely
• 10′000 feet – 20°C – Well… possible on warm summer day
• 12′000 feet – 0°C – Possible
• 14′000 feet – -30°C – In any case

Loosing one engine at 13′500 feet over the Aletsch glacier with a temperature of 5°C would result in a rate of descent of -100 feet per minute, down to approximately 11′000 feet, where a zero rate of climb will be achievable. This is not an enviable situation, but it makes an escape much more easy and comfortable than with a single engine aircraft. It is probably not possible anymore to decide on which side of the Alps to land, but flying to an airport in the valley or even on the plateau area is possible. With a single engine, gliding in down to the valley is possible, but then an emergency landing is the only option.

This scenario is probably a bit dramatic, but not unrealistic. Light twins make a controllable descent to a lower cruise altitude and then flying home possible. As long as home is not behind a mountain. This is much more than what a single engine aircraft can offer.

Cruise performance

I already mentioned the relatively low cruising speed of the DA42 in the post about V-speeds. Cruising speed at 10′000 feet under standard condition ranges from 127 KTAS to 164 KTAS. The recommended 75% power setting results in a speed of 153 KTAS. Compared to a Baron, or other light twins, this is quite slow. Even high-performance single engine can reach such speeds. One must however note that the DA42 fuel flow is much lower, thanks to the TDI engines, and the total price of fuel is much lower because it sips Jet-A1. Another advantage for European pilots is that as the MTOW is below 2′000 kgs, we don’t have to pay for IFR en-route fees.

Landing performance

In the conditions I described for the take-off performance (MTOW, 20°C at 1′500 feet, no wind), the landing distance from the 50 feet barrier is 640 meters, and the ground roll is 340 meters. With the extended MTOW of 1785 kgs, the landing distance increases to 780 meters and the ground roll to 440 meters. This again sounds similar to what I know from many single engine aircraft. The DA42 requires more runway than a Piper SuperCub, but not much more than a Saratoga or Bonanza.

Safer than a single engine…

Safer but not faster than a high-performance single engine. The more I learn about the DA42 TwinStar the more I think that this is the correct descrption of this aircraft. The extra engine makes management of an engine failure easier, but it does not make the performance much better. The point about the fuel efficiency is also important, but if you expect a speed above 180 knots, you should better consider a Baron, or may be a Mooney…