To date, HFE International has operated the DA 70 and DA 150 at altitudes of 20,000 ft. In both cases, the engines performed as designed. Degradation in power was relative to air density as estimated. Both the Mil Spec and commercial EFI systems were flown at 20,000 feet showing that the EFI systems are compensating for fuel with barometric pressure change. In the case of the DA 150, the engine was shut off at 20,000 feet (on purpose) as a test of the starter system. The engine was then re-started to determine if the alternator had enough torque and capacity at 20,000 feet. The engine re-started without issue. This test was completed multiple times with no issue.
Altitude testing can be a challenge for aircraft engine manufacturers. Tests are limited to an altitude chamber, finding a mountain, or actually flying at the specified altitude. Some companies regulate manifold intake pressure to simulate the altitude pressure change. The altitude chamber would appear to be the most likely candidate. However, most altitude chambers are sealed vessels. It is difficult to run an engine in a sealed environment for any length of time. The experiment does not allow for engine exhaust and air exchange. Only large wind tunnel like facilities are able to achieve this task. The option of flying is not an easy task either. Flying at 20,000 feet must be accomplished in controlled military airspace or in a cleared commercial airspace with a COA. As a result, HFE International had to leverage their international partners to fly and operate the testing in a foreign country where the air space is easier to work in.
In conclusion, both engines performed as designed at 20,000 feet, validating the flight envelope specified by HFE International.