Engine Air-Start Survey

Here is a video of a Remos GX performing a in air restart after engine stopped.

The Remos GX aircraft has a 912 ULS and a 3 bade Neuform propellor installed.

https://www.youtube.com/watch?v=VwPAiuu1dxw

I’ve successfully restarted a 912ULS hundreds of times in a Diamond Xtreme motorglider while setting altitude records in the flight levels and demonstrating the procedure as a Diamond check pilot for new Diamond Xtreme motorglider owners. Successfully demonstrated with both electric and hydraulic constant speed 2-blade props. Must put prop in takeoff pitch, dive to 120 kts and will loose between 1500-2500’. While soaring wave above 20,000’ for an hour or longer, the battery and engine would be too cold soaked for a restart and would glide to airport, land, and let things warm up 30-60 min to restart the engine. This a little more dicey with an airplane with a lower L/D and retractable gear. I’ll be doing air start flight tests in the new high performance Rotax powered Pelegrin Tarragon.  It is a turbocharged EFI engine but the Rotax engine has two stators which should provide sufficient electrical power to the ignitions for a restart in addition to the ECU backup battery. As long as the engine remains warm, should be able to airstart a Rotax in most applications. Flight testing the Tarragon 1st quarter of 2022. - ATP, Rotax Tech, LSRI, “Gold Seal” CFI, USAF (Ret)

An interesting question!

My ship is a Super Ximango AMT-200S motorglider with a tractor-mount 100 hp 912S4 (ULS) engine and Hoffman 2-blade, mechanically selectable 3-position prop. The official factory manual specifies an airstart speed greater than 110 knots. My experience doing this is far less extensive than that of Mr. Elves, only a half-dozen times, but I find the following so far.

When airstarting, I'm not quite following the procedure in the manual. Beginning at 60 kt with engine off and feathered, I first un-feather, which puts the prop into climb (fine) pitch. Throttle closed, fuel on, electric fuel pump on, mags on. Stone cold, I hold the choke open. If warm (oil temperature gauge reading something above the bottom point), choke remains closed. Then dive *quite abruptly*. So far, the prop starts spinning around 85-90 kt and the engine will start well under 100 kt. This will clearly depend a lot on how cold the engine gets. The swoop down probably takes only a few hundred feet with some of that recovered by converting airspeed to altitude. What really takes time/altitude is warming up the engine afterwards. Cold starting on the ground, it takes me on average about 6-7 minutes to get 50C oil temperature. Airborne, one has a 60 kt wind blowing on the nose and lower OAT which really slows things down. Simply waiting with closed throttle may have one running out of fuel before reaching 50C. Instead, I use the oil pressure gauge, nudging the throttle up until just touching the bypass opening pressure ~80 psi, waiting until it comes down to ~70 psi, the opening a little again. All the while, I'm drifting downward until about 3,500 rpm at which it maintains altitude. After about a 1,500-2,000 ft altitude loss, and as much as 15 minutes I feel able to climb again at a reasonable rate. If in gliding range of an airport, I'll wait to as low as 2,500' agl before restarting, if not I get nervous below 3,500. I need more practice to be more specific.

From someone who owns both a Ximango and a Dimona, he observes also that the air-start speed is considerably lower on the Ximango. We think this is because the MT-hydraulic prop has a much thinner blade chord than the fat Hoffman.

Closing an already too-long answer, I'll add that even with modest airspeed and a nearly-dead battery, the starter will spin the engine much more easily and faster than it normally ever does on the ground.

Here's a Ximango air-start with altitude and speed recorded.

https://www.youtube.com/watch?v=6CAX14ZjSWk

Appreciate your tip on using the starter with an air start even with a depleted battery. Anything helps to get the rpms up to energize the Rotax stator/ignitions.