Introduction

Folks often ask me "How low can you start the motor?".
Let's think about managing margins and the nature of motor-gliders; please bear with me...

Margins...

Flying safely is all about managing margins and keeping a safe landing option, which is all about judgement. Hard rules are always suspect; we don't always use the same pattern airspeed, land in the same direction, nor use ground references for our landing patterns, right? When we think about how low to thermal, lots of different factors enter into our judgement about what is appropriate and safe, for example:

We evaluate these and more about our current situation, and judge how big our safety margins must be, and ultimately how low is safe before giving up and landing. To be safe, we plan this well before we get low.

Motor-Glider Reliability

Poor reliability is a major consideration in maintaining adequate safety margins during an air-start. After one of my many, many, submissions to EASA about motor-glider defects, EASA reminded me:

...please let us point out that the pilot of a powered glider shall always have in mind, that it might be necessary to operate his aircraft as a pure glider. The engine of a powered glider, predefined by airworthiness requirements JAR/CS-22, does not meet the same safety standards as a "Part-E" engine of a motorplane.

The flight training for powered gliders shall take into account, that loss of engine power may occur anytime, and result in a scenario, which is comparable to a cable break during a winch launch or an aerotow. This deviation to the operation of a motorplane is reflected in several paragraphs of the airworthiness requirements JAR/CS-22, applicable for a powered glider. Examples are the specifications for engines, used for powered gliders (JAR/CS-22 Subpart H) that are less stringent than those for powered aircraft (CS-23). Moreover, requirements for software are not mentioned in the JAR/CS-22 at all - contrary to the specification for large aeroplanes (CS-25).

If a motor-glider pilot crashes because his engine fails during take-off and there was no good landing option, it is officially pilot error. If the pilot crashes after a failed in-air restart, it is officially pilot error. The motor is considered an optional accessory! Aside from cases where a motor failure damages the airframe, the certification authorities take no notice of quite frequent motor-glider engine failures. These are not Toyotas! The EASA statement above is lunacy; the failure of most motors presents a much-worse situation than a cable-break, as the glider sink rate and handling is typically much worse with engine out and stopped.

Electric power is not immune from reliability problems either; the Lange Antares 20E I recently disposed of averaged 1 propulsion system failure for every 10.4 propulsion system operations (over 14 years). If you're interested in why motor-gliders are inherently unreliable and some failure statistics, watch my 2020 OSTIV-SSA presentation (YouTube R--m0NDR0j8).

We are required by the certification criteria to plan for the possibility of the motor failing at any time, and fly such that no matter how and when and where it fails we can make a safe landing, because fail it will. This planning can be tricky during an air-start...

Always remember: Plan A is Landing. If the thing starts, wonderful! But as in all flying, plan for the worst, and hope for the best. The worst (for pylon-power gliders) is invariably the motor stopped and stuck out in the breeze (retract failure) - been there more than once. Think "when", not "if" you'll have a motor and/or pylon retraction failure.

OK, how do we do this safely? How low can we go? It depends on the glider, and on the situation...

Motor-Gliders Are Not All The Same

Different types have very different characteristics, especially...

Sink-rate with the engine out and not running.

In the pattern, while the engine is extended and before it starts, you're going to want to fly at the yellow triangle or faster (not the slower blue-line). At yellow triangle, some gliders have sink rates we lovingly refer to as plummet mode (really high drag from radiator, stopped prop, etc.). FES gliders don't have this problem, and jet-sustainer or some electric gliders like Antares also have a really low sink rate with the engine out and stopped. In several newer electric glider models, a controller failure allows the prop to windmill with a sink rate >500fpm (controller over-heating and prop windmilling contributed to the glider landing in a lake in Uvalde). Some older motor-gliders also have sink rates well over 500fpm, which means you need a lot more altitude for an air-start. My new Ventus 3M has a yellow-triangle engine-extended sink rate around 450fpm (glide ratio 13:1), but the manual advises flying it faster than yellow triangle with motor stuck out - plan on 500fpm. Additionally, pylon gas machines have higher drag when the prop isn't vertical (a vertical prop hides radiator and pylon); this may not be reflected in your book values.

Handling with the engine out and not running.

The proprietor of one motor-glider vendor told me I really did not want to stall the glider with motor out and stopped. This particular glider needs strong nose-down stick-push (more than can be trimmed); difficult while fiddling trying to start the engine. Turns to base and final look abnormal as the nose needs to be well below the horizon. The proprietor advised me no turns with engine out and not running, so set up on a long very high final before trying to start the motor (he lost a couple of friends and customers to spin-ins which appeared to be pattern turns to base or final with engine out).

Difficulty of engine start.

 Electric gliders generally require little fussing in the cockpit. Newer infernal combustion types and jets have few controls to fiddle and highly automated start procedures, also not much fussing and heads-down time - except for emergency override procedures. Some older gliders have a dozen controls scattered about the cockpit and a fiddly start sequence, so demand lots of heads-down time.

Dive-start engines.

 Sustainers that need a dive to start can be much too exciting if you try starting low. One common model has an engine red-line close to the rather high required dive speed; if you don't pull up promptly when it starts the engine overspeeds then the motor controller turns off the engine (to protect the engine ;-), but if you pull too soon the engine also stops. After the engine doesn't start, you'll have burned 300-500 feet, and you still need to be high enough for a safe landing with the engine out.

Time to deploy, start, and reach full power.

Different powerplants take very different times til full power. Your pattern plan must take into account the time, distance, and altitude loss for the complete sequence, whilst always maintaining a safe landing option.

Air-Start Example Situations

First a few real-life situations with factors that influence decision height.

Lined up on a big 5000+ foot long airport with no traffic, on a calm day, in a modern glider.

 Suppose you can just straighten out, start the engine with a few switches, and land directly when it doesn't start. Line up on the runway and begin the deployment/start sequence at 500 feet - no problem. When it fails, just land straight ahead.

Next to a big airport with no traffic, on a calm day, in an older more difficult glider.

 Suppose you can just straighten out to land, but starting the engine is fiddly, the glider handles badly during deployment, and the sink rate is truly plummet mode. You'll need more like 1500 feet and a planned high final to safely try a start.

Next to a short field bounded by trees and no other nearby landing options.

 Suppose you start the motor and start to climb away, then the motor quits. Now you no longer have a safe landing option. In one accident scenario, the motor starts on final, runs for a short while, then quits. The glider overruns the field and crashes (example: NTSB: ERA10LA325 pilot statement). For a difficult short field you need plenty of altitude so that when the motor fails and you can't retract the engine, you can still fly a safe and precise landing pattern with the higher sink rate. Maybe 2000 feet or more for a more difficult glider. Even gliders with a low engine-deployed sink rate can get into plenty of trouble with a failure near a short field.

In addition to adding the altitude needed to get in position, you want to do your landing checks before messing about with the motor. Remember to add in the landing check time (and distance) in addition to the motor start time (and distance and altitude loss).
Always remember: Plan A is Landing.

What if it starts?

Great, it started! Can't just motor off over the trees, you're still low! Plan an orbit of the field always remaining in a position to safely land when the thing quits, until good and high. Don't motor off over unlandable terrain with a tailwind and a low climb rate (for example FES gliders and especially pylon sustainers), as you'll promptly be out of reach of the field when it quits. Even with a great climb rate, you have to always keep a field in reach for when it quits and you have a retraction failure...

OK, How low can you start the motor?

The motor adds additional factors into our judgement about what is appropriate and safe, for example:

This stuff always takes more planning, altitude, time, and distance than a pure glider! One can never go as low as a pure glider, unless willing to forgo messing with the motor and just land out (the best option in many circumstances). Before messing about with the motor, I need to be ready with:

Making it up on the fly is likely to end badly. It is really easy to get overloaded when things don't go well, like when the motor controller starts flashing error messages and beeping and the thing won't start. So plan it first, then follow the plan. And each circumstance needs a slightly different plan. Don't even think about touching the motor controls before you've got it planned.

Really, How low can you start the motor?

It depends! As above, in some situations 500 feet is no problem, and in others 2,000 feet is not enough. As always, flying safely requires judgement, not blind rules. So stop asking me stupid questions already.

Addendum

Thanks to the many folks who helped me write this and contributed examples; some relevant references and examples follow below.

References

Nordic Gliding: The turbo never starts!

Dave's 2020 OSTIV-SSA presentation: Motor-glider Unreliability: Examples, Systemic Problems, Ideas (YouTube R--m0NDR0j8).

Video showing planning for failure at various stages of launch, then an actual failure.

Examples

No Data From ECU

The first 5 examples were during the first 3 USA contests of 2022, with a combined total of 50 motorgliders and 17 days of flying including practice.

  1. As the pilot got low, he prudently modified his flight path to get close to a large airport. Trying to start his FES, he accidentally pressed on the power knob (throttle), and the controller went into "menu mode". Turning the power knob changed menu choices but did not increase prop RPM. He couldn't figure out how to get the controller out of "menu mode" and landed at the airport. Note: this user-interface feature is also present on jet sustainers.
  2. Pilot is flying knowing his sustainer is inoperative. Before the contest, the compression release blew out of engine while running (thread stripped in head), leaving the engine running but producing minimal power. He's had this happen twice, both times in the air near an airport.
  3. Pilot did practice run of sustainer at the contest airport just after launch. After engine run, pylon would not retract and pilot landed with engine extended.
  4. As pilot got low, he prudently modified his flight path to get close to a large airport. Jet sustainer would not start and he landed at the airport and aero-towed home.
  5. Jet sustainer controller sometimes displays "No data from ECU" on the panel engine control, leaving jet inoperative. Pilot is flying assuming jet will not start.

So 5 of 50 motorgliders had an issue during the 3 contests (plus at least one additional failure between the contests I did not list above). Two were known issues before flying, but 3 were in-flight surprises. Remember these odds before depending on your motor...

A few more examples, illustrating common problem scenarios:

  1. Dive-start sustainer pilot waited until too low to attempt a start, then didn’t go fast enough for a successful air start because of the proximity of terra firma. An immediate landing was the only option.
  2. Pilot started motor on downwind to an airport, and continued flying on downwind heading. Motor exploded (failed connecting rod puntured side of case), but pilot was a bit far from the airport as he had not planned for a failure after a start. Landed short of airport threshold but no damage (other than engine and underwear).
  3. Pilot bought a used motor-glider and got a self-launch endorsement. On his first self-launch, he climbed to 3000 ft, throttled back, cooled the engine, and shut off the ignition. When trying to stow the engine, the prop would not stop at vertical. When near vertical it would pop past center (the automatic brake tried but wouldn’t stop it). Pilot finally got it stopped and held with the manual brake and put the engine away. During this time the glider had descended to 1400 ft (1.5 mi from the airport). The pilot attempted a re-start, but the engine ran for a few seconds and quit. 2nd try got same result. Now down to 1100 ft AGL, the pilot was too far from the airport, and made a last-minute decision to land out in a field with engine out, fortunately safely.