A tank you can read to three digits.
An electric powerplant knows its energy state because the battery management system integrates current — coulomb counting — and cross-checks against cell voltages. No float, no slosh, no guessing what the mixture did to consumption. The number is honest in both directions, though: it will also tell you, precisely, that the pack has faded with age and cold. A typical FES or sustainer pack stores 4–6 kWh — the energy of about half a litre of petrol. Electric flight works anyway, because the motor wastes almost nothing and the airframe it pushes is a glider.
Climb costs mgh, and nothing forgives it.
Lifting mass costs m·g·h — physics does not negotiate. What the machine decides is the exchange rate: from battery terminals through controller, motor and propeller to flightpath, roughly 60 per cent of the stored energy becomes height; the rest leaves as heat. For a 350 kg glider that prices a 100-metre climb near 0.16 kWh, so a 4 kWh pack holds around two kilometres of total climb — not per flight, in total, until the next charge. That is the whole self-launch and save-the-day budget, written as one number on one screen.
The pack, spent two ways.
Both strategies end at the same marker, and that is the point: climb-and-glide and level sustain spend the identical energy per kilometre, because both are paying the same drag bill — E·η·(L/D)/mg either way. Improve the glide ratio and both profiles stretch together; add mass and both shrink. The 30% reserve is already held out of everything shown. The climb segments are drawn vertically for clarity; a real climb drifts forward while it spends.
Sawtooth or sustain: the physics is indifferent.
Since the arithmetic ends in a tie, the choice is practical, not energetic. Sawtooth flying banks height you can spend on decisions, lets the motor run at its efficient full-power point, and — on a FES — folds the blades for a clean glide between climbs. Sustaining avoids the restart risk entirely and keeps you out of the failure mode that matters: the climb that does not start. Every electric pilot plans like a pure glider pilot within reach of a field, because a motor that spins up nineteen times in a row says nothing about the twentieth.
The reserve is a health policy, twice over.
Landing with 30 per cent is not timidity. It is, first, one honest go-around or one relocated circuit held in hand. It is, second, chemistry: lithium packs age with depth of discharge, so the reserve you refuse to spend today is capacity you still own in five seasons. And it absorbs the honest bad news the gauge delivers — a pack at 0 °C gives up a fifth of its energy before you ask, sag under full climb power briefly dips the voltage further, and calendar fade takes a few per cent a year regardless. The planner above quietly assumes a warm, healthy pack; your winter self should not.
In your cockpit.
Fly the pack the way engine pilots fly EGT: as trends, not moments. Watch state of charge against the plan, cell spread for the first sign of a weak group, and pack temperature before a climb you are counting on. The discipline transfers directly from the fuel article — the tank got honest, the arithmetic stayed the same: fuel is measured in minutes. And the pack that drives the motor obeys the same sag-and-capacity rules as the little one behind the panel: the power systems article.