I flew over the Baltic at Olando Kepurė at dusk. Here is the GPS path.
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Blue: GPS lock. Red: telemetry dark, position dead-reckoned. Here is how it got like that.
The Decision
A few weeks earlier I had flown a 2 km round trip in heavy field winds and
landed with 20% remaining. The link held, the battery held, the drone came back.
Same drone, same pack, six minutes of cruise in moderate winds, voltage never
below 3.56 V. I wanted to know what clear-horizon ELRS range looked like over
open water. No turbulence, no obstructions, just the Baltic at Olando Kepurė at dusk. It
seemed like the easier version of what I had already done.
It was not.
The Flight
The outbound leg was easy. 2.47 km, 66 km/h, 11 V pack. The horizon was flat,
the link was clean, the battery was barely moving. There was a tailwind I did
not specifically register as a tailwind. I registered it as good conditions.
I turned back. Climbed a bit for a better view. The speed dropped. The battery
started moving faster. The return leg, the same airspeed into the same wind,
cost 183 mAh/km: 1.45 times more expensive per kilometre than the outbound.
The tailwind is a headwind now and every metre home costs half again what
getting out did.
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Consumption rate in mAh/km, computed over a 150 m rolling window. Outbound
(left half): ~126 mAh/km with a tailwind. Return (right half, after turnaround
at ~2.5 km odometer): ~183 mAh/km into the same wind. The gap between ~1 and
~2.5 km is the main 150 s blackout, no measurement, no points.
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Current draw across the flight. Higher and more scattered on the return leg:
the drone is working harder into the headwind.
The Blackout
ELRS telemetry runs at 17 mW. The Pavo20 has a T-style antenna at the nose.
Clear line of sight over open water the whole way out. The chart tells a
different story.
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1RSS and link quality vs distance from home. RSSI was at −84 dBm at 60 m
outbound, already 48 dB below the launch value. RQly held 100% throughout until
total blackout. There was no RF alarm wired. If there had been, it would have
fired at 235 m outbound.
Two-ray multipath over seawater at grazing incidence: the sea is a near-perfect
RF reflector, and at low altitude the direct and reflected paths arrive nearly
equal in amplitude and opposite in phase. Climbing breaks the cancellation.
0 m: silent. 75 m: restored.
Telemetry went dark at t = 79 s, altitude 0 m, roughly 920 m outbound. It did
not come back for 150 seconds. During that gap the pack crossed 3.8 V per cell,
crossed 3.6 V, crossed 3.5 V. The radio was silent for all of it. EdgeTX logical
switches sourced from telemetry sensors are forced FALSE during a blackout. A
level-comparison switch cannot fire while the link is down.
Why the Battery Drained So Fast
The visible telemetry tells a boring story: 7.2 A outbound, 6.8 A return,
nearly identical. The wind slowed the return to 40 km/h versus 55 km/h
outbound, which explains the 1.45× mAh/km difference through time alone.
Nothing surprising there.
The blackout tells a different story.
The FC's Capa counter keeps integrating current even when telemetry is down.
At the last good reading before the main blackout: 156 mAh consumed.
At the first good reading after: 555 mAh. That is 399 mAh in 139 seconds.
Working backwards: 10.3 A average during those 139 seconds.
The OSD screenshot at 2.47 km, inside the blackout window, shows 10.09 A.
That frame confirms it. The blackout covered exactly the phase of the flight
where current was highest:
the turnaround at 2.47 km (throttle spike against the headwind)
the climb from 0 m to 75 m
the first 70 s of return at near-maximum throttle
Those 139 seconds at 10.3 A consumed 399 mAh: 59% of the entire pack.
The radio was blind for all of it.
The yard comparison now makes sense. At 6 A average (a mix of cruise, hover,
and slower manoeuvres), a 680 mAh pack lasts 6.8 minutes, which matches the
5–6 minute figure. The ocean flight at 7.2 A cruise would have lasted 5.7
minutes if that was all it drew. But 139 seconds at 10.3 A pulled 122 mAh
more than cruise rates would have, burning a full extra minute of flight
budget in the one phase nobody could see.
The pack was effectively empty before telemetry could say anything about it.
The pack was a LAVA II 680 mAh 3S LiHV. The flight controller had it configured
for 821 mAh, which overstates capacity by 20% and silently inflates every SoC
reading. Battery SoC read 85% remaining when the link first went dark.
Outbound: 126 mAh/km. Return: 183 mAh/km. Total telemetry dark: 171 s out of
350 s. 49% of the flight, invisible to the radio.
When telemetry restored, the FC showed 34% SoC remaining, around 231 mAh on
the actual 680 mAh pack. The point of no return, at the measured return rate
with a 10% margin, was 1147 m from home. The drone was 1946 m from home.
Already 799 m past it.
The rth callout fired at t = 229.8 s. Correct, in the sense that it fired.
Useless, in the sense that it fired 150 seconds after the decision had already
been made by physics.
The return leg ends where it ends.
What I Am Changing
Each row below is a failure mode the telemetry identified. The implementation
comes in a separate post.
Condition
Old behaviour
New behaviour
Battery voltage crosses 4.2 / 4.0 / 3.8 V/cell on descent
Single voltage-threshold logical switch, fires once on the first crossing, gated on the battery button
Spoken number ("one", "two", "three") on each 0.1 V crossing below 3.8 V; no tone (beep volume is 0 on this radio)
Battery < 3.6 V/cell
lowbat track via logical switch
Same, but also from the background script, first line of defence even through a blackout
Return leg costs more than 1.3× the outbound per km
No callout existed
Wind asymmetry warning after ≥30 s outbound + ≥15 s return: "warning close {ratio}%"
Specific power elevated + specific speed normal (internal fault: motor, prop, bearing)
No callout existed
"warning power {ratio}%" once, after a cruise baseline is established
Specific power elevated + specific speed depressed (external: headwind, drag)
No callout existed
"warning speed {ratio}%" once
Link RSSI below −92 dBm
rssiSource: none, no RF alarm wired at all. RQly held 100% until total loss
siglow + dBm value; sigcrt at −100 dBm. RSSI is the ramp; LQ is the cliff
Link RSSI degrading + altitude < 25 m
Nothing
"tolow", meaning climb. Measured: link died at 0 m, restored at 75 m, two-ray multipath over water
Telemetry dark for > 4 s
Logical switches frozen FALSE; all battery callouts silenced
Background Lua script continues dead-reckoning distance and SoC, re-announces state on restore
Point of no return approaching (outbound, GPS available)
Nothing
"close {metres remaining}" counting down while PNR − dHome < 400 m
Arming with GPS rescue not ready (FC says "?")
Satellite count against a hardcoded threshold in the script
FC's own verdict: Betaflight appends ? to the CRSF flight-mode string when numSat < gps_rescue_min_sats. Script reads that directly.
Impedance rises beyond what depth-of-discharge explains (excess ≥ 1.4×)
No callout
"warning bad {mΩ}" once, while SoC > 25%
The Pavo20 learned to dive. The telemetry did not. One of those is more useful
for not repeating the experiment.
Still need a cinewooping daily driver. Pavo20 4S or DeepSpace Stellar25. Can't decide which one gets the next swimming lesson.
