Knowledge, solutions & tips around drones – from the community.
Flight Controller broken – Symptoms & Initial Diagnosis A Flight Controller (FC) can fail in various ways. Here you will find the most common symptoms and how to proceed systematically. Typical Symptoms FC is not recognized (no USB device, no COM port) Status LED stays off or remains solid Betaflight / INAV does not connect When connecting battery > immediate smoke / hot chip Gyro/Baro deliver implausible values (e.g., CPU temp > 80°C while idle) Quad does not arm or disarms immediately Initial Diagnosis Steps 1. Check without battery: Connect via USB only to see if the FC is recognized. 2. Visual inspection: Burnt smell? Blown capacitors? Cold solder joints on the STM32? 3. Measure voltages: Check the 5V and 3.3V rail on the board. 4. Read boot log: Run `status` and `dump` in the Betaflight CLI. 5. STM32 reset: Disconnect FC from USB, briefly bridge the boot pin, reconnect. When is the FC really broken? No response from USB anymore Short circuit detected (current limitation on the USB port triggers) CPU temperature rises sharply while idle Permanent failure after replacing the BEC/ESC See also: How to properly test an FC.
How to properly test a Flight Controller Before installing an FC or reusing it after a crash, you should test it in isolation. Before installation 1. USB Test: Connect the FC via USB only. Is it detected? LED on? 2. Open Betaflight Configurator: Are there any error messages when connecting? 3. GyroCheck: Observe the gyro curve in the Sensors tab – all axes must react. 4. BaroCheck (if available): Pressure sensor shows a plausible altitude value (e.g., ~500m above sea level). 5. CLI `status`: Check CPU load, temperature, and voltages. 6. Test motors: Spin them up individually in the Motors tab (without props!). After a crash Check GyroOffset: Place the board flat, check the Motors tab – are the values jittering at zero? Calibration: Recalibrate the gyro. Does the drift persist? > Gyro might be damaged. Measure capacitors: If the FC gets hot after a crash when plugging in the battery. UARTDiagnosis: Test VTX / RX / GPS via CLI. Important CLI commands ``` status dump get gyro_hardware_lpf get cpu_overtemp ``` Conclusion A clean, isolated test (USB only, no props) prevents most consequential damage.
FC not booting – USB not recognized If your Flight Controller no longer shows up as a COM port when plugged in, go through this list: 1. Cable & Port Different USB cable (not just a charging cable, a real data cable!) Different USB port (preferably directly on the motherboard, no hub) Different computer to rule out issues 2. Drivers Install STM32 Virtual COM Port driver (Windows) Look for a yellow exclamation mark in Device Manager Use Zadig to reset drivers 3. Bootloader Mode Bridge STM32 BOOT0 to Ground, then plug in USB In Betaflight, select Firmware Flasher > Load Firmware [Local] DFUMode should be detected 4. Hardware Defect Measure voltage at the USB port (5V) If the FC gets extremely hot > CPU defective / short circuit If nothing else helps > FC is likely permanently broken > Tip: Sometimes `flash_full_chip_erase` in DFUMode helps.
Gyro / Baro defective – how to identify? Gyro symptoms Values jitter while standing still One axis does not respond Extreme offsets (> 10° / sec at idle) Gyro not responding in CLI / Log Testing the Gyro 1. Place the FC flat, open the Betaflight Sensors tab 2. Observe for 30s – all axes must stay at ~0 (±0.5) 3. Tilt the board slowly – values must follow plausibly 4. CLI: `status` > check Gyro CPU temperature Baro symptoms Altitude jumps when plugging in the battery GPS mode unstable despite enough satellites Baro: not detected in log Testing the Baro 1. Sensors tab > observe Baro graph 2. Warm the Baro with your hand – the value must rise 3. Compare outdoors with a smartphone altimeter What to do? Gyro: Replace the FC, rarely repairable. Possibly reflow the GyroIC, but the success rate is low. Baro: Check if the solder joint on the BaroIC is cold. Otherwise, replace the FC. CLI commands ``` get gyro_hardware_lpf set gyro_lpf1_type = PT1 set baro_hardware = auto save ```
How to test ESC & motors without an FC Sometimes you want to test an ESC or motor in isolation, e.g., to clarify if the FC is actually the problem. Method 1 – ESC tester / servo tester Connect a 5V power supply or BEC to the tester Connect the tester plug to the ESC signal cable (usually white cable) Slowly turn up the rotary knob on the tester The motor must start up smoothly without stuttering Method 2 – FC with dummy setup FC connected via USB, ESCs connected to 4S / 6S battery (caution!) Props OFF! Cycle through them individually in the Betaflight Motors tab Method 3 – Multimeter Measure resistance between 3 motor phases – should be roughly the same In case of a large deviation > motor winding is defective Important safety instructions Never mount props during a bench test! ESC can burn out if controlled incorrectly Always keep the battery at a safe distance > If the motor runs smoothly with a tester but not with the FC – the problem lies with the FC or the signal path (cable, solder joint, FC output).
FC after a crash – Checklist before the next flight After every hard crash, you should systematically check the FC before the next battery: Immediately [ ] Switch off the quad, unplug the battery [ ] Visual inspection: Smoke? Smell? Melted wires? [ ] Feel the FC temperature with your hand (carefully!) At the bench [ ] Props off, connect via USB only [ ] Open the Betaflight Motors tab – are all values plausible? [ ] Observe the Gyro in Sensors – no spikes while stationary [ ] Plug in the battery (without props!): The FC must not get hot [ ] Receiver check: RSSI, LQ stable? [ ] Switch on the VTX briefly > Is the video clean? [ ] Test the ESCs (Motors tab, I understand…) Test flight [ ] Hover for only 30s at 2m altitude [ ] No flip / roll, only hold position [ ] Unplug the battery, touch the FC again – is it cool? Abort criteria If any of the following occur: DO NOT continue flying! FC gets hot Gyro drifts significantly Motors twitch at idle VTX video flickers > A 5minute check after a crash costs less than a completely destroyed quad.