Right after the impact
Disarm first. A crashed quad with props jammed in grass or against a branch still gets motor output from the flight controller, and UAVMODEL's crash guide warns that this can burn out ESCs and damage motors. The same guide advises against re-arming to fly out of a crash when a prop or motor may be damaged. Unplug the battery when you reach the quad.
Look around before you pick it up. Note what it hit: concrete points to frame damage, branches to props and antennas, water means the electronics stay unpowered until they are dry. Collect anything that came off, including the action camera, its mount and prop fragments.
The battery
The battery hangs on the outside of the frame and takes many hits directly. Oscar Liang's view is short: it is risky to use dented batteries, especially when charging, and a swollen battery cannot be fixed and should be disposed of. His other warning signs are one cell that keeps ending a flight far off the others, and a pack that gets much hotter than the rest of your batteries.
A dented corner is the case people talk themselves into flying. Oscar's rule for that moment: if you ever ask yourself whether a battery is still safe to use, the answer is likely no. If you do keep charging a pack after a crash, watch it. A LiPo that gets warm while charging at 1C has a problem, and he says to stop immediately. Storage voltage, internal resistance and retirement are covered in the LiPo health article on this site.
Props
Replace any prop with a white crease line, usually near the hub. It may look as if you could bend it straight. Oscar Liang's advice is to replace it anyway, because a creased prop can break apart in the air, and it is most likely to go at high throttle. It has happened to him twice: once the quad flipped and broke an arm, the second time he landed it while it spun around the yaw axis. He also suggests avoiding white props, because creases are almost impossible to see on them.
Hairline cracks do not always show on a prop at rest. UAVMODEL suggests flexing each blade slightly and looking at the hub for cracks running out from the centre hole, and at the tips for chips. Props are the cheapest part on the quad, so if a prop is borderline, replace it.
Motors
Do this with the props off and the battery unplugged.
- Spin each motor by hand. It should turn smoothly and coast to a stop. A gritty feel points to dirt or damage in the bearings. A click or a catch in one spot suggests a damaged bearing (UAVMODEL). Inspired Flight's maintenance guide for its IF1200 uses the same test: all motors should coast down and sound the same, so compare the four against each other.
- Wiggle the bell sideways. Inspired Flight wants zero or minimal play, about the same on every motor. GetFPV says to compare against a new motor and consider new bearings or a new motor if the play is clearly larger.
- Watch the gap between bell and stator. It should be even all the way round. If it varies, or the bell wobbles as you turn it slowly, the bell or shaft is bent (UAVMODEL).
- Look at the bell and magnets. GetFPV's first check is that the magnet ring is still attached to the bell. Also check the motor wires where they leave the base: UAVMODEL warns that nicks in the enamel can cause shorts.
GetFPV adds a test with power: arm without props, spin the motors and rest a fingernail under each one. If one vibrates far more than the others, the shaft may be bent. A bent shaft has to be replaced. Rough bearings can sometimes be cleaned and re-oiled with high-speed bearing oil, or pressed out and replaced. GetFPV notes that bearings rarely need replacing, but heavy impacts can make them fail early.
Frame and screws
Oscar Liang's quick check before every session also works after a crash: make sure every frame and motor screw is present and tight, then try to bend each arm by hand and feel for play.
Then look more closely at the arms. UAVMODEL lists the signs of delamination: layers separating at the edges, a whitish or fuzzy edge, visible cracks between the carbon layers, or an arm that flexes more than usual or crackles when pressed. The arm root where it meets the centre plate takes the most stress, so look for cracks running out from the screw holes. Check that the standoffs are straight, too. A bent standoff loads the top and bottom plates unevenly.
GetFPV says splintered carbon can be glued back with cyanoacrylate glue, but the part stays weaker than a new one. Their advice is to replace frame parts that look badly damaged or weakened. Otherwise the frame may fail to protect the electronics in the next crash, and that crash costs more.
If a motor screw came loose, Oscar Liang uses blue threadlocker (he names TL242) on motor screws. Avoid the red type, which is meant to be permanent. Never put threadlocker on the motor shaft: it can weaken the plastic of the prop hub.
Electronics, connectors and antennas
With the battery still unplugged:
- Shorts to the frame. Carbon conducts. GetFPV sets a multimeter to continuity and checks the frame against both battery terminals and against each motor wire. There should be no continuity anywhere. For motors, Oscar Liang's version is one probe on a motor screw and the other on any of that motor's three wires, which are connected inside the motor. A beep means the screw is touching the windings.
- Visible damage. GetFPV lists blown capacitors, frayed wires, unplugged connections and bad solder joints. UAVMODEL adds an XT60 with melted plastic or bent pins, a dented or bulging capacitor, and cracked joints on the battery leads, which can work on the bench and fail under high current in the air. Reseat the small plug-in connectors.
- Antennas. Check that the video antenna connector is tight and its centre pin is straight and still there. On the receiver antenna, check that the exposed tip has not been cut or pulled out (UAVMODEL). If the video antenna screws straight onto the VTX, Oscar Liang recommends an extension lead mounted to the frame. A crash can otherwise rip the connector off along with its pads, and he calls that damage irreversible and very difficult to fix.
Power-up and a props-off test
After a hard crash, plug in the first battery through a smoke stopper, the same way you would on a new build (see the first power-up article on this site). Then check that the video comes up in the goggles and that the radio shows a link and telemetry.
Keep the props off and use the Motors tab in Betaflight Configurator. Raise each motor slowly. It should accelerate smoothly, without stuttering or new noises. UAVMODEL reads stuttering as a possible damaged ESC or shorted winding. In the Sensors tab, the gyro traces should be flat while the quad sits still.
A short hover before the real pack
Fit fresh props and hover in front of you, in line of sight, before you put the goggles on. UAVMODEL suggests about 30 seconds at eye level, watching for oscillation or drift, then small roll, pitch and yaw inputs and gentle throttle blips, listening for new motor noise. With the goggles on, check that the video has no new noise that rises and falls with the throttle. If the quad hovers clean and sounds like it did before the crash, fly it.
What to carry for this
Oscar Liang's field list covers most of it: spare props and prop nuts, a spanner for the prop nuts, hex drivers, spare frame, motor and camera screws, spare battery straps, zip ties, electrical tape and rubber bands, scissors and pliers, and a portable soldering iron with solder. Add a multimeter and a smoke stopper if they fit in the bag.