H501S - FAILSAFES

There are multiple failsafe mechanisms built into the H501S, and there is much misunderstanding of how they work and when they are invoked.
In this article I'll have a look at each failsafe and attempt to describe the mechanism that triggers them and what actually happens once invoked.

Failsafes are obviously there to protect your craft against loss or damage and also to reduce the risks to people and property in the vicinity. Whilst no failsafe is perfect, and at times it may be better not to be there, the majority of times it mitigates what could turn out to be potentially dangerous or expensive situation. 

1. Loss of control signal (LOS) (2.4Ghz).

This is the obvious one at the top of the list. What happens if you lose control signal?
When control signal drops below a useable level, the Quad automatically failsafes to RTH. 

This will be noticeable when flying on the edge of your control range. The quad stops, orientates itself heading away from you, and starts to return backwards. If the quad is lower than 10m it will first rise quickly to that height.
On recovery of the control signal, as it gets closer, it drops back into whichever flying mode it was previously in and is commanded once again from the controller (you!). This can have the impression that it is having a mind of it's own and not really responding to your commands (which it is for some of the time).
RTH is an autonomous mode. Once it is invoked you have no control over the quad until the autopilot relinquishes control.

Loss of control signal will also occur due to 2.4 GHz interference and loss of controller power. Loss of controller power is easy to test and should be checked form time to time during preflight checks. Just turn the controller off. The quad will enter RTH mode and perform a full RTH procedure.

Once the motors are armed loss of signal (for whatever reason) will NORMALLY invoke RTH. This is regardless (except in a couple of special scenarios - see later) of flight mode which can be GPS, ALT HOLD or MAN.
A word of CAUTION.
If you arm the motors, leaving the quad sitting on the ground, and then kill the Controller power, the quad will enter RTH. The first part of RTH is always to rise to 10m if it is lower. The quad will therefore immediately take off. If it had a good GPS lock it will just fly overhead and then slowly descend. If it didn't have good position lock it will just fly off in a random direction to wherever it thinks home might be!
Always treat an armed quad with the respect it deserves.
Here's a sample of what can happen with an armed quad.

2. Low Voltage (LVF) (Quad Flight Battery)

When the flight battery voltage falls to 7V the Controller displays a red "battery bar" and the rear lights on the quad begin to flash Red in unison.
This is an indication that it's time to consider getting home and landing. The red bar is a little  inaccurate because it uses a smoothing algorithm and therefore suffers from latency. It has a hard job coping with the voltage sag of the 10C battery and so will initially flash on and off in unison with the quad power requirements and the sagging terminal voltage.
At 6.8v the LEDs will stay permanently on. This is the final warning that you are approaching LV cutoff (LVC)
At this point the quad will quickly start to lose power and subsequently altitude. More throttle will be required to keep it aloft. You need to land quickly.
If the quad has not landed by LVC, [6.4v] the quad disarms the motors and landing is rapid 😱
Flying to LVC is not recommended! If you're a long way from home you'll have to go and search for the quad, and it ultimately leads to a reduction in battery capacity, longevity, along with potentially dangerous cell heating.

3. Manual Mode Failsafe (MMFS)

This is an undocumented feature, that took sometime to understand and get a response from Hubsan as to its intended purpose.
This occurs when the quad continues to gain altitude (or the atmospheric pressure drops) whilst the operator is commanding it to descend and applying full down throttle. MMFS places the Flight Controller into manual mode as indicated on the controller display.

This normally also causes the quad to momentarily pitch or yaw wildly at the instant the failsafe is invoked.
This failsafe is not the same as manual mode that you would select from the controller. In MMFS, even with a centred throttle stick, the quad will descend quite rapidly. You will need to apply full throttle to keep the quad in the air and even then it will probably not be enough (as I've found from bitter experience). It is NOT equivalent of flying in Manual Mode.
MMFS overrides LOS FS. Killing the controller power will NOT invoke RTH. It is more akin to the final stages of LVF.
The only way to recover form MMFS is to reboot the quad. There is no mechanism to re-enter GPS or barometric assisted modes.
This failsafe can take people by surprise, so it's critical that you should be aware of its existence and plan for how you will respond should it occur.
You can also try to mitigate against it ever happening in a couple of ways.
It will most often occur if you fly aggressively in Alt Hold or Manual mode and like sporty flying.
For instance flying fast with a lot of pitch and entering a sharp turn or letting go of the elevator, will cause a rapid rise in altitude. The natural reaction is to pull back sharply on the throttle to to kill the ascent. This is the exact scenario which invokes MMFS.

This can be countered by either resisting the pull back instinct, or by simply adding a physical interlock to the throttle stick to prevent minimum throttle. This can be achieved with a simple rubber band or a piece of sleeving, (see video below)
The second most common occurrence is when the quad gets caught in rapidly rising air, or a pressure drop occurs within the quad due to the venturi effect created by the airflow over the barometer.
The indications of this are a rapidly ascending quad which does not appear to respond to throttle reduction. The safest option to prevent MMFS is to just leave the quad alone and engage RTH.
If you do try to fight it down and it enters MM, you'll need to maintain high levels of throttle as the descent can be quite rapid and it you'll also have to claw it back against any wind. You will also be without any GPS assistance, a good reason to practice flying in Manual mode to ensure you can handle these unexpected scenarios.
Whilst this is not a common failsafe, it does occur with some regularity and is a common subject for discussion on online forums.
At least if you are aware of it you can be prepared to take action as soon as it triggers 😄

4. Loss of GPS Lock

This Failsafe only occurs in conjunction with LOS FS.

Really the entry in the manual says all that is required!

If whilst in RTH, GPS lock is lost for more than 20 seconds, the quad automatically descends in a similar fashion to LVC.

This is an extraordinarily difficult failsafe to test (unlike the others above), so I have no first hand knowledge of it. At some time in the future I will modify the onboard GPS so that it can be switched off remotely to check the full functionality. Until then we only have the manual and a small number of reports of this happening. It's unlikely to occur in normal everyday use, unless you fly in areas prone to poor GPS reception (among high rise buildings, deep gorges, amongst trees) or there are active GPS jammers (eg government, security or military establishments).

5. Motor Disarm

The motors automatically disarm if the angle of pitch or roll exceeds 90º for greater than 5secs.
This was introduced in FC1.1.22 and remains a feature in all subsequent revisions.
This failsafe is to prevent motor/ESC burn out should the quad crash and land upside down on its props. The 5 sec delay also allows the execution of loops and extreme angles of P&R during normal flight without disarming ( a feature that happened in the earlier FC1.1.21).
The disarm feature is particularly useful when hand recovering your quad. You can just tilt the quad 90º and the motors disarm without any other action. More recent firmware incorporated the disarm by holding the throttle low, but the disarm by tilting, is still probably the most useful additional feature in the last 2 years.

6. Controller Low Voltage

This is a controller low voltage failsafe.
This is invoked when the controller reaches its failsafe voltage.
This is to enable the pilot time to complete their flight and land safely.
Failure to do so, within the failsafe period, will cause the controller to lose all power.
The quad will then enter LOS FS (as above)
Each controller has different failsafe voltages, and they are also dependant on the battery chemistry in use!  [See Controller Batteries.]
The purpose of the failsafe is to remove the heavy current usage devices within the controller, namely the FPV display and VRX. The normal current requirement of the controller (H901A) is ~550mA.
By switching off the monitor and VRX this reduces to ~90mA. This allows plenty of time to complete your flight with full control with the remaining battery capacity. Obviusly there is no access to FPV or telemetry dat so you must fly LOS.
The failsafe is indicated by the power LED flashing RED alongside the shutdown of the display.

The controller remains in full operation until the battery can no longer operate it. You can fly as normal (I've flown for over 10 minutes and then given up through boredom) until the controller dies.
This is important to know. Just because you have no display does not mean you have lost control. The quad will happily respond to any command given and will respond to an operator invoked RTH from the controller.
Obviously when the controller dies completely (RED flashing LED goes out) the quad will RTH under normal control loss FS.