One of the most costly incidents that we experience is an Overspeed. It is imperative that we, as a helicopter community avoid any more such incidents in the future. Despite frequent discussions and reminders during continuation training, overspeeds occur far too often. Consequently, here is a reminder of how they can occur and how they can be avoided.
An overspeed occurs when either ERPM or RRPM or both exceed the red lines on the RPM tachometer.
Types of Overspeed
- Engine RPM overspeed on start up
- Engine & Rotor RPM during lift into the hover
- Engine & Rotor RPM during flight
- High Rotor RPM in auto rotation
Let’s look at each type individually and how they can occur and how to prevent them.
Engine RPM overspeed on start up
Engine is started – either by turning the key or pressing the starter button – without the throttle being in the fully closed position.
ERPM rises rapidly to exceed 104 %; governor does not have sufficient information to intervene before overspeed occurs.
Scoring of cylinders, damage to push rods and piston seals and crankshaft. Damage to magnetos misalignment of the cooling fan.
Follow the Company start up check list properly. Do not rush and do not skip steps.
The throttle must always be fully closed before and during the start process. Always ensure that the throttle is fully closed before you start the engine. If the engine is reluctant to start you may prime by opening the throttle then closing it fully again before a second attempt in an R22 or using the key in an R44. On no account should the engine be encouraged to start by cracking the throttle open whilst cranking the starter.
Engine & Rotor RPM during lift into the hover
Lifting without the governor switched on / governor faulty or non operational.
Without the governor switched on ERPM & RRPM will rise as the lever is raised. The mechanical system of Correlation does not work outside the range of 17-21” MAP. Consequently the needles will both continue to rise above 104% as the lever is raised. Whilst this may be counter-intuitive, it is nevertheless fact and will cause significant damage.
Scoring of cylinders, damage to push rods and piston seals and crankshaft. Damage to magnetos, Brinelling (crushing spherical bearings into egg shapes) of feathering bearings in the rotor head.
If the helicopter is flown in this condition for any length of time, overstressing of journals, rotor head and rotor blades, damage to drive shafts and main rotor and tail rotor gearboxes & hydraulic systems may also occur.
Follow the Company check list properly;
Switch the governor on before start as directed;
Allow governor to set RPM to 104 % during wind up;
Allow governor to recover RPM after Low RPM warning horn check;
Check both sets of Warning Lights are out when directed.
Perform pre takeoff checks properly every time you lift. Things may have changed since you trained, our recommended mantra is now:
- Upper Warning Lights Out
- RPM 104% & governed
- Lower Warning Lights Out (INCLUDING GOVERNOR)
- T’s, P’s & Keys (set to both magnetos)
- Fuel (Sufficient for flight)
- Carb Heat Set
- (R44 Hydraulics On)
- Hatches & Harnesses
- Area Clear, Left, Right & Above
Take off technique
When raising the collective to lift, pause at 17” MAP to check RPM is holding at 104%. If it is not, then the governor is not doing its job but you will have spotted it before you exceed limits and cause damage.
Engine & Rotor RPM during flight
Governor failure, inadvertent switching off of governor, mis-handling of throttle, poor re-engagement technique following recovery from auto.
ERPM & RRPM do not remain within the Power On range: R22: 97 – 104%, R44: 98 – 102%
Scoring of cylinders, damage to push rods and piston seals and crankshaft. Damage to magnetos, Brinelling (crushing spherical bearings into egg shapes) of feathering bearings in the rotor head, tail rotor drive shaft failure followed by loss of tailcone.
Regular scan of instruments must include RPM tachometer and warning lights.
Governor usually ensures RPM is correctly maintained. Pilot must still monitor that the governor is working correctly.
Low RPM: Warning horn sounds indicating governor has failed to maintain RPM;
High RPM: No such warning received. Pilot must spot and gently roll off throttle till RPM back in the permitted range.
If RPM correctly set prior to take-off, governor failure is unlikely to take RPM far enough outside limits to cause significant damage. Use manual throttle to correct RPM gently.
NB: During auto recovery, ensure needles joined in the power-on range before raising lever.
High Rotor RPM in auto rotation
Insufficient check up on lever to contain RRPM rises due to disc loading / airspeed changes.
As rate of descent increases, updrafting air increases RRPM. Failure to raise lever to contain RRPM rise may lead to RRPM needle exceeding Power Off Limit of 110%.
Brinelling (crushing spherical bearings into egg shapes) of feathering bearings in the rotor head, over-stressing of journals and rotor blades, damage to drive shafts and main rotor and tail rotor gearboxes may also occur.
Having entered autorotation, never forget to check up on collective to contain RRPM rise.
Avoid aggressive cyclic inputs during auto – fly smoothly & monitor RRPM throughout exercise.
Feel for increasing pressure on your backside signalling increasing disc load and check up again if necessary. Monitor needle re-engagement carefully during recover to climb.
Do not fly a helicopter above its maximum permitted all up weight.
What to do if you do overspeed a helicopter.
You must get RPM back within normal limits as soon as possible, land & shut down immediately.
Whilst the helicopter can probably continue flying, after damage has occurred, there is no guarantee, so be safe and make a sensible controlled precautionary landing.
You are legally required to report any unserviceability to the operator, so you must tell us what has occurred:-
- Certain engineering checks have to made to ensure helicopter is airworthy.
- Providing we can tell the insurance company what has happened and that it is Pilot Error, it is an insured risk and will usually be covered by the underwriters.
- No other pilot will be put at risk by your error.
If you do not report an overspeed:
- Repairs will not be covered by insurance and the owner will have to bear the costs, making them unlikely to hire out helicopters in the future;
- An un-airworthy helicopter could remain in operation which could suffer a catastrophic failure at any time causing injury or even a DEATH for which you would be responsible.
It is therefore vital that any incident is reported. We always try to address such matters positively and retrain pilots when errors occur and are admitted. That makes for better pilots. You won’t be popular but providing you engage with us in rectifying the problems it need not affect your flying in the future.