Turbine engine startup gone wrong

[baobab72]

Hi folks

I am a plank driver with a keen interest in helicopters and while i was watching a video on you tube on how to start different turbine helicopters i have noticed that in the vast majority of them - r66, bell206 and nh500 - the pilot briefed that in case of a hot start he would have closed the throttle and kept the starter button pressed in until the itt or equivalent would drop below a certain number of degrees stressing the fact that whatever happens do not let go on the starter button!
Now my question, although it might sound silly, is why doesn the starter switch lock in the engage position as it does on all jet engine airplanes - a speed sensing switch disengages the starter at a preset n2 once the engine becomes self sustaining - to prevent the pilot from accidentally disengaging the starter which might potentially lead to a hot start or in case of a hot start to prevent the engine from being cooled by the air rushing through it?

Many thanks

Baobab72

[lelebebbel]

why doesn the starter switch lock in the engage position as it does on all jet engine airplanes - a speed sensing switch disengages the starter at a preset n2 once the engine becomes self sustaining

It does on some machines, like the R66. Others, like the 206 or MD500, are simply older designs. These days, pretty much any newly designed machine will have some sort of a FADEC system that takes care of all the starting magic.

[Ascend Charlie]

The b212 and 412 also have a starter which locks on, which then allows the pilot to make the mistake of forgetting to turn it off.

Only when the Gen will not come on line do they work out what the problem is.

[Nigel Osborn]

I see you have done that too!!!!!

[BOBAKAT]

on "old ladies", as Alouette 2 and 3 the starter is on OR Off
in flight = on
on the ground = off...

[CYHeli]

Some have an auto relight switch, but that is used post start.

[Stallion85]

on this old ladies you sometimes need the "pilot feet switch" to start the turbine!

[Fareastdriver]

It's a question of size. Helicopter turbine engines are fairly small compared to turbofans and they are much more sensitive with regard to how much fuel is arriving during the start.
On a turbofans an extra bucketful or so of fuel doesn't make a lot of difference. The nearest a fixed wing would have to a helicopter is an APU which doesn't have much of an operating range between idle and maximum desired so neither power unit is desperately sensitive when starting.
Most small to medium helicopters using the Allison or Arriel have a pilot modulated start which can be more than interesting if the engine is hot. The bigger helicopters are much more expensive so they get far more monitoring circuits to look after the engines.

Then you can get back to kicking the tyres and letting the engines fire up whilst you are strapping in before some whinging co-pilot arrives.