AAIB(H) UK September 2025
 

R22 at top of approach, student pulls mixture instead of carb heat, instructor immediately puts it in auto with the stopped engine and they walk away despite a partial rollover on touchdown - link

The student had been specifically briefed about the possibility of confusion, but Robinson identified this as a potential problem in 1981. Human performance discussion, go. The usual suspects will have a pop at Robinson, but let’s not forget how it’s possible to start a 109 with the rotor brake selected on and it only grabs once hydraulic pressure builds as the rotor speeds up…

I'm surprised more attention wasn't given in the report to where the mixture guard actually was, in this case, and how the mixture control was easily accessible. As the report states, this issue has been known for 44+ years! I'm sure this will result in an interesting conversation with the insurance company.....

Being clever after the event here, but.......

A red knob for mixture ?? All our lives we have seen red on water taps and car heater controls; meaning heat or hot.

So a student, new to this type, with a hundred other things on their mind is reminded to apply heat, while also flying and controlling the aircraft in the descent.

Their conscious processing capacity is completely used up, with no spare capacity, so they use their deep memory and reach for the red control which in their deep memory means heat. Even though they were briefed about the mixture control.

And Robinson have apparently known about this problem for years. So why therefore haven't they - and other piston engined aircraft manufacturers - changed the colour of the mixture knob ? Black and white stripes, for example.

Having a normal flying control coloured red is a bit odd anyway - red controls are generally for emergency situations, such as engine fire.

Luckily both occupants were not injured, owing to an excellent recovery by the instructor :ok:


Regarding the A109 rotor brake; surely that is fixable by following a pre-start paper checklist ?.

You can have all the checklists you want, but…A109 with smoke coming from the rotor head at Battersea

You could give it any colour, even put blinking LED's on it, that won't change anything. The fact is that Fuel cut off and carb heat are very close to eachother and each require a pulling action to activate. I'm sure the student didn't even look down, just grabbed what he thought was the carb heat and pulled. So the red color is not a factor here.
And if we do look at colours... red is danger, red is stop, so don't touch it.
Before you touch carb heat, cabin heat, mixt cut off, look down and identify it. It takes 1 second. But hind sight is easy, it's a mistake easy made and will be made in the future again.
Good job on the FI landing it without injuries.

The standard colours for civilian piston engined aircraft have been, for decades:

• Throttle - black
• Mixture - red
• Propellor (if a VP) - blue

Actually, the use of red is precisely to highlight the danger of misusing the control! I see you have a FW ATPL, so I don't know what basic trainer you used, but here's a 60 year old Piper Cherokee as an example (no prop control)

https://cimg1.ibsrv.net/gimg/pprune....05d3ed5be0.png

The fact that Robinson R22 AND 44 have had a plastic tube attached with a chain to the console, to put over the mixture once in, the knob being RED and of a different shape than the CARB heat isn't enough? It also requires a dual action, so a push on a central knob and a pull on the lever to activate. Even advanced military aircraft with highly advanced systems have had buttons and levers pushed because of rush.

To change the lever yet again will solve nothing.

I am not a heli pilot, so I obviously don't know, but if there is a guard, then how did the pilot accidentally operate this control ?

Cessna 152, PA28, PA34.

I fully acknowledged I am being a 'Monday morning quarter-back', or whatever the phrase is :)

I have a foot in both engineering and piloting camps, and am always wondering about design issues and how accidents happen.

Here we have a situation where a control coloured red is routinely used during normal flight. My point was, is that sensible, or would it be better to only have emergency controls, e.g. fire push buttons, coloured red ?

....you have to use them - by reading them, not by memory - and/or make sure they include important items such as a rotor brake !

Your report says the A109 pilot completed their checks and started the engine(s). Well, no, they obviously didn't complete their checks, or the checklist missed-out checking the rotor-brake !

It's NOT routinely used during normal flight, that's the part you're missing. You don't manually lean these.

I'd bet the guard wasn't in place personally, the report was missing a lot of detail there for sure.

Ah, OK, I see.

Just for my interest, why then is there a mixture control at all ? In what circumstances is it used ?

To cut the fuel supply to the engine entirely when stopped, and to allow fuel to flow when the engine is running. So it should be moved at startup to "on' and at shutdown to "off" and at no other time.

There's no procedure, as in fixed wing, to allow the engine to be leaned if at a high altitude strip. In the R22/R44, it's either in or out, on or off, nothing in between.

Oh. So it is effectively a fuel cut-off, not really an operational mixture control at all - I was assuming the latter.

Understood. I will now crawl back into my shell :)

As the R22 is rated up to 14,000ft the mixture can be leaned at higher altitudes.

I no longer have a copy of the R22 or R44 POH but online PDFs have caution sections saying it's not recommended - if you forget to push full rich on the way down to lower altitudes, the engine may stop due to being over lean ...

Strewth. Remind me never to fly in one...

all our POH’s are available for free on our website, the link below goes to the section you are referring to. I’ve also attached a screenshot to explain what you are talking about. The preferred approach is to not lean at all. But if you do, you will need to adjust back if you change operating altitudes.
https://cimg2.ibsrv.net/gimg/pprune....68eff7318.jpeg


https://robinsonstrapistorprod.blob....6581e57af0.pdf