Cessna 310 Fire Hazards from Hot Starts
 

Entitled "An Augmentor Tube Hazard", the following 1964 Aviation Safety Digest report on a fire to a Cessna 310 is worth studying by pilots currently operating the type.

At Canberra recently, an engine of a Cessna 310 caught fire while it was being started for a flight to Cootamundra. A loud explosion inside the engine nacelle caused the cowlings to be blown open and the fire followed, but it was quickly extinguished by the fire crew and the damage was confined to the cowlings themselves.

At the time the engine was hot and the pilot had at first tried to start it without priming. When the engine would not catch, he primed it for two or three seconds and the explosion occurred on the next start attempt.

Investigation revealed that the aircraft was standing tail into wind at the time and this fact, in combination with the hot and over-primed engine, had allowed the nacelle to fill with fuel vapour. When the engine fired, the exhaust flame in the entrance to the augmentor tubes ignited the vapour inside the nacelle, blowing out the cowlings and starting the fire.

The incident is by no means isolated. In December, 1960, the Digest reproduced a warning issued by the Beech Aircraft Corporation on the dangers of fire when starting augmentor tube equipped aircraft, and in 1961 the Cessna Aircraft Company sent a Service Letter to all operators of Model 310 aircraft setting out the precautions that should be taken against this possibility.

The problem is not confined to Cessna and Beech aircraft but applies equally to the starting of all aircraft that are fitted with augmentor tubes. Pilots can do much to eliminate the problem by at least observing the two golden rules of avoiding engine flooding, especially with a hot engine, and ensuring proper ventilation of the nacelle area by avoiding, as far as possible, downwind or excessive cross-wind start-ups.

Centaurus, great read as always, not sure I'm familiar with Exhaust Augmentor Tubes? Is this something that I would have seen a thousand times but never really had explained or pointed out? Does it go by a different name these days? Perhaps an uncommon feature these days?

Can't seem to find a lot of info when googling so might be looking for the wrong keywords?

The early C310 models had twin over-wing Augmentor tube exhausts. I remember them on C310B models VH-REL and VH-DBA that I flew in the very early seventies. Wouldn't be many of those old tarts left now. VH-DBA was last spotted in CBR in all natural metal finish, looked a million dollars. I wonder what happened to it?

Oh....yes. The augmentor tubes. As Centaurus pointed out, I have learnt at the very early stages of my Queenair days not to attempt to start up with even the slightest of tailwind. Two things I always remembered with Queenairs....make sure the fuel drain next to the augmentor was fully closed after sample taken and make sure there were no signs of fuel leak at the spar attachment which was right next to the augmentor.
I think there was a fatal crash of a Queenair near Roma in the seventies due to a fuel leak near the augmentor. The wing caught fire on take off and by the time the pilot attempted to turn back it has burnt through the spar causing it to crash.

Augmentor Tubes
 

Ixixly, if you google up "ADF Serials - DeHaviland Caribou" and scroll through the various photographs you will find examples of Augmentor tubes on the 'Bou. If you look at A4-225 Picture 21 of 33 you will see clearly the augmentor tubes, two per engine, running back over the wing.

The Dove Mk8 had them, whereas the Mk6 did not. I think the theory was that there was a small amount of extra thrust gained by the air in the tube being heated and ejected with the exhaust gases.

Thanks Old Fella, managed to find a couple of vague references to them in some old articles, but not much about what they were really meant to do or how, seems like an idea someone had that was proven to be more hassle than they were worth perhaps? or perhaps just snake oil in the end?

Could someone explain to me why the augmentor tubes make a difference?

I would have thought that over-priming any hot engine would result in fuel vapour being ejected, and with a suitable wind direction that could well end up inside the cowling.

Is the difference simply that the augmentor tubes provide the ignition source?

Augmentor tubes
 

It has been claimed that they "augment" thrust by vectoring the exhaust gasses. A bit dubious I expect.

The Convair 440 Metropolitan operated by the RAAF No. 34 Squadron based at Canberra in the Sixties were equipped with augmentor tubes to the P&W R2800 engines. In fact they were a major selling point on Convairs when they were first produced I believe. They were advertised to augment more exhaust thrust and thus extra IAS.

sms777
Perhaps VH-CMI, the Connair accident at Alice Springs ?

A former Chief Pilot of Connair told me the time between the engine failure and wing separation was very quick....those old accident reports were a great learning resource - professional and detailed.

150hrs on c310’s. Just shy of 500hrs on 402’s....the rest is history. WTF are you idiots talking about!!?

What is your point ?
 

Defensestrator. Just what is your point? Hoo Ha, close to 650 hours on Cessna C310/C402, you must be an "Ace". Centaurus posted referring to an Aviation Safety Digest article which led to some questions regarding Augmentor Tubes. So, again, what is your point?

This link clearly shows the Augmentor tubes on a Caribou.


Caribou Augmentor Tubes

Credit: Picture from Petester's Aviation in Vietnam.

From a Convair manual.

The augmenter system of engine cooling was used on the CV-240 throughCV-440 aircraft - they are the two large "exhaust pipes" that come out of the rear of the nacelle.

I think Defenestrator may be too young to have seen a 310 with tubes.

See https://en.wikipedia.org/wiki/Cessna...al_Airport.jpg

Eckhard already has.

And if you found the above extract explaining the stack of the gas ejecting bell mouthed Siamese twins in multiples of eight segregated into two single stacks up against a firewall and ejecting gas at twice the speed of sound a little confusing try this...

By way of expansion (excuse the pun) the exhaust pipe (often fluted) exits the cowl through an outer pipe surrounding the exhaust for some length which feeds back into the cowl along the exhaust. The exhaust pipe heats the air between the exhaust and the inner area of the surrounding pipe, this heated air having less density has a lower pressure and draws air from the cavity of the internal cowled area out along the pipe to the stub end of the exhaust.

Fluting along the exhaust, or inverted gills (think like a cheese grater turned inside out) if you like helps to draw air in the pipe into the hot low pressure exhaust flow and also in an outward direction. All this helps to offset cooling drag and it was said that good exhaust augmentation could offset cooling drag to the extent of zero-ing cooling drag, or at least so they claimed.

Principals of common dog xxxx when you think about it but likely not as good as claimed in theory.

Hope that helps

Thank you Obidiah. It does help.
Maybe a distant relative of Meredith E. Fect?

https://www.google.com.au/search?q=c...G0h21m3DQY8BM:
As can be seen from the above image, the C310H has an even larger area for a tailwind to blow back up the nacelle. Augmenters work by using the flow of exhaust gases into a much larger tube (augmenter - typically at least 2” larger diameter than the exhaust outlet) to draw additional air around the engine and through the augmenter. Especially when hot, any excess fuel vapours can be forced back into the nacelle by any tailwind, igniting by anything from a generator spark to the engine exhaust and potentially creating spherical cowlings and blowing the side inspection panels completely off.
A “normal” exhaust is closed (think blowing into a test tube) and a much smaller diameter, whereas the augmenter system is open and has much larger potential to allow vapour to flood the entire nacelle.