Fusible Plugs:

[Prince of Dzun]

Hot wheels, flat tyres and fusible plugs have been in the news of late but I wonder how many of those involved in the hands on side of aviation know that fusible plugs have been around for more than 100 years.
Early 19th century English steam locomotive engineers installed fusible plugs in locomotive boilers after a series of catastrophic boiler explosions. These explosions were related to an unnoticed drop in water level within the boiler which then raised both boiler temperature and pressure and as there was no such thing as a blow off safety valve the boiler would explode. The problem was solved by installing lead plugs in the top of the boiler nearest to the fire box (the hottest part of the boiler). When the temperature within the boiler became excessive the lead plugs would melt and allow the steam to escape and a possible explosion was averted. The engine lost power but could still continue to operate.
Clever people those old time engineers.

Prince of Dzun

[Captain Dart]

erm...yes, they were.

[H721]

Informative...Prince

You mentioned the use of fusible plug to preclude the possibility of catastrophic boiler explosion... what about the ones that are installed on our beloved heavy metals... what temperature / pressure are they designed to be triggered... what about if they are not performed as per the design... becoming big riskssss to to the fire fighters / maintenance & engineering staff...

[Prince of Dzun]

H721

As far as I can recall from my days on the 747 (a long time ago ) the fusible plugs on the main wheels melted at 400 degrees Fahrenheit. I don't think the plugs had anything to do with pressure which is a separate issue. Those plugs always seemed to do their job OK but an inflated tyre on a hot wheel is certainly dangerous. However I've always felt that fusible plugs are not infallible which is the reason pilots are advised never to approach a hot wheel from the side. No such restrition on the steam locomotive as when those lead plugs let go the " red hot " steam went skywards.

Prince of Dzun.

[Centaurus]

Fascinating subject, Prince and especially the history of fusible plugs as you explained it. The Lancaster bomber and its larger version the Lincoln (which I flew) had expanding strong canvas (I think) bags that pressed against brake drums and inflated by 450psi pneumatic pressure. Every squeeze of the brake lever on the pilots control column dropped the total available pneumatic pressure and one had to be careful of not running out of steam as it it were, after a long taxi in a crosswind.

If the brakes were used excessively and continually such as during an aborted take off or even a flapless landing on a short runway, the canvas brake bag would burn through with friction and look Ma - no hands no brakes. Not too bad on a nosewheel aircraft but no fun on a heavy tail-wheel type such as the Lincoln where it was easy to ground-loop. And talking about a no brakes landing: The long-in-tooth among us will recall the former RAF flight safety magazine called "Air Clues".

A story appeared about a Vampire pilot that had brake failure on landing and was in real trouble of going off the end of the runway. In his incident report the pilot said he decided to wind back the Vampire canopy to create more drag to slow up. It didn't work that well and he finished up in the mud unhurt.

The editor of Air Clues was in fact a former respected Fleet Street journalist or well versed radio play actor (I forget which) and he had a droll way with words. He used the pseudonym of Wing Commander Spry, when commenting on flight safety matters.

Commenting on the pilots action of opening the canopy for drag, he offered the suggestion if the pilot had thought about it at the time, more drag could be attained by the simple process of the pilot tying the four corners of his handkerchief and holding it outside the cockpit in the airstream where it would it would act as a braking parachute.

Now back to the original subject of fusible plugs. Taxiing a 737 at Kai Tak airport Hong Kong, I was appalled to see an Air India 747 at the blocks with huge plumes of white steam rising from a set of wheels as engineers directed stream of cold water from a high pressure water hose upon hot brakes suffered after a long taxi-in after a heavily braked landing. I waited for the inevitable wheel rims explosion but it didn't happen, thank goodness.

[Milt]

Centaurus

Did I ever tell you how to taxi a Lincoln backwards?

[Centaurus]

Haven't heard that one, Milt. Intrigued - go ahead.
Cent

[bonajet]

A very unusual subject for FH Prince of Dzun!
The idea of fusible plugs in the crown of the firebox was, in 20th century British locos, to melt and let the steam into the firebox when the water level dropped below the crown. There it wouldn't have the power to douse the fire but could provide some audible warning to the crew. It didn't always work, in particular if the firebox door was shut at the time - there was a fatal boiler explosion on the French railways from this. The pressure relief valves or blow-off valves (up to four) were provided as well - these operated at different pressures and were on top of the boiler casing and let the steam escape to atmosphere. Whilst these would be used every trip, it was hoped that melting the firebox plugs was a never in a career job!
Now Milt - taxi-ing a Lincoln backwards.........

[Basil]

engineers directed stream of cold water from a high pressure water hose upon hot brakes
I've a photograph somewhere which I took of the engineer hosing the hot brakes of a Russian aircraft at Cairo.
Upon approaching (from forward) and expressing an interest he said "On this aircraft is OK!"

[Prince of Dzun]

Centaurus;

Very interesting what you say about the Lincoln and its canvas bag brakes. I've never heard of them and can't but wonder if any other aircraft had such a strange set up. Why didn't the designers use drums or pads and pressure them like everyone else and keep things simple??

bonaset;

I'm not to sure that you are correct in saying that the blow off steam was used to douse the fire. The fire is not the problem, the problem is the pressure and once that is released the danger is removed. I'm thinking about it and will get back to you.

Prince of Dzun.

[bonajet]

Hello PofD
The fusible plugs let steam into the firebox when they melt - not to douse the fire but to provide warning to the crew. They were there to help protect the firebox from overheating and collapse if it became uncovered. So fusible plugs are for the protection of the firebox and safety valves are to protect the boiler.

A very good current source is How Steam Locomotives Really Work - Semmens and Goldfinch Oxford Press ISBN 019 860782 2. A good anorak's read!

To quote the story of the French Railways accident:
"The magnitude of a locomotive boiler explosion was well demonstrated in 1935, when the crew of a PLM 2-8-2 working a Geneva-Paris express up a 15 mile bank allowed the water level to fall and uncover the top of the inner firebox. This overheated, losing its ability to withstand pressure and the crown sheet burst downwards, discharging the contents of the boiler through a tear in the plates, which had an area of 17sqft. The whole boiler was ripped from the frames and the cab, killing the crew and causing the brake to be automatically applied as the air pipes were severed. This brought the train to a halt with the rest of the locomotive still on the track and attached to its coaches. Fortunately the train was going around a curve at the time and the boiler's trajectory sent it off unto the line side fields. It first hit the ground, firebox first, 272 ft from the point of explosion, after clearing a telegraph line and a row of trees in the process. It then cartwheeled three more times, finally finishing up 512 ft from where it had taken off"

The inquiry found that the fusible plugs had been blowing for 9 mins prior to the explosion after the water level dropped too low.

[Brian Abraham]

Very interesting what you say about the Lincoln and its canvas bag brakes
Expander tube operated brakes (air or hydraulic) were par for the course in those days, DC-3 for example. Was a while before disc brakes made an entry.

[HAWK21M]

Fuseable plugs are for High temperatures.Most wheels have relief valves for Excess pressure.

[Milt]

Centaurus

Taxying a Lincoln Backwards. The procedure is secret!!!

Extract from memoirs -
Then there was the time I was converting a pilot to fly Lincolns. We had landed and wished to taxi back to the other end of the runway for another takeoff. The pilot made too wide a turn and almost too late I notice that we are about to run over an electric flare light on the edge of the runway. By the time I grabbed for the brakes and stopped the aircraft the main wheel was about three feet from the light. To proceed further would flatten the light and run the risk of damage to the tyre. It looked as though the only thing to do was to call the tower to have a tractor come out with a tow bar to move us back away from the light. I could see that if I could move the main wheel backwards a few feet, we could then proceed safely. But how to do this in an aircraft having only engines which were at all times trying to move us forward. No reverse pitch on these propellers. But I had an idea. The brakes could be applied independently to each main wheel. It was the right wheel that was close to the light so I locked up the brakes on the left main wheel and ran up the power on the No 1 left engine outboard of that wheel. With a lot of power on that engine the aircraft started to swing around the locked wheel and at the same time rolling the right wheel backwards.

Having moved the right wheel backwards I then locked its brakes and repeated the exercise using the outer right engine and soon had plenty of room to continue the turn without hitting the light. I cancelled the call for tractor assistance and left all concerned wondering how on earth I had moved a Lincoln backwards.

Of course, the procedure was a big strain on the main undercarriage struts as the locked wheel would be twisted around on the spot and the screwing tyre would tend to dig into soft ground but I did use the technique on several occasions to demonstrate to experienced disbelieving Lincoln pilots just how to taxi backwards.

The other secret procedure is how to take a Gooney off on one engine. Succeeded only once and never want to try it again. Bill Simmonds in the right seat was petrified.

[Old Fella]

On those aircraft so equipped, the airbag did not contact the brake drum but rather expanded to push the brake shoes against the brake drum, exactly the same way in which conventional hydraulic drum brakes do. MC, really enjoyed your Canberra bomber demonstration flights.

[Prince of Dzun]

bonajet;

You are right and I am wrong. The fusible plugs do let steam into the firebox and not into the atmosphere as I thought. It's reasonable to assume that if the plugs were directed into the atmosphere the escaping steam would be seen by the crew but only during daylight. It would not be easily seen at night which is the very time that the original problem of low water in the boiler would go unnoticed. I wonder what the operational procedure was when the crew realised the firebox was filling with steam?? If the fireman opened the firebox door to fuel the fire the cab may have been filled with scalding steam. Perhaps in such a situation they just let the whole engine run down. Any ideas on this?

Prince of Dzun.