The crack widened steadily — a foot, two feet. Then the farther half of the roof split into fragments and folded inwards, sinking down towards the lava in a heat blast of dust. And as it fell I saw the stone housing of the doorway disintegrate.

I scrambled towards it. It was a chance in a million. Through the choking dust I saw the wooden stairs intact leading down to the room below. I hesitated. I think any one would rather die in the open than be caught like a rat in a trap inside a building. But it was still a chance and I took it. I swung myself over the edge, dropped on to one foot and jumped the whole length of the stairs. I landed in a heap on the boards of the room below. The farther wall was missing and through it I could see the heat curling up from the top of the lava.

The stone staircase, thank God, was behind me. I scrambled to the top and slithered down. On the second flight I almost broke my arm as I fell against the side of the archway at the bottom. I could feel the building trembling now and the room I was in was full of a vicious, suffocating heat.

As I picked myself up I saw through the choking dust clouds, a long face with ears twitching and eyes rolling. It was the poor wretched mule, lashing out with its legs as it strained at the halter that held it. A long-bladed butcher’s knife lay on the floor. I grabbed it, hopped over to the animal and slashed it free. I had a childish fear that if I let the creature die, I should die too.

God knows why I did it. Some pilot’s instinct, I suppose, to have a mascot. But the mule was nearly the death of me. It leapt free and ran careering and screaming round the room, hoofs lashing and teeth grinding in its fear. Then it found the ramp and went thundering down, slithering the last part on its haunches. I was so close behind it that I saw the sparks kicked up by its hooves as it pawed itself to its feet at the bottom.

Those ramps were easier than the stone stairs. They were slimy with dung and I slithered down them, lying on my back and thrusting myself forward with my hands. I could feel the building rocking as I descended and at each floor I could see the burning face of the lava where the farther wall had once been. As I reached the ground floor there was a series of splintering crashes and I knew the house was disintegrating above my head. The way to the street, the way the cattle had been brought in, was gone and in the ragged gap I saw the white heat of the lava face and felt the scorching breath of it singe my hair.

The mule had gone out by a window, crashing through it and taking the whole frame with it in its terror. I followed and as I fell to the ground I realised I was in the garden of the house and there lying almost beside me was my leg.

It was one of those strokes of luck that fate is kind enough to offer once in a while and looking back on it I can’t help having an instinctive feeling that it was all because I paused to free the mule. I know it sounds stupid. But there it is. We had odder beliefs than that when we were flying night after night over Germany.

I picked up the dented limb, hopped to the wall and scrambled over. And as I fell into the next garden, the house I’d been imprisoned in disintegrated, filling the narrow space between the houses with noise and dust. I got through the next house and came out into a narrow street that was blocked at one end by the lava. The place was a cul-de-sac, and there was my mule, standing at the end of it with his face to the lava and whinnying.

I dropped my trousers and strapped the leg in place. The lava grit embedded in the stump of my leg hurt like hell when I put my weight on it. But I didn’t care. It was such a relief to be able to stand upright like a human being again. It’s a horrible feeling to have only one leg and to be forced to crawl around like the lower order of creatures. To stand upright again and move normally gave me a sudden surge of confidence and for the first time that morning I felt I might win out in the end.