I saw what he meant. There was not much clearance at the end of each wingtip and beyond the plane the gully narrowed sharply and if the aircraft had rolled a few feet further the wings would have been ripped off. I said as much. That's not what I mean,' said Byrne. He turned and studied the terrain with narrowed eyes. This airplane is in a goddamn box.' He pointed to the wall of rock at the wider end of the gully. 'So how did it get in the box?' He shook his head and looked up at the sky. 'He must have brought it down like a helicopter.'

'Is that possible?'

'Unlikely. Look, the guy is in trouble; it's night time and something has gone wrong, so he has to put down. He can't see worth a damn, his landing speed is sixty miles an hour, and yet he sets that thing down right way up on its wheels in a space that should be impossible.'

I looked around. 'No wonder it wasn't found. Who'd look on the Tassili anyway? And if they did it's in an impossible place.'

'Let's go get the gear,' he said. 'We'll set up camp here.'

He called out, telling Paul to stay there, and we went to round up the donkeys, load them, and take them back to the plane. It was difficult to find a way in but we found a cleft big enough to take one donkey at a time, and unloaded and set up camp in the clear space just behind Flyaway. After that the donkeys were taken out again, hobbled, and turned loose to feed on what sparse vegetation they could find.

When we got back Paul had recovered, although his eyes were still red. 'Sorry about that.'

'That's all right, Paul,' I said. 'I didn't expect an icy calmness.'

Byrne was pacing the distance from the rock wall at the end of the gully to the tail of the plane. I walked towards him. 'Sixty yards,' he said, and blew out his cheeks expressively. 'I still don't believe it. Paul okay?'

I nodded and put my hand up to touch the rudder. 'She looks ready to fly.'

'You'd have to lift her out of here with a crane,' said Byrne. 'And then build a runway. But there's more. Look!' He pointed down to the tail wheel which was flat. When he kicked it, it fell apart in a powdery heap. 'That's the weak link. The airplane is fine – all metal. 24ST Alclad according to the specification, and the desert wouldn't hurt that. The engine will be fine, too; it'll just need the dried oil cleaning out and it'll run as sweetly as new. But all the sealings will have gone, and all the gaskets, and anything made of rubber. And I guess any plastic parts, too. I hear those early plastics weren't too stable chemically.' He sighed. 'No, she'll not fly again – ever.'

As Paul joined us Byrne said, 'Mind if I take a look in the cockpit?' Paul looked puzzled, as well he might, because this was the first time Byrne had asked his permission to do anything. Byrne explained, 'I guess this is your airplane – by inheritance, Paul.'

Paul swallowed, and I saw the glisten of tears in his eyes. 'No,' he said huskily. 'I don't mind.'

Byrne walked around the tailplane and put his foot on the step on the wing fillet, then swung himself up to look into the cockpit. The cockpit cover was slid back and he looked down and said, 'Fair amount of sand in here.'

I left him to it and walked back to get my camera. I spent some time cleaning the lens, which wasn't easy because the air was dry and the static electricity such that you could see the fine dust jumping on to the surface of the lens under its attraction. I did my best and then loaded the camera with a film and went back to take pictures.

Byrne had got into the cockpit and was fiddling around with the controls. The rudder moved, but with a squeaking and grating noise, and then the ailerons went up and down with less disturbance. Paul was standing on one side, doing nothing but just looking at Flyaway. I have never seen a man look so peaceful, and I hoped he would now be cured of what ailed him, because there was no doubt that he had been a man badly disturbed to the point of insanity.

I used up the whole roll of film, taking pictures from various angles, including two of the faded name on the side of the fuselage. Then I rewound the film into its cassette and packed it away with my unused shaving gear.

Presently Byrne called me and I went back to the plane. He. was still in the cockpit. 'Come up here.'

I put my foot on the step and hoisted myself up. He had his pocket prismatic compass in his hand. 'Look at this!' He tapped an instrument set at the top of the windscreen.

'What is it?'

'The compass. It reads one hundred eighty-two degrees.' He held up the prismatic compass so I could see it 'Mine reads one hundred seventy-five.'

'Seven degrees difference. Which is right?'

'Mine's not wrong,' he said evenly.

'An error of seven degrees wouldn't account for Billson being fifteen degrees off course.'

'Maybe not.' He handed me the prismatic compass. 'I want you to go back there – well away from the airplane. Take a sighting on the rudder; I want you lined up exactly the way the airplane is. Then take a reading and come back and tell me what it is.'

I nodded and climbed down, then went back as far as I had left the baggage. I sighted on the rudder and got a reading of 168°. I thought I'd made an error so I checked my position and tried again and got the same result. I went back to Byrne. 'A hundred and sixty-eight.'

He nodded. 'Fourteen degrees difference – that would be about right to put him here.' He tapped the aircraft compass again. 'Look, Billson is flying at night, right? So he's flying by compass. Let's say he sets a course of one eighty degrees. He's actually going one sixty-six and way off course.'

'His compass was that much out?'

'Looks like it. And it must have gone wrong in Algiers because he got that far without trouble.'

I said, 'Why did your compass give different readings in here and out there?'

'Magnetic deviation,' he said. 'Remember what I told you at Assekrem about iron in the mountains causing trouble? Well, there's a lot of iron here. Up front there's a goddamn hunk of iron called an engine. That affects the compass reading. Now, that's a Wright Cyclone with nine cylinders and, in flight, all the spark plugs are busy sparking and sending out radiation. They tell you they can be screened but I've never seen anyone do a good job of screening yet. And there'll be other bits of iron about the airplane – the oleo struts, for instance.' He tapped the metal of the fuselage. 'This don't matter – it's aluminium.'

I said, 'What are you trying to tell me?'

'I'm getting to it.' Byrne stared thoughtfully at the compass. 'Now, you build an airplane, and you take a perfectly good compass and put it in that airplane and it gives you a wrong reading because of all the iron around. So you have to adjust it to bring it back to what it was before you put it in the airplane.' He pointed to the compass. 'Built in back of there are some small magnets put in just the right places to compensate for all the other iron.'

'And you think one of them fell off? Because of vibration, perhaps?

'Nope,' he said shortly. They're not built to fall off; they're screwed in real tight. And there's something else – any compass, no matter how good, will give a reading that's a bit off when you're flying on different courses. You see, the needle is always pointing in the same direction, to magnetic north; so when you change course you're swinging all your iron around the needle.'

'It's getting more complicated.'

This is the real point. Every compass in every airplane is tested individually because all airplanes have different magnetic characteristics – even the same models. The airplane is flown along different known courses arid the compass readings are checked. Then a compass adjuster does his bit with his magnets. It's a real skilled job, more of an art than a science. He works out his calculations and maybe adds in the date last Tuesday, then he makes out a deviation card for the residual errors he can't get rid of on various courses. I've been looking for Billson's deviation card and I can't find it.'