Program alarms part II

Apollo 11 had already had a brief encounter with the computer throwing out program alarms during the braking phase. As Armstrong and Aldrin brought Eagle through the approach phase, the computer began to play up again.

"Eagle, Houston. You're Go for landing,'' said Duke in the Capcom chair.

"Roger. Understand. Go for landing. 3,000 feet,'' returned Aldrin, as they passed 1,000 metres altitude.

"Copy," said Duke.

Just then, Aldrin made a call that the computer was acting up yet again. "Program alarm. 1201.''

"1201," repeated Armstrong, then to Aldrin, "Okay, 2,000 at 50.'' They were 700 metres up, still flying forward and descending at 15 metres per second. The program alarm was distracting both crewmen from their practised task as Armstrong was looking at the DSKY for information instead of looking out of the window. Bales quickly told the flight director that the 1201 alarm was similar to the earlier 1202 alarm and that they could proceed. Duke passed it on. "Roger. 1201 alarm. We're Go. Same type. We're Go.''

"Give me an LPD,'' asked Armstrong with his eyes out of the window again. He wanted to know where the computer was taking them.

"Into the AGS, 47 degrees,'' Aldrin was working the secondary computer, the abort guidance system, as well as feeding numbers to Armstrong.

"47," replied Armstrong. "That's not a bad looking area. 1,000 at 30 is good. What's LPD?'' he asked Aldrin again. Armstrong didn't realise that the LPD was being interfered with by the propellant sloshing about in the tanks. As the great weights of liquid moved from side to side, they altered the LM's attitude enough to set the RCS thrusters into excessive activity trying to correct it, and the computer could not keep up with the attitude excursions.

"Eagle, looking great,'' said Duke from mission control. "You're Go.'' Then when another alarm appeared, he confirmed that the flight controllers were seeing it also. "Roger. 1202. We copy it.''

The program alarms were found to have been caused by a procedural error that had left the spacecraft's other radar, the rendezvous radar, in a mode that sent false information to the computer. Although the computer had indicated the existence of a data overflow by showing the code, its programming was intelligent enough to ignore the data and continue with the more important duties associated with the landing programs.

Since the computer was still doing its primary job flawlessly, despite the alarms, the crew returned to their roles; Armstrong looking out, and Aldrin keeping him abreast of the numbers. "35 degrees. 35 degrees. 750 [feet]. Coming down at 23 [feet per second].''

"Okay."

"700 feet, 21 [feet per second] down, 33 degrees."

"Pretty rocky area," said Armstrong. The erratic LPD angle had swung by a huge amount to 33 degrees and it was indicating that they were heading towards an area just outside a large crater known informally as West Crater for being situated west of the centre of the landing ellipse. It was common for the ejecta blanket around such a crater to include a scattering of large blocks. This did not look like a place he wanted to set down. Armstrong had never used the ability of P64 to redesignate his landing site. He was too preoccupied with computer alarms and the inability of the LPD to ever give him a trustworthy idea of where he was going. Nevertheless, he took control, made his decisions and carried them out.

"PICKING UP SOME DUST": P66

"600 feet, down at 19." Aldrin continued his litany of data while Armstrong weighed up his prospects. The computer was still behaving and otherwise the descent seemed to be going well. But he had to decide what to do about the blocky ejecta around West Crater.

"I'm going to..." he told Aldrin, and assumed manual control of the LM's attitude by changing to P66. He then pitched forward to an almost vertical attitude that allowed Eagle to maintain its horizontal speed and let him fly over West Crater and its boulder field. Once clear of the field, he pitched the LM backwards again to resume cancelling the craft's horizontal speed, and searched for somewhere to bring it down.

P66 looked after the LM's vertical speed, also known as its rate of descent (ROD), by adjusting the throttle to maintain a desired value. The commander had a ROD switch that he could flick up or down momentarily to increase or reduce the rate of descent by fixed increments. At the same time, his hand controller let him adjust the vehicle's attitude, which gave him control of horizontal speed, very much in the manner of a hovering helicopter. Tilt to the left and the engine would aim slightly to the right, pushing the LM towards the left.

"100 feet, 3% down, 9 forward,'' called Aldrin, "Five per cent. Quantity light,'' he added.

A light had come on to indicate that they had only 5.6 per cent of their propellant remaining. From pre-flight analysis, planners had decided that, from this point, they could only fly safely for another 114 seconds before either getting the LM onto the surface or aborting. A 94-second countdown began in mission control that would lead to a call for the crew either to abort or land. If the commander felt he could get the ship down in the remaining 20 seconds, then he could continue, otherwise he had to get out of there by punching the abort button.

However, Apollo 11's slosh problem had fooled them again. By triggering the quantity warning light early, it made them believe they had less propellant than was actually available and it came very near to causing an unnecessary abort. A set of fold-out baffles were retro-fitted to Apollo 12's LM but they were not very effective. It wasn't until Apollo 14 that the slosh problem was resolved.

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