Which system has a restriction of six consecutive starts with a one-hour rest afterwards?

Which system has a restriction of six consecutive starts with a one-hour rest afterwards?

• A. APU
• B. Oxygen system
• C. Electrical generator
• D. Hydraulic pump

Answer: (A)

APU systems have start-cycle limitations because starting and stopping the turbine puts significant thermal and mechanical stress on the unit. Limiting to six consecutive starts within an hour, with at least a one-hour rest afterward, gives the turbine time to cool and the bearings and lubrication to return to normal conditions. This protects the APU from overheating and fatigue, reducing the risk of damage during high-demand periods. The other systems—oxygen, electrical generator, and hydraulic pump—do not have this six-start-per-hour restriction in standard practice. They may have other limits or operational considerations, but they aren’t governed by this specific start-and-rest rule. Therefore, the system with this restriction is the APU.

APU systems have start-cycle limitations because starting and stopping the turbine puts significant thermal and mechanical stress on the unit. Limiting to six consecutive starts within an hour, with at least a one-hour rest afterward, gives the turbine time to cool and the bearings and lubrication to return to normal conditions. This protects the APU from overheating and fatigue, reducing the risk of damage during high-demand periods.

The other systems—oxygen, electrical generator, and hydraulic pump—do not have this six-start-per-hour restriction in standard practice. They may have other limits or operational considerations, but they aren’t governed by this specific start-and-rest rule. Therefore, the system with this restriction is the APU.