20. Control Chart: Landing Gear Hydraulic pressure in the main cylinder of the landing gear of a commercial jet is very important for a safe landing. If the pressure is not high enough, the landing gear may not lower properly. If it is too high, the connectors in the hydraulic line may spring a leak.

In-flight landing tests show that the actual pressure in the main cylinders is a variable with mean 819 pounds per square inch and standard deviation 23 pounds per square inch. Assume that these values for the mean and standard deviation are considered safe values by engineers.

(a) For nine consecutive test landings, the pressure in the main cylinder is recorded as follows:

Landing number 1 2 3 4 5 6 7 8 9 Pressure 870 855 830 815 847 836 825 810 792 Make a control chart for the pressure in the main cylinder of the hydraulic landing gear, and plot the data on the control chart. Looking at the control chart, would you say the pressure is “in control” or “out of control”? Explain your answer. Identify any out-of-control signals by type (I, II, or III).

(b) For 10 consecutive test landings, the pressure was recorded on another plane as follows:

Landing number 1 2 3 4 5 6 7 8 9 10 Pressure 865 850 841 820 815 789 801 765 730 725 Make a control chart and plot the data on the chart. Would you say the pressure is “in control” or not? Explain your answer. Identify any out-ofcontrol signals by type (I, II, or III).