ArduPlane:bug fix #1453

Job	Run time
build (base)	3m 57s
build (clang)	4m 27s
build-gcc-heli	4m 12s
autotest-heli (sitltest-heli)	11m 32s
autotest (sitltest-copter-tests1a)	13m 6s
autotest (sitltest-copter-tests1b)	10m 54s
autotest (sitltest-copter-tests1c)	13m 6s
autotest (sitltest-copter-tests1d)	10m 44s
autotest (sitltest-copter-tests1e)	10m 48s
autotest (sitltest-copter-tests2a)	11m 5s
autotest (sitltest-copter-tests2b)	33m 53s
	2h 7m 44s

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