By Tommy H. Thomason

Thursday, December 10, 2009

The Reason for Those Lines on the Vertical Fin

I thought I'd mentioned this before in this blog but apparently I hadn't, because I can't find it. Back before the pilot and the Landing Signal Officer (LSO) were provided with indicator lights to insure that the airplane was at the correct angle of attack on approach, the left side of the vertical fin on most carrier-based airplanes* was marked with precisely positioned stripes. By noting which ones he could see with respect to a fixed point on the airplane, like the wing leading edge, the LSO could determine the airplane's angle of attack and signal the pilot to speed up or slow down if required.  These stripes were somewhat exaggerated on the mockup of the Douglas F4D Skyray and I'm not sure that the top one is properly located, but they illustrate the concept.

A closeup shows that the lines were marked in degrees.

The more subtle angle of attack markings used operationally are just evident on the leading edge of the fin of this F9F Panther.

These markings weren't of use at night, of course. In that case, the LSO relied on an "approach light" in the leading edge of the left wing. There were three colored lenses in front of the light so that he saw green for too slow, red for fast, and amber for on-speed based on the attitude of the aircraft at the proper approach altitude. The light only came on when the hook was lowered so it provided a positive indication that the hook was down at night. (A field carrier-landing switch was provided for practice night approaches ashore.) LSOs also used the relative orientation of the running lights (and on propeller-driven airplanes, the flames from the exhaust stacks) to determine the speed (actually angle of attack) of the approaching airplane.

Within a few years after angle-of-attack measurement and cockpit display were added to Navy carrier-based jets in the early 1950s, the LSO was provided with an angle-of-attack indication via three lights. Again slow (actually, too high an angle of attack) was green, on-speed was yellow, and too fast was red. The lights were generally located in a box mounted on the nose landing gear as shown here.

*I don't know why, but F2H-2 Banshees didn't have these stripes even before angle of attack indication became available. On the FJ-1 and F2H-1, they were placed on the nose - on both sides of the FJ-1, probably because North American didn't know what they were for...


Erik said...

It's almost impossible to believe those would be visible in any but beyond-ideal circumstances... one has to think LSOs had more stringent vision requirements than the pilots they were guiding!

Anonymous said...

Dave says...
I'm confused - you mentioned green for too high AND too low?!

Tailspin said...

Oops - When I did an edit to revise the order in which I listed the colors, I failed to change one. Thanks for catching that. I've fixed it.