9 May 2012: I've made some changes over the past few days based on new information and analysis. Stay tuned if this subject is of interest because I'm sure that this is not the last I'll have to write about this.
4 May 2012:
Major update and correction. I had naturally assumed that the color indicated by the approach light first introduced on carrier-based airplanes in the late 1930s had the same meaning as the approach lights do today: red = fast, green = slow, amber = on speed. However, Larry Webster of the Quonset Air Museum provided me with an F3D maintenance manual excerpt that clearly illustrated the opposite.
Note that the angle lines on the vertical fin were used to establish the 14-degree setting for the amber beam.
I had thought it to be a very unlikely error but still an error until I went back and reread Gerry O'Rouke's account in his book,
Night Fighters over Korea, of watching night field-carrier landings of VC-4 F3Ds in late 1952/early 1953:
As the low, moving lights came around, settling even lower toward the trees, the dim silhouette of the airplane had almost to be imagined. It was there, outline by the wing and fuselage lights, almost shrouded by the multicolored approach light that told the LSO what the plane's flight attitude was. Too slow was red, on speed was amber, too fast was green. The sound of the engines mounted, and small changes in throttle settings were detectable through ever-so slight changes of pitch in their whine.
The LSO stood with arms fully extended, then slowly lowered them slightly. The plane's engines picked up a new whine and the lights seemed to rise. The LSO's arms rose to level. The approach light, initially amber, switched to red momentarily, then to amber, as the pilot raised, then flattened the plane's nose attitude to stay on altitude.
It is unlikely that O'Rourke misremembered, since red for slow was the reverse of his subsequent and extensive experience with the red-for-fast three light AOA-driven system.
It appears that for some reason, the convention was reversed when the angle-of-attack driven lights were introduced. I'll note that red seems more appropriate for slow initially, since the concern would be for being too slow, risking a stall on approach and crashing into the sea, rather than too fast, which at worst meant a crash into the barriers forward of the landing area.
30 April: More revisions and additions
29 April: Added an illustration
27 April: And a few more revisions made today
26 April: As a result of comments and more research on my part, I somewhat revised this entry today...
Like the American Indians who supposedly preferred not to fight at night, Navy carrier pilots before and even during World War II only landed on a carrier at night if absolutely necessary. The increase in degree of difficulty was significant.
However, it was a necessary capability so the airplanes were equipped for it and the pilots trained for it, beginning with shore-based evaluations at night of deck-lighting concepts concurrent with the commissioning of the U.S. Navy's first carrier,
Langley. Actual night landings were accomplished aboard
Langley for the first time in April 1925 off San Diego. However, night landing qualification of carrier-based Naval Aviators was not required until January 1929 and to be current, only four were required per year. Most were reportedly accomplished by the light of a full moon or the twilight following the setting sun.
During daytime, the Landing Signal Officer (LSO) could deduce an aircraft's speed, both actual and any trend toward faster or slower, from its attitude (the relationship of the wing and horizontal tail), the sound of the engine, and whether it was climbing or descending. Stripes were eventually added on the vertical fin to add more precision to the determination of attitude. See
http://thanlont.blogspot.com/2009/12/reason-for-those-lines-on-vertical-fin.html.
At night, the existing wing-tip-mounted position lights provided a sense of roll attitude along with the glare from the exhaust stacks. What was needed was a means of determining pitch attitude. Initially, this was a light or lights on the aft fuselage and/or stabilizer. Admiral Chick Hayward described the technique used at night in 1934 in his book,
Bluejacket Admiral.
For starters, we were assembled by the landing signal officer's platform on the flight deck (of the Langley) to watch the LSO, Lt. Walter Holt, and a night-landing veteran, Lt. Giles E. Short, in a Boeing F4B, show us how to do that. As Short flew in, Holt, with lighted batons, was to signal him, "Too high...low...level off," whatever, until two lights on the F4B were lined up. When they were, that told the LSO the F4B was "in the groove," and he'd wave it on in. Finally, he'd signal the pilot to cut his engine when that would drop the plane down into the arresting cable.
Unfortunately, Short was apparently cut too early on this particular evening, because he crashed into the water. The plane-guard destroyer picked him up unhurt. According to Hayward "It did not stir in us any enthusiasm for night landings on the
Langley." As it happened, it was subsequently determined that it was safer for Hayward's squadron to night qualify on the much bigger
Saratoga.
It's not clear what lights that the LSOs initially used at night for determination of pitch attitude. (Strictly speaking, "lining up the lights" would also only work if the airplane were on a descending approach, not the level approach then used, at least in daytime.) There are two candidates evident on carrier-based airplanes at that time, on the turtle back and on both sides of the horizontal stabilizer. This BFC-2 at the National Museum of Naval Aviation at Pensacola has both:
These are generally referred to as position or formation lights, which they may in fact have also been and simply adopted as a reference point for night carrier landings. (The turtle back light was also used at night to signal instructions before inter-plane communication by radio became standard.)
Some of the early monoplanes had what had come to be known as the approach light mounted just in front of the stabilizer, a more appropriate location given the use of the stabilizer as a reference in daytime conditions.
The aft-mounted approach light was subsequently augmented and then replaced before World War II with an approach-light unit mounted in the leading edge of the left wing* that placed red, yellow, and green-colored lenses in front of the light bulb. Depending on the angle it was viewed at, the light would appear to be red, yellow, or green to the LSO. Red meant too slow (nose up, tail down); green, too fast.Yellow was neither too slow or too fast.
This is the approach light installation in the XF4U-1:
The operation of the light was automatic. (A bypass switch was provided so the light would come on even when the tailhook was up and it could therefore be used by an LSO during field carrier landing practice.) Initially, the light came on when the tailhook was lowered. Before I looked at an F6F Hellcat wiring diagram provided by Larry Webster, I had thought that the exterior lights had to be on as well but apparently not on the F6F; it appears that the exterior lights had to be on for the F3D approach light to receive power.. Later, the operation of the light was enabled by the lowering of the landing gear: otherwise, the light remained off regardless of the position of the tailhook; when the landing gear was down, the light flashed if the tailhook was up and was on steady if the tailhook had been lowered. This provided a positive indication at night that both the landing gear and hook were down.
This is the light as installed in an F9F Cougar on display at the National Museum of Naval Aviation in Pensacola. Note that a high nose angle results in the light shining through the red lens from the LSO's view point. The box looks like it might be installed upside down because the angle markings that were used for adjustment to show an amber light at the correct approach angle aren't visible and there is no hole on the bottom of the box for the screwdriver used to adjust the angle of the box (see the F3D approach light illustration above.)
Here is the F2H maintenance manual illustration showing the relationship of the lenses and single light bulb and the means of adjusting the approach light, which was Government Furnished Equipment provided to the airplane's manufacturer for installation, to the proper angle for the airplane that it was installed in:
This gadget had a limitation with respect to providing the LSO with an accurate assessment of the aircraft's speed/angle of attack, since other factors like height, distance from the ramp, changing pitch, etc. would affect what color the LSO saw. According to the Boeing XF8B-1 pilot's manual, the LSO would see yellow when the airplane was 100 feet astern, 25 feet above the deck, and 10-12 knots above stall speed.
When the Navy added an angle-of-attack gauge in the cockpit of jets beginning in the mid-1950s, the three-lens approach light was replaced with a three-light system that provided the LSO with an indication of the approaching airplane's angle of attack regardless of its position relative to the desired glide slope and trend to or from that angle. It is still in use today—day and night—and consists of three separate lights mounted on the nose gear strut, in the leading edge of the left wing, or in the nose (E-2).
For some reason, the meaning of the lights was reversed: now red
indicated fast (low angle of attack) and green, slow (high angle of
attack), with yellow being "on speed," or strictly speaking, at the
desired angle of attack. My guess is that the convention was reversed roughly when the angled
deck, descending approach, and angle of attack indication for jets were
introduced, i.e. the early to mid 1950s. My thinking is that red for
slow was important for the level approach at minimum speed to axial-deck
carriers because of the risk of stall. Angle of attack indication and a
descending approach somewhat minimized that risk; fast may therefore
have then been more of a concern due to arresting system and aircraft
hook loads at the higher approach speeds of jets as well as the fact
that being fast on a fixed glide slope meant a higher sink rate,
something to be avoided as well.
Ed Barthelmes confirmed that the AD Skyraider three-lens approach light interpretation was reversed from the F3D convention, at least later on, and the same as the current one, i.e. red indicating fast.
The switch in convention undoubtedly applied to all carrier-based airplanes at the same time, at least in the same air group. It appears to have been of importance only to maintainers and LSOs and was therefore not communicated to those who had no need to know since I can find no mention of it in Naval Aviation News or other documentation.
The E-2 approach lights are mounted horizontally in the nose.
However, it also has a blue position light on the lower left aft fuselage that LSOs can use to determine pitch attitude at night, reminiscent of the original approach light implementation.
For a little more on LSO history, see
http://thanlont.blogspot.com/2009/05/waving-them-aboard.html
*An exception was the A4D-1 installation, which was on the
right side of the nose landing gear strut. Presumably mounting it "externally"—rather than behind a hole in the wing covered by a plexiglass panel—was lighter (it also appears to be miniaturized) and it couldn't be on the left side of the strut because of the location of the shrink strut that compressed the nose gear as it was being retracted.(On later models, the three-light presentation was moved to the leading edge of the left wing.)
Ron Lewis reports that the red lens was on top from the first production A4D-1s, further evidence that the change was as early as 1954.
