By Tommy H. Thomason

Thursday, January 27, 2011

The F-111B versus the F-14A, One More Time


I recently had the incentive to revisit this diatribe in the process of responding to a request from another author about the F-111B program. As before, this assessment was made using the respective Standard Aircraft Characteristics charts, the F-111B’s dated 1 July 1967 and the F-14A’s, dated April 1977. While the argument can be made that the F-111B SAC did not reflect its final weight, I believe that the same argument can be made concerning the one used for the F-14A, so it’s at least a pretty close apples-to-apples comparison. Note: In the following discussion, the F-14A is penalized with the weight of an internal gun and ammunition whereas the F-111B is penalized with the weight of the original Airborne Missile Control System (AMCS) design, roughly the same.

Much has been made of how terribly overweight the F-111B turned out. And it was, compared to a totally unrealistic specification. Many think that the F-14A was far lighter than the F-111B, primarily because most comparisons neglect to do so using the F-111B’s design mission for both aircraft. The F-14A is still lighter, of course, because the Navy changed its requirements so that it would be. Deleted were the escape capsule, bomb bay, and swiveling wing pylon stations among other things. The Hughes Airborne Missile Control System, given a few more years of development, was lighter. The structure was designed for 6.5 gs at 49,548 lbs, about 10,000 pounds less than the F-111B’s design gross weight at that g level. In effect, the six Phoenixes and 3,800 lbs of fuel were treated as an overload for the design of the F-14A structure. At combat weight (13,800 lbs fuel and six Phoenix missiles) the F-111B therefore had a load limit of 5.8 g and the F-14A (12,000 lbs of fuel and six Phoenix missiles), a lower (but not particularly constraining) 5.2 g. The result, however, is a somewhat lower structural weight for the F-14A.

According to the F-111B SAC, when it was loaded with full internal fuel and six Phoenixes, it weighed 77,566 lbs and required 11 knots wind-over-deck on a tropical day for launch; the F-14A, not surprisingly, weighed almost 7,000 lbs less but, surprisingly, required 16 knots wind-over-deck, five knots more than the F-111B. Moreover, at its takeoff gross weight the F-111B was carrying 3,000 lbs more fuel than the F-14, making the difference in takeoff gross weight for the same amount of fuel and weapons carried only 3,866 lbs, or 5%, not exactly the amount or percentage difference that most would have guessed given all the negative publicity garnered by the “Sea Pig.” With that additional fuel, the F-111B could loiter on station for 1.5 hours with the combat fuel allowance assuming an acceleration to 1.5 Mach; the F-14A with the two external tanks of overload fuel, and with the same combat Mach number (one has to read the SACs very closely), could only loiter for 1.1 hours.

As for landing, they were both heavy. In fact, the maximum arrested-landing weight limit of the F-14A precluded it from landing back aboard with all six Phoenixes, whereas the F-111B had a 5,000 lb margin, all fuel, between its maximum landing weight and the landing weight with the standard landing fuel load of 2,417 lbs of fuel and six Phoenix (56,980 lbs). One does not need to be a Naval Aviator to appreciate being able to land with three times the required fuel. On a tropical day at the standard weight, the F-111B needed 15 knots wind-over-deck for landing; the F-14AA could only land with five Phoenix, and even then needed 17 knots wind-over-deck at its maximum landing weight of 51,830 lbs. The F-111B was also less of an handful following an engine failure since its engines were not as widely separated as the F-14A’s.

For a little more on the F-111B at-sea trials often used to condemn the F-111B's carrier compatibility, see http://thanlont.blogspot.com/2009/03/f-111b-carrier-trials.html

This is not to say that the Navy didn’t do the right thing in getting the F-111B program cancelled and replacing it with the more versatile F-14, particularly since the Hughes AMCS wasn’t ready for prime time (the Navy's Phoenix program was about two years behind schedule and production F-111Bs were about to be delivered). However, with respect to its Fleet Air Defense design mission, it got an airplane that could not loiter as long or land with its full complement of missiles, had a higher stall speed, required more wind-over-deck for takeoffs and landings, and was more difficult to bring aboard with two engines running, not to mention with one inoperative.

So which is the real "Sea Pig" then?

My answer is neither of the above. The F-111B could do, pretty much, the Phoenix-based Fleet Air Defense (FAD) mission that it was intended to do while weighted down with Air Force low-level supersonic mission and other requirements. The F-14 could not do the FAD mission quite as well—but well enough if needs be—and it could also accomplish the carrier Navy’s other, equally important, fighter missions.

2 comments:

Captain Mark said...

I note that the F-111 remained in service with the RAAF until 2010 while the F-14 was withdrawn in 2006. [I ignore the Iranian F-14s, if any remain, because they don't have a choice.] This supports the notion that the Navy erred in rejecting the F-111 and requiring the F-14.

Jeff J said...

Interesting comment Capn. I left McDonnell (Field Rep Avionics) in late 67 to go with Grumman A-6A. All I heard from Grummies was how bad the F-4 was. Used to really tick me off. I never saw what use the Phoenix was going to be after being in Thailand with 8th TFW . No BVR in all Route Pak's in N Viet allowed.