|The Army Lockheed AP-2E Neptune on display at Fort Rucker|
During the Vietnam War, the Army had a whole host of aircraft for signals intelligence (SIGINT) and electronic warfare based on general aviation aircraft. These aircraft were loaded with electronic equipment and studded with external antennae and fairings to house the sensors. Operated by the Army Security Agency, missions flown by these unusual aircraft were to detect, identify, and localize enemy radio transmitters and then use electronic warfare to disrupt the networks. The most common types in use for this role were based on the U-8 Seminole (itself based on the Beech Twin Bonanza), the U-21 Ute (based on the Beech King Air 90) and the Grumman OV-1 Mohawk. These three airframes formed the backbone of Army Security Agency SIGINT/EW aircraft in Southeast Asia. However, as early as 1965, the Army realized that bigger aircraft would be needed, particularly as operations expanded in scope to include Laos and Cambodia. A bigger aircraft offered a greater payload capacity as well as more range to cover an expanding theater of operations. At first, thought was given to modifying the De Havilland CV-2 Caribou for the signals intelligence role, but given that these transports were soon to be handed over the USAF, it made little sense to fund the modification of an aircraft that would given to the USAF as the Army shifted focus on heavy-lift helicopters like the CH-47 Chinook from fixed wing transports.
The US armed forces during Vietnam was a different organization in many ways from what it is now with a stress on joint operations with the different branches contributing to common goals. Inter-service rivalries were much more pronounced in those days and went back to the Second World War. With the Army losing fixed wing aviation to the newly independent United States Air Force in 1947, the USAF wasn't exactly who the Army wished to turn to in a need for a larger aircraft that could be modified for signals intelligence, particularly since the USAF had been lobbying hard to take over the Army's Caribou transport fleet. Throughout the postwar period up to Vietnam, the Army's various attempts at developing its own aviation resources had an unlikely ally in the US Navy. So when the Army needed a larger aircraft for the SIGINT mission that wasn't going to provoke the USAF, the Navy was ready to help with the offer of Lockheed P-2 Neptunes. The Neptune had been in service with the Navy since 1946 and over a thousand had been built by the time the Army was looking for a new platform. The aircraft had already flown intelligence and electronic warfare missions for the Navy and even the USAF in limited numbers. With several Navy patrol squadrons in Southeast Asia still using the Neptune for maritime patrol, the logistical and maintenance infrastructure was already in place. Given that Neptune was a lot larger and more complex than any other fixed wing type the Army operated, Navy assistance was vital. As several stateside Naval Reserve squadrons were already operating the Neptune, several could be "loaned" from those units without adversely affecting frontline readiness in Southeast Asia.
Twelve P-2E Neptunes were passed to the Army by the Navy for use in Vietnam with the first aircraft delivered to the Army Security Agency in 1966. The aircraft were extensively modified in the interior to house the necessary electronic equipment with the only external clues to their role being extended wingtip tanks to house sensors, some extra antennas, and a solid nose in place of the usual transparent observer's compartment. The white over gray colors was kept with only "ARMY" titles aft of the national insignia. The aircraft were designated AP-2E, but if the Army were following the designation rules correctly that had been established in 1962, the Army Neptunes should have been RP-2E given their intelligence tasking, but the Army designated them AP-2E to avoid attracting the ire of the USAF. With a crew of 15 and jammed with electronic equipment, the Army AP-2Es were the heaviest and most complex of the Neptune variants to fly.
|1st ASA Company's "Crazy Cat" emblem|
The Neptunes were based at Cam Ranh Bay where the air base there already had a sizable Navy presence, so the Army Neptunes wouldn't have attracted much attention. From July 1967 to April 1972, they were operated by the 1st Army Security Agency Company which had the cover designation 1st Radio Research Company. Many sources refer to the unit by the cover designation, but they were officially the 1st ASA Company and were nicknamed the "Crazy Cats". The unit also operated RU-8 Seminoles and RU-21 Utes, which led to the the AP-2Es to be fitted with some of the same equipment as well as early versions of later SIGINT/EW suites. For what I can determine, some of the equipment included what was called "Left Foot"- this was a 360-degree SIGINT system that started out in 1970 under the code name Left Bank which then was improved to be come Left Jab on the RU-21s. Left Jab was the first airborne SIGINT system to use a digital computer and combined both intelligence and navigation in the same system to enhance its accuracy. Left Foot came next and combined the digital computer from Left Jab with the direction finding system from what the RU-21s used which had the code name Laffing Eagle.
Another system installed on the Neptunes was called CEFLY Lancer. CEFLY stood for Communications and Electronics Forward Looking Flying. CEFLY Lancer was also used on the RU-21s, but it's believed that the increased room on the Neptunes, development work on CEFLY Lancer took place during the missions. This sensor suite was designed to intercept communications. It's also believed that the early versions of another system called CEFIRM Leader also flew on the Neptunes. This system was designated AN/ULQ-11 and was a direction finding and communications jamming system. Officially CEFIRM Leader didn't become operational until 1973 as the Army's first multi-mission intelligence and electronic warfare suite, but early versions were likely test flown in Vietnam.
At the end of 1972, the last of the AP-2Es were returned to the Navy with a single example going on display at the Army Aviation Museum at Fort Rucker. By that point, developments in signals and communications intelligence as well as electronic warfare as well as the end of US involvement in Vietnam made the AP-2Es redundant. Many of the systems used on the Army Neptunes laid the groundwork for the later multirole Guardrail system that's still in use today by the Army.
Source: US Army Aircraft Since 1947 by Stephen Harding. Specialty Press, 1990, pp 171-173. Photos: Wikipedia.