On many occasions, resupply by air has saved a beleaguered outpost. Often the difference between success and failure in an operation is the aircraft available to move supplies in, move the wounded out and redeploy the troops.

T

THE HONORABLE HAROLD BROWN, Secretary of the Air Force

HAT the defense of the Marine base at Khe Sanh would prove one of the most important campaigns of the Vietnam war, militarily and politically, was a foregone conclusion from the start. Even before the siege was lifted on 8 April 1968, nearly three months after it began, Khe Sanh was being called a clear-cut victory, a dramatic reversal of earlier nonmilitary warnings of disaster and predictions of another Dien Bien Phu. The successful defense of Khe Sanh was maintained primarily by the dogged determination of a small group of U.S. Marines, supported by Army and Army of the Republic of Vietnam (ARVN) forces, and a massive and skillful application of air power, which included, as an integral component, tactical airlift. The demands for aerial resupply tested tactical airlift in its classic role and vividly illustrated that airlift is, and must be, an extension of the battle, not divorced in any way from the realities of success or failure of an engagement.

The evolution and operation of the sustained air lifeline to Khe Sanh were part of a shifting tactical situation not uncommon at the time in South Vietnam. There had been, since late 1967, indications of extensive enemy buildup below the western part of the demilitarized zone, the area around Khe Sanh, signaling the possibility of a major offensive. From the beginning of the northern monsoon season and throughout the Christmas truce, enemy activities increased in all parts of the Republic. The major threat appeared to be in the northern I Corps area, and Khe Sanh,

a strategic point on a vital North Vietnamese infiltration route, appeared to be a key target. In early January, an estimated two North Vietnamese divisions, 15-20 thousand troops, were reported in the Khe Sanh area, occupying the high ground surrounding the base. To counteract the threat in the northern provinces, several U.S. and ARVN units were airlifted to that area, including the 1st Air Cavalry Division and two brigades of the 101st Airborne Division. Marines and ARVN forces at the Khe Sanh base had been reinforced to a defensive 6000.

What happened during the next three months will constitute a lasting chapter in both the history of airlift and the final chronicle of the Vietnam war. This article will trace the decisions taken during those three months, the rationale behind them, and their consequences, in the hope of providing an insight into that chapter on the air resupply of Khe Sanh. Further, it will attempt to put the operation into proper perspective by revealing its impact on the overall airlift system in Vietnam and assessing the significance which the lessons learned will have on the future development of tactical airlift.

Evolution and Operation
of the Resupply

On 21 January a rocket and mortar attack destroyed the Khe Sanh ammunition storage area, scattering shrapnel over half of the runway and preventing C-130 Hercules from landing. Under a tactical emergency designation (the highest airlift priority), six C-123 Providers were diverted from their scheduled missions, loaded with 26 tons of ammunition, and flown to Khe Sanh. They landed on the potholed runway, by the light of artillery flares, under heavy automatic weapons and mortar fire. Thus began the saga of Khe Sanh, which eventually ran into 78 days of emergency aerial resupply sorties to the embattled outpost.

At the outset the Commander, Seventh Air Force, then General William W. Momyer, directed the 834th Air Division, airlift head

craft and aircrews; and (3) minimize load recovery problems for the Marines.

In addition to enemy activity, a major operational consideration was the weather. At that time of year the spring monsoon brought chronic fog and rain to northwestern Quang Tri Province. The particular geographical situation of the base compounded the problem. Even when visual meteorological conditions (VMC) prevailed in the surrounding area, frequently the runway was fog-shrouded below minimum instrument landing conditions each day through mid-morning. Khe Sanh's field elevation is 1500 feet above sea level; surrounding mountains tower 5581 feet on the north and an average of 3000 feet in other directions. A ravine on the runway's east end drops approximately 800 feet and acts as a trough through which prevailing winds carry the warmer, moist air to the airstrip. This is Khe Sanh's perpetual "fog factory" during the monsoon season, accounting for relatively short periods of ground-controlled approach (GCA) minimum visibility conditions each day.

This weather condition, coupled with daily mortar and rocket attacks that closed the runway, reduced the productive resupply day to approximately 2% to 3 hours. Successive aircraft landings brought an increase in the intensity of the attacks, indicating the enemy's determination to destroy the aircraft on the ground.

By early February, landings had become extremely hazardous for both the jet-equipped K-series C-123 Providers and the larger, heavier C-130 Hercules. While any aircraft attracted mortar and artillery fire at Khe Sanh, the smaller Providers, requiring shorter runway space to land, were less susceptible to the intense small-arms fire than the Hercules. On 12 February all C-130 landings were suspended. Events culminating in the edict underscored the Hercules' special vulnerability on the runway in that environment: enemy fire destroyed a U.S. Marine Corps C-130 and hit one belonging to the Air Force, grounding it for two days before sufficient repairs could be made to enable its crew to fly it to Da Nang.

The suspension forced the obvious: the

primary method of resupply would be by airdrop, delivering ammunition, POL, and rations. Airland operations were reduced to delivery of cargo considered not airdroppable (special ammunition and medical supplies), troop deliveries, and evacuation of casualties. The C-130 and C-123K were designated for the airdrop role; the C-123K was designated as the airland vehicle, with the C-7A Caribou as backup as needed. In view of the Caribou's relatively limited carrying capacity and the extremely congested air traffic situation at Khe Sanh, C-7As deferred to the more efficient C-123Ks to get twice as much cargo on the ground for practically the same risk and effort.

development of the IMC technique

Our two major antagonists in this logistics drama-weather and enemy fire-were formidable obstacles throughout the entire operation. Climatology data indicated from the start

that there would be days at a time when the weather would be below minimum conditions. Thus, concurrent with the initial airlift sorties, planning for possible contingencies began. With airdrop a certainty as the primary mode of resupply, procedures had to be developed to drop in instrument meteorological conditions (IMC). To that date, no techniques had been established for dropping solely by instruments. Several techniques were, at the time, being explored, but none were operational.

A survey of Khe Sanh revealed the fol

lowing:

-The drop zone was 300 x 300 yards, 200 yards shorter than the standard for container delivery system (CDS) drops. It was located outside the Marines' defense perimeter, 1000 yards from the west end of the runway and within three degrees of the runway heading.

-The runway was 3900 feet long and 60 feet wide, declining on a three percent gradient from west to east.

-The airfield had GCA capability and a TPQ-10 radar site.

devised, accounting for wind effect, providing the navigator the lapsed time from the mark

-The terrain was hazardous, rising steeply on three sides of the field, and enemy gun emplacements were on all sides.

-Turbulence was expected to be moderate. An airlift planning session at the 834th Air Division resulted in several alternatives and, eventually, a formulation of the instrument procedures to be used. Dropping on the runway was ruled out because of the possibility of damaging the surface at a critical time and thus precluding any emergency airland operations. The initial inclination was to have the GCA operator vector the aircraft to the computed air release point (CARP), about 600 yards short of the point of impact (PI) for CDS drops, with the ground controller providing a countdown to initiate the drop. A quick flight test, however, revealed that the GCA, because of design limitations, was incapable of vectoring the aircraft to a point located behind the radar itself, which in this case was the location of the CARP. The GCA could vector the aircraft to a point above the leading edge (threshold) of the runway, so a method of accurately steering the aircraft from that point to the CARP had to be resolved. Dead reckoning (DR) was ruled out; flight test proved the aircraft could be as much as 500 yards off track when using DR, which was totally unacceptable because of the size and location of the drop zone. Cross track was resolved by use of the C-130's Doppler system in the drop function. By combining the features of the GCA and the capabilities of the Doppler, the IMC technique was developed and put to immediate use.

In practice, the GCA Operator vectored the aircraft over the threshold of runway 28 at 500 feet drop altitude and gave the crew a "mark." Ground speeds and winds were determined during the approach by direct readings from the Doppler instrumentation, and the desired course to the CARP was set into the Doppler instrument, establishing the correct heading. A precomputed timing sheet, based on a 130-knot indicated air speed, had been

For example, at 135 knots ground speed, it took 24.8 seconds to fly that distance. The "green light" was given at the precise lapsed time, and a CDS drop performed. However, instead of using the standard pullup at the CARP, which is used in a visual meteorological condition, we decided that a smooth application of power combined with the effect of a load moving aft would adequately eject the containers and be more suitable for instrument conditions.

other planning

Although this IMC/GCA technique served as the primary method of instrument delivery, we had to plan for all possible eventualities. Clearly, GCA was the key to the functioning of the entire IMC technique. But it was vulnerable to incoming enemy fire, and airlift, as Khe Sanh's only means of supply, could not cease even for one day. A backup system had to be developed.

Fortunately, a TPQ-10 radar system, normally used to direct fighter strikes, was located on base and was operational. As in the GCA/IMC formulation process, initial flight tests investigated the TPQ-10's ability to vector the aircraft directly to the CARP. But the aircraft passed at such close proximity to the radar antenna that the antenna azimuth drive could not vector the aircraft down a predetermined glide slope; it could only position an aircraft at a point in space. Therefore, additional navigational aids were required, to guide the aircraft down the glide slope to the proper altitude at the runway threshold. Although Khe Sanh did have a tactical air navigation station, TACAN reception on C-130 aircraft is not considered reliable enough; and an airborne radar approach (ARA), using radar reflectors previously positioned along the runway, was not acceptable by itself. The combined inputs from the TPQ-10, TACAN, and ARA, with the addition of a radar beacon transponder and low-frequency beacon, however, provided an acceptable method of descent to