The 707 was designated the 720 when it was modified for short-to-medium routes and for use on shorter runways. Engineers reduced the fuselage length by 9 feet, changed the leading-edge flaps and later installed turbofan engines. Boeing built 154 of the 720s between 1959 and 1967. United was the very first operator of the Boeing 720, first flying the jet on July 5, 1960. Through the 1960s, American used a mix of Boeing 720 and 727 aircraft. In the 1960s, El Al's fleet comprised a mix of Boeing 707 and Boeing 720B aircraft. The 720's short-to-medium-range role was later filled by 727s and 737s.
Noise-abatement landing approach patterns, which were explored initially by NASA on the Boeing 367-80, were investigated in an extensive program conducted by an Ames team in collaboration with American Airlines and United Airlines. The purpose of the work was to develop an avionics system that would allow commercial jet transports to perform two-segment landing approaches under instrument flight conditions. The tests, flown in 1971 using an American Boeing 720 and in 1973-74 on a United Boeing 727 and Douglas DC-8 aircraft with crews from the airlines, FAA, and NASA, showed the effectiveness of a two-segment descent profile in substantially reducing noise on the ground under the approach path.
In 1984 NASA Dryden Flight Research Center and the Federal Aviation Administration (FAA) teamed-up in a unique flight experiment called the Controlled Impact Demonstration (CID), to test the impact of a Boeing 720 aircraft using standard fuel with an additive designed to suppress fire. The additive FM-9, a high molecular-weight long chain polymer, when blended with Jet-A fuel had demonstrated the capability to inhibit ignition and flame propagation of the released fuel in simulated impact tests.
Although the 720 was considered obsolete, its structural design and construction were still representative of narrow-body transport aircraft in use at that time. Two major objectives were included in the tests: (1) to test an antimisting kerosene fuel in an FAA program to reduce the severity of aircraft crash fires and (2) to study structural crashworthiness.
The additive, FM-9, a high-molecular-weight long-chain polymer, when blended with Jet-A fuel had demonstrated the capability to inhibit ignition and flame propagation of the released fuel in simulated crash tests. This anti-misting kerosene (AMK) cannot be introduced directly into a gas turbine engine due to several possible problems such as clogging of filters. The AMK must be restored to almost Jet-A before being introduced into the engine for burning. This restoration is called "degradation" and was accomplished on the B-720 using a device called a "degrader." Each of the four Pratt & Whitney JT3C-7 engines had a "degrader" built and installed by General Electric (GE) to break down and return the AMK to near Jet-A quality. In addition to the AMK research the NASA Langley Research Center was involved in a structural loads measurement experiment, which included having instrumented dummies filling the seats in the passenger compartment.
Before the final flight on December 1, 1984, more than four years of effort passed trying to set-up final impact conditions considered survivable by the FAA. During those years while 14 flights with crews were flown the following major efforts were underway: NASA Dryden developed the remote piloting techniques necessary for the B-720 to fly as a drone aircraft; General Electric installed and tested four degraders (one on each engine); and the FAA refined AMK (blending, testing, and fueling a full-size aircraft). The 15 flights had 15 takeoffs, 14 landings and a larger number of approaches to about 150 feet above the prepared crash site under remote control. These flight were used to introduce AMK one step at a time into some of the fuel tanks and engines while monitoring the performance of the engines.
On the morning of December 1, 1984, a remotely controlled Boeing 720 transport took off from Edwards Air Force Base (Edwards, California), made a left-hand departure and climbed to an altitude of 2300 feet. It then began a descent-to-landing to a specially prepared runway on the east side of Rogers Dry Lake. Final approach was along the roughly 3.8-degree glide slope. The landing gear was left retracted. Passing the decision height of 150 feet above ground level (AGL), the aircraft was slightly to the right of the desired path. Just above that decision point at which the pilot was to execute a "go-around," there appeared to be enough altitude to maneuver back to the centerline of the runway. Data acquisition systems had been activated, and the aircraft was committed to impact. It contacted the ground, left wing low. The fire and smoke took over an hour to extinguish.
The plane was instrumented for a variety of other impact-survivability experiments, including new seat designs, flight data recorders, galley and stowage-bin attachments, cabin fire-proof materials, and burn-resistant windows. Crash forces were measured, and a full complement of instrumented crash test dummies was carried on the flight.
The aircraft was remotely flown by NASA research pilot Fitzhugh (Fitz) Fulton from the NASA Dryden Remotely Controlled Vehicle Facility. Previously, the Boeing 720 had been flown on 14 practice flights with safety pilots onboard. During the 14 flights, there were 16 hours and 22 minutes of remotely piloted vehicle control, including 10 remotely piloted takeoffs, 69 remotely piloted vehicle controlled approaches, and 13 remotely piloted vehicle landings on abort runway. It was planned that the aircraft would land wings-level and exactly on the centerline during the CID, thus allowing the fuselage to remain intact as the wings were sliced open by eight posts cemented into the runway.
The Boeing 720 landed askew and caused a cabin fire when burning fuel was able to enter the fuselage. It was not exactly the impact that was hoped for, but research from the CID program yielded new data on impact survivability which helped establish new FAA rules regarding fire prevention and retardant materials. Although proponents argued that AMK prevented a hotter, more catastrophic fire during the CID, FAA requirements for the additive were put on the back burner.
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