Boeing's CH-47D Chinook - A small problem with the Droop Stops.

Boeing CH-47 helicopter - A Vietnam Experience.

The Boeing CH-47 in Vietnam.

The Boeing CH-47D and Droop Stop failure.

Two Tail Numbers, Same Results.

Hillclimber

DATELINE: Beaumont, Texas, October 2000. 47 hours out of On Condition Maintenance (OCM) conducted at Corpus Christi Army Depot (CCAD), Chinook 89-00149 slung an Aft Blade Fixed Droop Stop. The aircraft was picked up from CCAD on 2 October 2000 and proceeded to conduct operations at the Joint Readiness Training Center (JRTC). During the return mission to home station in Hawaii, the B Company, 214th Aviation Regiment aircraft landed at Beaumont for upload on a surface ship. Upon arrival and during engine shutdown, one of the Fixed Droop Stops was missing and, as the rotor blades slowed, a blade struck the fuselage.

13 November 2000. Summary of problem: An accident occurred during shutdown of a CH-47D. An Accident Investigation was conducted and the findings indicated that two incorrect bolts were installed in the Droop Stop on the Red Blade of the Aft Rotor Head Assembly. A shorter bolt, P/N NAS624H5, was installed instead of the correct bolt, P/N NAS624H7. There is a 1/8 inch difference in the length of the bolts. Approximately the last three threads were removed from the NAS624H5 bolts causing the Droop Stop to separate from the Pitch Varying Housing. It appears the excessive force on the threads, along with loads seen during Droop Stop contact, caused the threads on the bolts to fail.

The photographs below show what can happen when a Droop Stop fails on the Aft Rotor Head. The Droop Stops normally prevent the Rotor Blades from dropping below a certain level during engine shutdown. Should a Droop Stop fail, in this case the mount bolts had broken off, then the Rotor Blade will droop low enough to contact the fuselage resulting in considerable airframe damage.

Each blade on the Chinook has a Droop Stop, for a total of 6 Stops per airframe. There are three fixed Droop Stops on the Forward Rotor Head and three fixed Droop Stops on the Aft Rotor Head.

Additionally, the Aft Rotor Head has a Centrifugal Droop Stop System. The Centrifugal Droop Stop System consists of a spring loaded Interposer Block that is positioned between the Fixed Droop Stop, mounted just below the Horizontal Hinge Pin, and the Rotor Hub. The Interposer Block moves out of the way at about 70 RPM due to centrifugal force. This results in improved performance and aerodynamics by allowing the Rotor Blade to flap up and down during it's rotation around the tip path plane.

NOTE: The Aft Rotor Blade is considerably lower than normal:

Boeing CH-47D helicopter 89-00149 - Droop Stop failure.

There is considerable damage to the left side of the fuselage just aft of the Forward Pylon, the removal of the Number Two Tunnel Cover, and the severe damage to the primary flight control push pull tubes rendering them useless:

There is extensive damage to an Aft Rotor Blade shown in the photograph below. When the Aft Red Droop Stop departed the aircraft, the next item to fail was the Kevlar wrapped Dampner Mounting Lug on the blade. Instead of a normal rotor blade lead and lag of approximately 4 feet at the blade tip, with the lug broke off it became 8 to 12 feet. The Red Blade played like a pinball bouncing between the two forward blades until rotor system came to rest. Fortunately, the rotor system did not de-phase, and nobody was injured in the accident.

NOTE: What an Aft Droop Stop Assembly should look like:

What a damaged Aft Droop Stop Assembly looks like is shown below. The only component missing is that 1/2 inch wide by 6 inch long piece of metal known as the Fixed Droop Stop. It is the one thing that prevents the Aft Rotor Blade from falling over ten feet during engine shutdown:

Below is a drawing of the Centrifugal Droop Stop System on the Aft Rotor Head. The arm that is attached to the Spring Assembly swings outward from the Rotor Hub once there is sufficient centrifugal force obtained by the rotation of the rotor system after engine start. This outward swing lowers the Interposer Block away from the Fixed Droop Stop and Rotor Hub. In doing so, the Rotor Blade is allowed to flap up and down in a greater range to increase rotor system efficiency and flight aerodynamics. This is, in part, the theory behind the Conservation of Angular Momentum.

Boeing CH-47D helicopter - Aft Rotor Centrigugal Droop Stop System.

Flipper

On 1 February 2001 at approximately 1430 hours EST, aircraft 85-24331, assigned to C Company - "Flippers", 159th Aviation Regiment, Fort Bragg, North Carolina, experienced Droop Stop failure on the Aft Rotor Head. One of the Fixed Droop Stops on the Aft Rotor Head was inadvertently installed improperly. Nobody was hurt and the aircraft was repairable.