NTSB Factual Report

On June 12, 2015, about 1215 central daylight time, a Grumman - Schweizer G 164B airplane, N48687, nosed over during a forced landing following a loss of engine power during an aerial application maneuver near Foster, Nebraska. The commercial pilot sustained minor injuries. The airplane empennage was substantially damaged during the nose over. The airplane was registered to and operated by an individual under the provisions of 14 Code of Federal Regulations Part 137 as an aerial application flight. Day visual flight rules conditions prevailed for the flight, which did not operate on a flight plan. The local flight originated from the Bloomfield Municipal Airport (84Y), near Bloomfield, Nebraska, about 1100.

According to the pilot's accident report, he indicated that the airplane departed 84Y with two hours of fuel on board. The terrain of the field he conducted the aerial application flight over was flat with scattered trees. He stated that the ground was muddy and water logged and that "all gauges were in the green." His report, in part, further stated:

During my last turn to conduct a pass on the pasture head land the engine

stopped producing power at an altitude of about 100 feet. I leveled off,

checked that mags were on, mixture rich, fuel valve on, [and] I moved the

throttle control to full. Noting that my airspeed falling I applied forward stick

pressure to regain airspeed. Knowing I didn't have altitude to make the

nearest road I was forced to land in a pasture adjacent to the one I was

spraying. As my wheels touched down, airspeed slowed reducing lift and

putting more weight on the main wheels. With the added weight, the wheels

sunk into the mud causing the airplane to nose over onto its top.

The pilot said that during a post accident inspection, the fuel quantity gauge, part number A5587-1, was found to be indicating incorrectly. He stated that when the airplane was refueled using the remote single point port to full fuel, as indicated on the fuel quantity gauge; however, the airplane was only 3/4 full and when the fuel quantity gauge read 1/4 full, the airplane fuel system was "actually empty."

Another operator was contacted and asked to describe his operational knowledge of the single point refueling system and the fuel quantity gauges. The operator stated that all models of the Ag Cat (G164, G164A, G164B, G164C, G164D) have relied on the simple mechanical fuel gauge in the bottom of the center section fuel tank. The Ag Cat airplanes have been produced with only a center section tank, one fuel tank in each wing in addition to the center section tank, and two tanks in each wing in addition to the center section tank. With each of these configurations, the tanks are connected together and the engine is supplied fuel from the center section tank.

The one fuel gauge, mounted in the bottom of the center section fuel tank, indicates the level of the entire fuel system regardless of any other installed fuel tank. There is only one fuel filler neck and cap in the center section to fill the entire fuel system. With the bottom point fuel system modification, there is a system of valves and hoses installed up to the center section, so fuel can be pumped up to the center section and fill the entire fuel system from the ground.

Operationally, the airplane fuel quantity gauges are generally checked to confirm their accuracy when they are full and when they are empty, but when the tanks are partially full, the readings fluctuate especially during heavy maneuvering and or unusual attitudes flown during agricultural application flights. Additionally, the operator indicated that he heard of Ag Cat fuel quantity gauges slipping a tooth in the gearing system and registering a false indication. The fuel filler cap located on the top of the center section can be opened and sticked to determine fuel level. Sticking the center fuel tank required someone to climb onto the top wing of the airplane. The operator reported that a purpose of single point fueling system was to eliminate someone climbing on top of the wing to visual check the center section fuel tank level.

According to a fuel gauge manufacturer's brochure, the accident gauge was a senior liquid level, bottom mount, side view dial gauge. The gauge's float operates the gauge dial through gear action.

The airplane flight manual, in part, stated:

4.5 NORMAL OPERATING PROCEDURES

(a) Pre-flight Check ...

2. Upper Wing Center section

Fuel Quantity Check

Fuel Tank Cap Secure

Fuel Tank Sumps Drain

Fuel Gauge Check

Fuel Lines Check

Vent Pipes Check

Cabane Struts & Rigging Check ...

4.7 AMPLIFIED NORMAL PROCEDURES

The remainder of this section supplies detailed information, explanatory

material, warnings and cautions for normal operating procedures necessary for

safe flight of the airplane. ...

4.9 PRE-FLIGHT CHECK ...

(b) Upper Wing Center Section

Open the fuel cap and check the fuel level and color visually (80/87) red, (100

Low Lead) blue, (100/130) green, (115/145) purple. Ensure the seal is installed

and the cap is tight. Observe fuel gauge for proper reading. Drain a small

quantity of fuel from each sump into a glass jar and inspect for water, dirt, or

sediment. ... Check fuel lines for evidence of leakage.

The National Transportation Safety Board aviation accident database was queried for accidents involving G 164 airplanes that showed fuel quantity gauges as an accident cause. The query showed that four accidents occurred before this accident and their case numbers are appended to the docket material associated with this case.

The airplane manufacturer was asked to report their service history in reference to the fuel quantity gauge. The manufacturer indicated that they acquired the "AgCat Program" in 1998, and that they sold 600 of the A5587- 1 units. They stated that 400 have been removed for various reasons including stained/illegible dial faces, "sunk" floats, movement obstructions, "or just worn out." The airplane manufacturer further indicated that any gauge should not be considered "fuel quantity verification" and AgCat flight manuals have included "fuel quantity" and "fuel gauge" checks as part of pre- flight procedures.

The accident pilot confirmed that he relied on the fuel gauge indication and that he did not stick the fuel tank before departing on the accident flight.

NTSB Probable Cause

The total loss of engine power during a low-level aerial application flight due to fuel exhaustion following the pilot’s inadequate preflight inspection that led to his assumption that sufficient fuel was on board for the flight based on his reliance on a fuel gauge that indicated the incorrect fuel level.