Pomona, CA, USA
N1628R
Grumman American AA1B
The engine lost power in the traffic pattern base to final turn during a post maintenance test flight and the airplane collided with a building short of the airport. The carburetor was overhauled about 11 hours prior to the accident. During post overhaul flights an intermittent carburetor rich fuel/air mixture was noted in the idle circuit, which prevented the engine from idling below about 1100 rpm without fouling and quitting from excessive fuel. The carburetor was removed and sent back to the overhaul shop who removed the installed needle valve assembly and replaced it with another identical new assembly. The carburetor was then installed on the engine. When the main fuel shutoff valve was turned on, fuel began to run out of the carburetor. It was again removed and returned to the overhaul shop. The shop owner suspected that a stuck or hanging up float may have been also been the problem. Since the carburetor was equipped with an Advanced Polymer float, which is larger than either the original brass or older style composite float, the shop owner carefully adjusted the float for lateral clearance between the float and the bowl wall, and, between the float clip and the needle valve. After several attempts on the test bench to achieve a stabilized fuel level, the carburetor finally passed. The pilot (an A&P) picked up the carburetor from the shop on the day of the accident, installed it, then flew the airplane for about 15 minutes. The accident happened on the second test flight. Post accident examination of the engine revealed sooted spark plugs typical of a rich fuel/air mixture. The carburetor was functionally tested by mounting it on a tilting test fixture. A trace amount of fuel was observed leaking from the discharge nozzle. When the fixture was rotated to a bank angle, fuel flowed freely from the nozzle. Tapping on the bowl stopped the flowage. Operational testing disclosed that the carburetor was operating at an excessively rich setting at idle speed. Disassembly disclosed that the Advanced Polymer float was clean and intact with no sign of damage. The float setting and bowl clearance was good. The needle valve seat assembly was inspected and measured and found to meet specifications. The float retractor clip and needle valve shoulder clearance was measured about 0.005-inch. The pivot pin/shaft that hinges the float assembly was found to be "tight" in the inside diameter of the Polymer Float hinge points; however, the float and shaft combination did rotate freely in the float bracket. The manufacturer's service manual (and incorporated overhaul instructions) were examined. There are three types of floats which can be installed. The originally designed floats are hollow brass chambers. The second type, no longer in production, consists of floats constructed out of composite materials. The third type is the Advanced Polymer floats, which are physically larger that either the brass or composite floats. With either the brass or composite floats, a typical 0.081-inch clearance exists between the float and the bowl chamber. The increased size of the polymer float reduces the float to bowl wall clearance to a typical 0.031 inches. The original sections of the service manual address the original brass floats and calls for a post reassembly minimum clearance of 0.005-inch between the float valve seat shoulder and the float valve retractor clip. Instructions E-955 (dated 03/18/99) have been incorporated into the manual and cover the installation of the polymer floats. This document requires the assembler only to "Insure that clearance exists between the float valve seat (shoulder) and the float valve retractor clip." There is no published minimum clearance limit. During final assembly, the carburetor fuel bowl and throttle body go together blind and without the ability to see the final internal clearances. Float clearance and height settings are critical to the proper metering of fuel proportional to airflow through the venturi of the carburetor. Any float drag against the wall of the bowl assembly could feasibly disturb the critical balance. The investigation measured several sources of free play in the carburetor removed from the accident airplane with a dial indicator. About 0.015-inch was measured rotating the bowl cover and fuel bowl halves with just snug bowl screws. A source of horizontal float centering free play not mentioned in E-955 is in the float hinge that is attached with screws to the bowl cover; the hinge can slide about 0.028-inch from either screw. Index marks were added by the investigator to each float tip, and the total sideways free play of the hinge measured about 0.229-inch at the float tips. The accident float flange was coated with a bead of black transfer material to test for rubbing on the bowl walls. The float was then reassembled into the accident carburetor bowl and throttle body assemblies following the service manual and kit instructions E-955. The carburetor was then rotated in various positions. Disassembly revealed black transfer markings on the inside of the bowl walls. The arc of float travel on the hinge point can bring the Polymer float against the bowl wall when the lateral movement is adjusted while installing the float bracket. An improperly centered float that rubs on the bowl wall may affect the float buoyancy and seating of the needle valve. The float retractor clip attaches to the needle valve. Without positive clearance from the needle valve seat shoulder a loss of float buoyancy or pressure to seat the valve can occur. Without proper seating of the needle valve due to float horn to bowl contact, positive fuel shutoff would be unlikely.
HISTORY OF FLIGHT On February 26, 2002, at 1650 Pacific standard time, a Grumman American AA1B, N1628R, collided with Barn No. 2 at the Los Angeles County Fair Grounds, Pomona, California, after a loss of engine power on base leg for landing at Brackett Field, La Verne, California. The airplane was operated by the pilot as a local maintenance test flight under the provisions of 14 CFR Part 91. The pilot sustained fatal injuries and the airplane was substantially damaged. Visual meteorological conditions prevailed and no flight plan had been filed. The local flight originated at 1645. The airplane was owned by a partnership, and all of the members were active in aviation. According to statements from both a partner in the airplane and a member of the pilot's family, the carburetor had been overhauled by Barkhorn Precision Engines, a Federal Aviation Administration (FAA) approved repair station (ARS), concurrent with an engine major overhaul. After the initial carburetor overhaul, the airplane was operated about 11 hours, with the pilots who flew the airplane reporting an occasional intermittent carburetor rich fuel/air mixture in the idle circuit. One of the partners stated that after the overhaul, he had on occasion idled the engine about 1100 rpm, rather than allowing the engine to idle any slower and "load up" from excessive fuel. On February 6, 2002, during a biennial flight review, one of the partners was asked by his flight instructor to perform a short field landing. He was required to throttle all the way back and the engine quit during the landing rollout. At that point, the carburetor was removed and sent back to the engine overhaul shop for resolution of the problem. The engine shop removed the installed Consolidated Fuels (CF) FAA-PMA approved needle valve assembly and replaced it with another identical new CF233-615 assembly (Consolidated Fuels manufacture's replacement parts for the Precision Airmotive Carburetors under an FAA approved Parts Manufacturing Approval). A static fuel level check for positive shutoff of the fuel on a fuel test bench was performed by the engine overhaul shop and the carburetor was returned to the partnership. The carburetor was then installed on the engine. Prior to running the engine, the main fuel shutoff valve was turned on and fuel began to run out of the carburetor. It was again removed and returned to the engine overhaul shop. In written and verbal statements, the engine shop's owner said the carburetor arrived on February 26 with a reported discrepancy that the float valve was leaking. The carburetor bowl was removed and a new float valve (manufactured under a PMA by Consolidated Fuels) was installed. The shop owner suspected that a stuck or hanging up float may have been also been the problem. Since the carburetor was equipped with an Advanced Polymer float, which is larger than either the original brass or older style composite float, the shop owner carefully adjusted the float for lateral clearance between the float and the bowl wall, and, between the float clip and the needle valve. After several attempts on the test bench to achieve a stabilized fuel level, the carburetor finally passed and it was repackaged for delivery to the customer. The pilot picked up the carburetor from the engine shop earlier on the day of the accident. The pilot, a licensed airframe and power plant technician, installed the carburetor and performed a ground run check. About 1400, the pilot test flew the airplane. An entry was made in an informal flight record booklet documenting a tachometer out of 73.25 and tachometer in of 73.63, for an engine run time of .38, with the notation of "test carb." At the accident site the tachometer indicated 1673.76. During that brief flight an oil filter by-pass light illuminated and the pilot landed. He cleaned the newly installed oil filter system, and asked a partner getting off work if he wanted to go flying; the partner was busy and declined. The pilot/owner then made the accident flight. According to the FAA control tower air to ground communications tapes, the pilot requested taxi for takeoff at 1639. He was cleared for takeoff at 1645. He reported an engine out at 1648, while turning base leg to land. PERSONNEL INFORMATION The pilot held a private certificate with an airplane single engine land rating and had accumulated about 415 total flight hours. His last documented third-class flight physical occurred on May 9, 2001, with a reported 415 total flight hours. He was also a certified airframe and power plant maintenance technician. A review of the pilot's log revealed a 1.5-hour biannual flight proficiency occurred on June 1, 2001, in a Grumman AA5A. AIRCRAFT INFORMATION The last recorded annual inspection occurred on January 22, 2002, at a recording tachometer time of 1,661.9 hours. A major engine overhaul was performed at the same date and tachometer time. The details of the overhaul of the engine and components, including the carburetor, are attached to this report. At the time of the accident the engine had operated about 11 hours since the overhaul; the accident tachometer indicated 1673.76. The recording Hobbs hour meter indicated 2,347.8 hours at the time of the accident. WRECKAGE AND IMPACT INFORMATION The Safety Board investigator examined the accident site soon after the occurrence. The airplane was impacted against a brick wall of Barn No. 2 in a horse stable area of the county fairgrounds on an approximate heading of 260 degrees magnetic. Fuel was observed in the tanks with some spillage. There was no post accident fire. The entire airplane was accounted for at the accident site. After a physical examination and photo documentation of the wreckage, the airplane was moved intact out of the fairgrounds area to a hangar at Brackett Airport, about 1 mile west. Prior to a formal examination of the wreckage, information was provided to the Safety Board investigator about an on-going carburetor problem. A post accident examination of the engine and airframe was conducted March 4, 2002, at Brackett Field. The carburetor, with minor damage, was removed and shipped to Precision Airmotive Corporation, the FAA type certificate holder, at Marysville, Washington. The request for flow bench testing and disassembly with the Safety Board investigator present was made. A blue colored fuel was found in abundance in the entire fuel system. The carburetor bowl was drained of fuel and found free of contaminants. The fuel lines were open and free of obstructions. The fuel selector was found on the left tank; it was removed and functional tested satisfactorily. The left wing root fuel hose (a mil-6000 hose cure dated 2Q99) was removed and cut open. It was found to be free of internal deterioration. The carburetor air box and inlet duct were free of obstructions on both sides of the air filter. The engine driven fuel pump was opened, the diaphragm and valves were examined. The electric autopulse pump was electrically functional tested. Both Slick magnetos were manually sparked. Compression was established on cylinders No. 1, 3, and 4. Cylinder No. 2 was impact damaged to the cylinder head. MEDICAL AND PATHOLOGICAL INFORMATION On February 27, 2002, the Los Angeles County Medical Examiner performed an autopsy on the pilot. During the course of the procedure samples were obtained for toxicological analysis by the FAA Civil Aeromedical Institute in Oklahoma City, Oklahoma. The results of the analysis were positive for Chlorpheniramine, an antihistamine, at 0.018 (ug/ml, ug/g) detected in the blood and urine. TESTS AND RESEARCH INFORMATION Post accident examination of the engine revealed sooted spark plugs. According to a Champion Check-A-Plug AV-27 chart, that observation is typical of a rich fuel/air mixture. The carburetor was taken to the type certificate holder, Precision Airmotive Corporation for examination and functional testing. A visual examination revealed a damaged mixture control arm preventing it from full travel into the idle cutoff position. The carburetor was mounted on a tilting test fixture and fuel was supplied to it. A trace amount of fuel was observed leaking from the discharge nozzle. When the fixture was rotated to a bank angle, fuel flowed freely from the nozzle. A tapping on the bowl stopped the flowage. The fuel and float level in the bowl was determined to be high at this point with fuel observed at the discharge nozzle tip and spillage would occur from a tipping angle. The carburetor was placed on a bench for fuel control limits. The idle speed limits were found to be high with air 18 pound/hour, with a limit of 6, and the fuel was 13 pound/hour, with 5.3 as the limit. The idle speed and idle mixture are set on the airplane during installation by the installer after repairs or service. The other three limits were within specifications. The carburetor was disassembled at Precision, and their report stated that the Advanced Polymer float was clean and intact with no sign of damage. The float setting and bowl clearance was good. The CF-233-615 Needle Valve Seat Assembly was inspected and measured and found in conformance with the FAA-PMA print requirements for the part. The float forked retractor clip and needle valve shoulder clearance was measured about 0.005 inch. The retractor clip transmits the float movement, which enables the float needle to seat and vary the fuel flow, which is critical in the idle circuit below 1100 rpm. According to Lycoming, above 1100 rpm the engine can consume an excess fuel amount and the pilot has the manual leaning option; this is not available in the idle circuit. The pivot pin/shaft that hinges the float assembly was found to be "tight" in the inside diameter of the Polymer Float hinge points. The overhaul manual states that the pin/shaft is to be free to rotate. The float and shaft combination did rotate freely in the float bracket. Following the functional testing and disassembly of the carburetor at Precision Airmotive, the investigation examined the Precision service manual (and incorporated overhaul instructions) for the accident carburetor part number. There are three types of floats that can be installed. The originally designed floats are hollow brass chambers. The second type, no longer in production, consists of floats constructed out of composite materials. The third type is the Advanced Polymer floats, which are physically larger that either the brass or composite floats. A photo is included in the docket which compares the three types of floats. With either the brass or composite floats, a typical 0.081-inch clearance exists between the float and the bowl chamber. The increased size of the Advanced Polymer float reduces the float to bowl wall clearance to a typical 0.031-inch clearance. The original sections of the Precision Airmotive service manual address the original brass floats and calls for a post reassembly minimum clearance of 0.005-inch between the float valve seat shoulder and the float valve retractor clip. Carburetor float kit instructions E-955 (dated 03/18/99) have been incorporated into the manual and cover the installation of the Advanced Polymer floats. The pertinent sections of this document require the assembler only to "Insure that clearance exists between the float valve seat (shoulder) and the float valve retractor clip." There is no published minimum clearance limit. Several new and used fuel valve seat shoulders were measured. The shoulder design target measurement is 0.204 inches with a tolerance of plus or minus 0.010-inch. The seat depth measurements varied about 0.007-inch from one to another. An additional observation of the used needle seats revealed a number were damaged in the slot area used to torque the float valve seat into the upper bowl cover. The overhauler of the accident carburetor used the recommended M-104 seat installation tool and still damaged the tool slot while installing the fuel valve seat. Installation instructions E-955 also provides the option for the assembler to use a screwdriver for this installation. According to overhaulers who were interviewed, this minor damage to the slot is not unusual and does not affect the operation of the needle valve. For the Polymer float installation, a tool slot "burr" above the shoulder of the needle seat will reduce the clearance between the retractor clip and needle valve shoulder. Measurements were taken of several shoulder "burrs" and were 0.001 to 0.002 inches in height. The shoulder height for previous brass or composite floats was not an issue as with the larger size Polymer float and needle valve clip. Minor differences were noted between the manufacturer of the float needle valve by Consolidated Fuels and the type certificate holder Precision Airmotive. Consolidated Fuels still supplies the brass float and needle valve and seat with a .031 aluminum washer. The CF assembly was measured and found in conformance with the FAA-PMA print requirements for the part. Precision Airmotive supplies the Polymer float kit, which included a .031 and a .015 aluminum washer for a wider range of needle seat shoulder heights. The Precision Airmotive Manual Illustrated Parts List shows only one float valve seat gasket P/N: 16-224. ADDITIONAL INFORMATION The Safety Board investigator contacted seven FAA Approved Repair Stations in several states that overhaul carburetors. The question asked was; What has been your experience with the Precision Advanced Polymer Float? All reported problems with the Polymer float pontoon to bowl clearance, carburetor bowl casting differences, clearance from needle seat shoulder to forked retractor clip, and general setup or fit problems. A memo of the interviews are in the docket. Historically, the carburetor floats in the MA series of carburetors were made alternately of composite and brass materials. The advanced Polymer float was introduced by Precision Airmotive, for the MA series carburetors, on February 26, 1998. It called for the float kit instructions (E-955) to become an addendum to the overhaul manual dated March 30, 1993, as floats were changed. Brass floats are still manufactured under an FAA-PMA, and remain as an approved viable alternative to the Polymer Float. Kelly Aerospace Power Systems, Inc., continues to provide brass floats and other FAA approved components for aircraft carburetors. Parts suppliers will fill parts orders from either the FAA type certificate holder parts or from FAA Approved Parts Manufacturing Authority (PMA) sources. Each float design has experienced manufacturing and operational defects or failures. The Polymer float has the least amount of pontoon to bowl clearance, a minimum of 0.031-inch clearance, down from 0.081-inch minimum clearance for the brass and composite floats. The carburetor final assembly of the fuel bowl and bowl cover go together blind and without the ability to see the final clearances. The use of a modified lower bowl P/N M-510 with a window to see the pontoon position is recommended to be used for setup and control of free play, which will change after the removal of the M-510 bowl. Float clearance and height settings are critical to the proper metering of fuel proportional to airflow through the venturi of the carburetor. Any float drag against the wall of the bowl assembly could feasibly disturb the critical balance. The investigator measured several sources of free play in the carburetor removed from the accident airplane with a dial indicator. About 0.015-inch was measured rotating the bowl cover and fuel bowl halves with just snug bowl screws. A source of horizontal float centering free play is in the float hinge that is attached to the bowl cover with two each 4-40 screws not mentioned in (E-955); the hinge can slide about 0.028-inch from either screw. Index marks were added by the investigator to each pontoon tip, and the sideways free play of the hinge measured about 0.229-inch. The accident float flange was coated with a bead of black transfer material to test for rubbing on the bowl walls. The float was then reassembled into
a total loss of engine power due to an excessively rich mixture setting in the carburetor. The overly rich operation of the carburetor was due to the overhaul shop's failure to obtain the proper clearance between the float and chamber walls, which allowed the float to rub and hang up. The overhaul shops failure to achieve the correct clearances was due in part to the inadequacy of the manufacturer's overhaul instructions concerning installation of the Advanced Polymer Floats.
Source: NTSB Aviation Accident Database
Aviation Accidents App
In-Depth Access to Aviation Accident Reports
