Beechcraft 50 pilot reported sensing a strong smell of fuel during flight. Upon inspection; it appeared that SCAT tubing was pressing against a fuel crossfeed line located in the landing gear bay creating a fuel leak.

Narrative

This was a FAA medical Flight Test for a field of vision restriction. During the day I was cleaning and waxing the aircraft. It was quite warm and all windows were open. When the FAA Examiner arrived I conducted a complete preflight with no discrepancies found. A normal takeoff and departure was conducted. After airwork had been completed and while extending flaps just prior to turning from base leg to final; the FAA Examiner asked if I smelled fuel? At that time I did not. However after lining up on final and extending the landing gear a very strong fuel odor became apparent. We were on a 1 & 1/2 mile final. Landed normally and exited the runway. There was no longer the strong smell of fuel. Taxied to the ramp and shut down. An extensive post flight exam was conducted. No visible signs of fuel nor was there much odor. The fuel boost pumps were turned on and all fuel system configurations were tried. No fuel leak or strong odor were observed. (I am the sole owner/operator and A &P mechanic for this aircraft I conduct most of the routine maintenance.) The next day I removed the cockpit and cabin floor panels. Pressurized the fuel system and tried all configurations. No fuel leak or strong odor were observed. A second Pilot/Mechanic was employed to run the engines and fuel system while I 'Poked & Prodded' all the fuel lines. When I pushed against the SCAT tubing that passes over the fuel crossfeed lines I detected a faint fuel smell. I then forcibly pried the SCAT tubing away from the fuel lines and the leak became readily apparent. Conclusion: I have owned; maintained and operated this aircraft for many years. Each year I carefully review all the AD's and SB's that I am aware of. I do not recall any that address the cause of this issue. The SCAT tubing is located in the compartment that housed the LG apparatus. I believe that during some LG maintenance activity I must have pushed down on the tubing causing it to interact with the fuel lines and failed to observe the error. There is very little clearance between the SCAT tubing and the fuel lines. I will be adding an anti-friction pad between the fuel lines and SCAT tubing as a primary defense and insure the SCAT tubing does not interact in a negative fashion with any other aircraft structures. Aging Aircraft maintenance: Beechcraft D50ESCAT Tubing air duct routed over fuel crossfeed lines with very little space and no secondary provisions to prevent SCAT tubing spring steel wire from interaction with aluminum fuel linesNOTE: Components are estimated to be original equipment. Based on the appearance and production part number markings. The SCAT tubing in question delivers heated or ambient temp air from the floor mounted plenum chamber to the main cabin air distribution box. The tubing is routed around the landing gear motor and gearbox. It then crosses above the crossfeed lines and connects to the air distribution box. Tubing appeared slightly distended/ oval shaped instead of round in the area above fuel lines. The fuel lines are supported with phenolic blocks and mounting brackets. A very small space exists between fuel line and tubing components. Ducting information may be found in the Illustrated Parts Catalogue (IPC); on Figure 20; page 2-42 ; Duct listed as item number 83. Listed as a DUCT and the Beech PN is 111716-AX-XX-6-48. This is a 3 inch ID and 26 inch long flexible tubing with spring steel wire wound within the fabric. Commonly described as SCAT tubing. Over time the tubing has remained ridged and did not appear to be interacting with the crossfeed fuel lines. However; it appears that the SCAT tubing may have been inadvertently pressed down at some time during my maintenance activity and subsequently rubbed against the fuel lines. This resulted in an inflight fuel leak discovered while turning from base leg to final during an FAA flight checkride. Immediately after landing; a post flight inspection of the fuel system was conducted. The fuel lines were pressurized using the aircraft fuel boost pumps. Normal pressure was achieved. However; no fuel smell or leak were observed in any fuel system configuration. Later after I removed the aircraft floor panels and landing gear motor inspection cover; the fuel system was again pressurized with no fuel leak observed. The aircraft was started by a second pilot mechanic; and with boost pumps on and engines running; I inspected fuel lines and fittings. After a few minutes I detected a faint fuel smell in the vicinity of the SCAT tubing. After the SCAT tubing was forcibly moved in the vicinity of the fuel lines; the fuel leak became readily apparent. I am very disappointed in myself for not foreseeing this interaction between the SCAT tubing and the structure. I pay close attention to SCAT tubing in the engine compartments and over the years have found numerous issues in that area. However; I did not apply the same rigorous discipline to this area of the aircraft structure. A thorough review of all the SCAT tubing interactions with all aircraft structures will be applied prior to any release to airworthiness is granted again. Resolution: Design of the space between the SCAT tubing and Crossfeed lines is almost nonexistent. Inadvertent pressure on the duct will likely result in the SCAT tubing being pushed against the fuel lines. Solution: (1)Apply anti-friction padding between fuel lines and SCAT Tubing. (2) Prior to closure of the landing gear motor compartment; a second Inspection of the area to confirm SCAT tubing does not interact with the fuel lines.