Date: 201307
Local time of day: 1801-2400
Location: ZZZ.Airport, US
Flight conditions: VMC
Weather elements / visibility: 100
Light: Daylight
Aircraft: Cessna 180 Skywagon
Operator: Personal
Operating under: Part 91
Flight plan: None
Mission: Personal
Flight phase: Cruise
Airspace: Class E ZZZ
ATC / advisory: CTAF ZZZ
Aircraft component: Fuel Tank
Component problem: Design
Reporter organization: Personal
Reporter function: Single Pilot
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Sea; Flight Crew Multiengine; Flight Crew Flight Instructor; Flight Crew Flight Engineer; Flight Crew Commercial
Human factors: Confusion; Situational Awareness; Troubleshooting; Workload
Anomaly: Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Fuel Issue
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Landed in Emergency Condition; Flight Crew Took Evasive Action; Flight Crew Inflight Shutdown; Flight Crew Became Reoriented; General Maintenance Action
Contributing factors: Manuals; Aircraft; Chart Or Publication
Primary problem: Ambiguous

Narrative: Normal preflight; C180 float plane; dipped fuel from dipstick calibrated specifically for the airplane; noted 16 gallons on right tank; 10 gallons on left. Added measured 10 gallons and re-dipped right tank which now read 26 gallons as expected for 36 gallons total. On start up fuel totalizer initialized and read 26 gallons; consistent with initial tank readings. [I] added 10 gal to read 36 gal total. Departed for several nearby wilderness lakes; [I] made left turns approximately 800 AGL while circling a 2;700 FT elevation; rather tricky lake; and determined it was too challenging to land. Added power to depart the area and the engine coughed; sputtered and died. I immediately turned back toward the lake; pulled carb heat on and adjusted the mixture and throttle; and the engine re-started fitfully. [I] declared an emergency and identified location; etc. with FSS and local helicopter operator flying nearby. Engine continued to surge; sputter; start and stop. [I] switched to the right tank and it caught and continued to run. I immediately began a climb; as I was not happy with the lake I was over; and hoped to gain options to other lakes. Engine monitor showed even cylinder temperatures and low EGT's; good oil pressure and oil temperature. [I] was climbing slowly under poor power; lower MP than normal for the altitude; with engine running rough and surging still. Gained enough altitude to head to a longer; lower elevation lake and did so; eventually hopping from lake to lake until over my 350 FT elevation home lake at 7;200 FT MSL. Began circling down over the lake and lost engine power at 6;900 FT MSL. Engine restarted briefly at 4;000 FT and then died again. [I] performed a dead-stick landing to the lake; uneventful [and] reported safe with FSS and local operators. [I] immediately dipped the fuel tanks and both were completely dry. Flight time had been 1.9 hours (time; not tach); including 25 minutes after the engine first quit. Fuel totalizer showed 18.6 remaining. Normal fuel usage is 10.2-12.8 GPH. For comparison; the previous day's flight under similar operating conditions was 1.4 hours and used 15 gallons; however I generally flight plan for 15 GPH just to be on the safe side. While investigating where the missing fuel was or where the error in math was; I made several important discoveries. Upon filling the tanks to the top; I was only able to pump in 72.4 gal. My 1969 POH says I have 84 gal capacity tanks with 79 usable; however; the bladders were replaced in 1999 and they are placarded at 88 gal capacity usable 84; 42 on the outboard and 37 on the inboard fuel points. I either had a bunch of fuel hiding in the bladders; or a reduced capacity. I did a search and found references to options for 88/84/65/60 gallon options on Cessna fuel bladders. Upon calling several fuel cell repair shops I was told ALL Cessna long range tanks are standardized at 80 gallon capacity; 75 usable and that Cessna POH and placards are wrong - reference Airworthiness Directive (AD) 75-16-1 which requires corrected placards and POH pages. I called the [Local] FAA office and they had no idea and referred me to [another office]; who also was unfamiliar. They found the AD; but could not read the specified placards due to poor reproduction. They noted the AD did not apply to my model. I found a note 'B' which said it applied to ALL models if they had fuel cells replaced after the mid 70's (which I had). They referenced a 1969 C180H POH which said total fuel capacity was 79 gallons and referred me to Cessna. I spoke with them; and they found the original service letter; which addressed the fuel cell replacement requirement for specific models in the 70's as well as to re-placard for older aircraft; but excludes the models from 1962-1972 from that requirement. They could not tell me anything about the fuel cells; subsequent AD nor could they read the required placard; and could only provide me with what their publication presented: 84 gallon capacity and 79 usable. I verified the part numbers for my replaced fuel cells are each 40 gallon capacity; 37.5 usable. So the first thing I found is that my fuel bladders are 8 gallons less than the 88 which its placarded for; and the 84 for which it was certified; and the usable is only certified at 75 gallons; not 84 as placarded. Of that 80 gallons actual capacity; I can say with great certainty that no more than 72.4 is usable. This is nearly an hour difference in cruise; and the issue is relatively unknown; even among Aircraft Certification Offices (ACO); well established Cessna maintenance shops and commercial operators; as well as at the manufacturer. When none of these folks can tell a Cessna 180 owner what the maximum capacity of their long-range fuel tanks is; there is a problem. I couldn't even find reference to it on online skywagon reference sites. Only the fuel cell repair shops seemed to know this. This explained where the missing 11.6 gallons (84 usable) was when I filled the tanks from their dry state; but there are still some 8 gallons missing from total capacity - perhaps bladder snaps; or collapsed corners or shrinkage? Unusable fuel below the sumps? The second thing I found out is the dip stick was calibrated for the outboard fuel port; not the inboard; as I had been told. This is the cause of the fuel starvation as the inboard readings indicate 5 gallons more per side; so I had always calculated my fuel with a 10 gallon error. Thus; I departed with 26 gallons; not the 36 I had verified from the dipstick; and its not totally unreasonable to assume I burned it all up in 1.9 hours. However; the fuel totalizer still didn't jive. While the total amount indicated (36 at departure and thus 18.6 upon landing) was based on my inputs directly derived from faulty fuel stick readings; the actual fuel flow has been very accurate over hundreds of hours; so after deducting the 10 gallon error; that meant I should still have had something close to 8 gallons remaining.This mystery was perhaps solved the next morning when I discovered fuel leaking from the left wing. It had not been evident until it dried; but was leaking visibly and streaked along the fuselage to the horizontal stab. I have since had the leak repaired; noted the AD which applies; and fuel capacity in the logbook and will order new placards for the fuel ports and have re-calibrated the fuel stick to the inboard filler ports. I knew there was a fuel flow issue; but did not consider I had under-fueled the airplane; as I had three methods of cross-check in place (fuel meter on the pump; dip-stick; totalizer) so I was biased against thinking I was actually out of fuel. I exclude the 4th method - my Cessna fuel gauges - because they are notoriously inaccurate in flight. Because the engine ran on the right tank; but not on 'both'; albeit not very well; I suspected fuel contamination; a blocked vent; or water sloshing from bladder folds. Had I realized actual fuel depletion I would have still attempted to climb in order to get to a different lake; but would have landed at the third lake I hopped to; as it was comfortably long. I would not have considered continuing on to any other lakes. With the problem undetermined; but the ability to climb and the continued operation over a 20 minute period of time; I made different decisions to continue to attempt to improve my options. I also discovered that with variations in glide speed I did not have much effect on my gliding distance or rate of sink. The Pilot Operating Handbook (POH) specifies 85 MPH as V-glide; and the float plane supplement only contains a statement that the supplement only identifies items which are different from the POH and if it is not included in the supplement; one may rely on the data in the POH. Therefore V-glide remains at 85 MPH; but I'm surprised that the same speed is used with floats; and with variations in float models. In any case; I was sinking like a rock; and will need to gopractice to establish a better glide speed. This may have helped my situation. The biggest outcome from the event is the realization that this compounded situation of dip-stick reading errors of 10 gallons; and a usable fuel of 72 instead of 84 gallons for a total fuel calculation error of over 20 gallons; was an accident waiting to happen. I have ended my flights many times with close to 20 gallons; thinking that was a very conservative margin; and now realize I may have been very close to fuel starvation on multiple occasions and would surely have attempted to continue flight with less at some point; resulting in a potential disaster. Fortunately; this situation had a safe outcome; and caused me to catch the underlying critical issues before an accident happened.




Synopsis: A 1969 C180 float plane lost power because of fuel starvation but made a safe dead stick lake landing with an estimated 20 gallons remaining. The wing tank bladders where changed in 1999 but the placarded quantity was inaccurate by twelve gallons; 72 usable; not 84.