Date: 201910
Local time of day: 1201-1800
Location: ZZZ.Airport, US
Flight conditions: VMC
Weather elements / visibility: 10
Aircraft: Zenith Undifferentiated or Other Model
Operator: Personal
Operating under: Part 91
Flight plan: None
Mission: Personal
Flight phase: Landing
Airspace: Class E ZTL
ATC / advisory: CTAF ZZZ
Reporter organization: Personal
Reporter function: Pilot Not Flying
Reporter qualification: Flight Crew Commercial; Flight Crew Flight Instructor; Flight Crew Multiengine; Flight Crew Instrument
Human factors: Confusion; Distraction
Anomaly: Deviation - Speed All Types; Deviation / Discrepancy - Procedural Published Material / Policy; Flight Deck / Cabin / Aircraft Event Other / Unknown; Ground Event / Encounter Other / Unknown
Detector: Person Passenger
When detected: In-flight
Result: General None Reported / Taken
Contributing factors: Human Factors; Aircraft
Primary problem: Ambiguous

Narrative: I went along as a passenger in my hangar neighbor's Zenith 801. It was explained by this owner/ pilot/ builder these model planes have counter intuitive characteristics to typical major certified aircraft; and though it is a STOL aircraft; it requires a good deal of power and acutely higher pitch up for proper landings. Because of this; [pilot] was practicing simulated emergency engine out landings should he ever experience and engine failure. Though riding as a passenger; I am a commercially rated pilot and CFI; so I'm filing this form for the intended purpose of this NASA safety program. Naturally these same reasons; I did not want to be acting as an instructor or crew for this flight.My observations; the first power off landing was a made with no flaps and I thought was just ideal in every way. This second power off landing; was done with some with flaps extended when the field was thought to be made; I estimate 10-degree flaps were deployed. I noted an acute reduction aspect ratio after flaps were lowered. Realizing on short final that a power off glide was going to be short; pilot said: 'perhaps I should go around' and introduced power; plane then quickly achieved what appeared the needed glide path; it looked ideal; and he said; 'I think I will try to make this work'. Unfamiliar with this plane; sight picture would seem very reasonable. Pilot continued his power off glide toward the runway with similar AOA as previous landing. As we approached close to flare altitude; I looked over to notice airspeed indicator at 54 mph and falling rapidly! Alarmed by the rapid deceleration; I yelled 'power!' by that time indicator was passing though 50mph.There was a throttle on my side of the airplane; but since I was not acting as a pilot or instructor and for the unfamiliarity of this aircraft; I thought it safer and only appropriate to only advise the observation of critical airspeed decay; so I didn't consider counter control input or trying to add power myself.The pilot dropped the nose and added power to recover; from my experience in certified aircraft; for a balked landing I would have added power and pulled the nose up; but this Zenith 801 is so different I thought best to only alert the pilot the urgency to take action for the decaying airspeed. There was some mild wind gust; and we were in the unfortunate phase of those gust as well. After exhaustive thought; for the unusual flight characteristics of this Zenith 801 with flaps deployed; I still cannot say definitively what the correct correction of pitch should have been for this situation of low airspeed and low altitude for this aircraft. [Pilot] is a very conscientious and exemplary pilot; I'm inclined to believe that this experimental model aircraft with flaps deployed; may simply be inherently hazardous for power off landings. Perhaps this could be very useful in advising other similar experimental owners; builders; and pilots of this designs insidious power off characteristics.




Synopsis: Passenger reported a significant loss of airspeed during final approach that would have resulted in a stall if corrective action were not immediately taken.