The Spirit of St. Louis

On May 19, 1927, after waiting for a week for the rain to stop, pilot Charles A. "Slim" Lindbergh (James Stewart) tries to rest in a hotel near Roosevelt Field on Long Island, New York, prior to a transatlantic flight from New York to Paris. His friend Frank Mahoney (Bartlett Robinson) guards his hotel room door from reporters. Unable to sleep, Lindbergh reminisces about his time as an airmail pilot.
Flying to Chicago in winter, Lindbergh lands his old de Havilland biplane in a small airfield to refuel. Despite the bad weather, he takes off, unaware the Chicago landing field has closed due to snow. After running out of fuel, Lindbergh bails out. Recovering mail from the crashed DH-4, he continues his journey by train and meets a suspender salesman who tells Lindbergh that two airmen just died competing for the Orteig Prize awarded to the first to fly nonstop from New York City to Paris. [N 1]
From a diner, Lindbergh calls Columbia Aircraft Corporation in New York, pretending to represent a group of prominent businessmen. Quoted the price of $15,000 (equal to $204,339 today) for a Bellanca aircraft, Lindbergh lobbies St. Louis financiers, with a plan to fly 40 hours in a stripped-down, single-engine aircraft. Excited by his vision, the backers dub it Spirit of St. Louis.
The Bellanca deal falls apart when the company demands their own pilot will fly the aircraft. Lindbergh then approaches Ryan Aeronautical Company in San Diego, California. Mahoney, the president of the company, promises to build him an aircraft in just 90 days. With Ryan's chief engineer Donald Hall (Arthur Space), a design is begun. To decrease weight, Lindbergh refuses to install radios or heavy equipment, even a parachute, and plans to navigate by "dead reckoning". [N 2]. Workers at the factory agree to work 24-hour shifts to complete the aircraft on time.
Lindbergh flies his new aircraft to St. Louis, and on to New York. Unable to sleep, he prepares his aircraft at Roosevelt Field, ensuring a full load of 450 gallons of fuel is taken on. In the cramped cockpit, the magnetic compass was positioned above his head but a young woman offers her compact mirror. Lindbergh fixes it to the instrument panel with chewing gum so he can see the compass. Furtively, Mahoney slips a Saint Christopher medal into the pilot's lunch bag.
With the weather clearing, theSpirit of St. Louis trundles down the muddy runway and barely clears the treetops at the end of the field. Every hour, Lindbergh switches fuel tanks to keep the weight load balanced. As he flies over Cape Cod, he realizes he has not slept in 28 hours. He recalls sleeping on railroad tracks, on short bunk beds, and under a windmill. When Lindbergh begins to doze, he is awakened by a fly. Over Nova Scotia, he sees a motorcyclist below, remembering his own Harley Davidson motorcycle traded as partial payment for his first aircraft, a war-surplus Curtiss Jenny.
Over the seemingly endless Atlantic, Lindbergh remembers barnstorming across the Midwest in a flying circus. After 18 hours, the aircraft's wings ice up and the Spirit of St. Louis begins to drop, but the ice breaks off in the warmer air and the engine restarted. Back on course, his compasses begin malfunctioning, forcing him to navigate by the stars. By dawn, Lindbergh falls asleep, with the aircraft circling and descending, but sunlight reflecting off the mirror awakens him in time to regain control.
Seeing a seagull, Lindbergh realizes he is close to land. He tries without success to hail a fisherman below. Sighting land, he has reached Dingle Bay, Ireland. Pulling out a sandwich, Lindbergh discovers the hidden Saint Christopher medal, hanging it on the instrument panel. Crossing the English Channel and the coast of France, he follows the Seine to Paris. Finally seeing the lights of Paris ahead of him, as he approaches Le Bourget Airfield in the dark, he is confused by the spotlights. The strange movements below him are actually crowds of people. Exhausted and panicked, Lindbergh makes his descent, whispering a prayer, "Oh, God help me!" After landing, hordes of people rush to Lindbergh, blind him with camera flashes, and carry him off triumphantly to the hangar. Tired and confused, Lindbergh eventually realizes that the crowds are cheering for his great achievement. When Lindbergh returns to New York, he is given a huge parade in his honor.

Question: What does the Spirit of St Louis barely clear the top of?
Answer: treetops