22 August 1914 dawned foggy and grey. Nevertheless, "Lieutenant J" and his observer boarded their airplane for a reconnaissance flight that took them from Sedan in France over the Belgian border. North of Bertrix, heavy rain forced them to descend to an altitude of 1000 m -- within range of French artillery. A barrage of gunfire ensued. Lieutenant J was hit first in the chest and then in the head; as the plane careened downwards, the observer "turned around and saw him lying there dead with a bullet through his forehead."
The observer took control of the plane, angling to land it in a wood beyond the reach of the enemy. Alas! A gust of wind forced him down among the French troops. They approached him guns blazing. With blood streaming down his face, he took out his pistol and shot three of them. Then he found himself with a bayonet at his breast and, an instant away from a glorious Heldentod, his life was spared by an officer shouting, "Let him live! He is a brave soldier!"
The observer was immediately taken prisoner and interrogated by the "commanding general of the XVII French corps." The general, however, was "of course unsuccessful" in his attempts to extract information. Later in the day, the observer managed to escape when German shells hit the house where he was being held. The French took to their horses; he crept under a bush and remained there until their retreat was complete. Then he dragged himself, head wound and all, to Bertrix. The town was now in German hands and on the following day he returned to Sedan by automobile.
The source of that story is the "brave soldier" himself -- the anonymous observer. This letter detailing the crash and the "miracle" of his survival was originally addressed to his father. It was shortly thereafter deemed sufficiently interesting to be printed in newspapers across Germany . In the letter, he evinces amazement at the fact that he did not die while at the same time overflows with bombast and self-assurance verging on arrogance. Reading it today, one cannot help but wonder whether (or how much) he might have embellished the truth or omitted inconvenient events in order to present himself as the superhero of the story.
An excerpt from Iwan von Stietencron's letter to his father as it appeared in the newspaper Erzähler von Westerwald, 5 September 1914
It was in fact possible to confirm at least some of the events.
Several German units engaged in battle in Bertrix that day. Soldiers saw the downed plane and an officer from IR 81 furthermore corroborated some of the details of the observer's escape.
- "In einem Einschnitt erkennen wir ein zertrümmertes Flugzeug, leider ein deutsches. Klar leuchten uns die Eisernen Kreuze entgegen." ("In a depression in the landscape we see a smashed plane — unfortunately a German one. The Iron Crosses shine brightly in our direction.") 
- “...sehen wir im Felde einen deutschen Flugapparat liegen, der uns wie ein Aar anmutete, dem die Schwingen gebrochen waren. Der Apparat war morgens bei einem Erkundungsflug von plötzlich aus dem Walde hervorbrechender französischer Infanterie heruntergeschossen worden.” (“…we see a German flying machine lying in the field, which looked to us like a eagle with its wings broken. The machine had been shot down during a reconnaissance flight in the morning by French infantry as they broke out of the woods.”) 
- "Das Haus wurde abgebrannt, nachdem die Verwundeten hinausgetragen waren. Vorher gelang es zum Glück noch einem deutschen Gefangenen, zu entweichen. Er war 'Beobachter' eines dicht bei dem Hause herabgeschossenen deutschen Flugzeuges gewesen, welches noch da lag. Der Flugzeugführer war tot.” (“The house was burned down after the wounded were carried out. Fortunately, a German prisoner managed to escape beforehand. He had been an ‘observer’ on a German plane that had been shot down close to the house. The plane still lay there. The pilot was dead.”) 
The downed airplane was also reported in French newspapers.
- “Ce matin-là des fantassins du 20e de ligne abattaient un taube qui survolait nos positions en face du bois de Luchy où le 11e lutta désespérément de onze heures à la nuit tombante.” (“That morning, the infantry of the 20th line shot down a plane that was flying over our positions in front of the Forest of Luchy, where the 11th fought desperately from eleven o’clock [in the morning] until nightfall.”) 
So it seems that the broad strokes of the story are true.
Wait! There's more! (At least a little bit.) In 1928, the French military published a dissection of their disastrous showing at Bertrix . It includes an account of the downing of the plane and the capture and escape of the observer.
Excerpt from Les Rescapés de Bertrix recounting the capture and interrogation of Lt. von Stietenkron
Yes, ladies and gentlemen, we now know the name and rank of the heretofore anonymous observer: Lieutenant von Stietencron. We also learn that he spoke "excellent French" and was, in his own words, the "son of a French mother."
In his letter to his father, von Stietencron bragged that he had let nothing slip during interrogation. This may be true, but he was nevertheless quite voluble. As the French told it, he was so full of patriotic pride that he couldn't keep from boasting about the "formidable" German artillery; whether they believed him or not seems unclear. He had, after all, sustained a head injury. The French source also substantiates the circumstances of his escape, though naturally their account is rather less effusive.
- Notre artillerie est formidable, reprend le Houzard, nous avons des canons de gros calibre, même du 50 centimètres ! - Les assistants sourient à cette boutade, et l'on emmène l'aviateur pour le faire panser. Il devait d'ailleurs s'échapper au milieu du désarroi de la retraite. (“‘Our artillery is formidable,’ continues the Hussar. ‘We have large calibre guns — 50 centimetres, even!’ The assistants smile at this pleasantry, and the aviator is taken away to be patched up. He must have escaped in the midst of the disarray of the retreat.”) 
With a surname, it was now possible to look up the observer in the Verlustlisten . A search for "Stietencron" produced a number of potential matches, including a member of the Feldfliegertruppe. That man's first name was Iwan and the details of his entry matched the story. The Verlustlisten also revealed the name of the pilot: Lieutenant Erich Janson, which dovetails with the semi-anonymized "Lieutenant J" of the newspaper's version of the letter.
Entries in the Verlustlisten for Iwan von Stietencron and Erich Janson. The latter is listed as killed in action at Sedan and buried at Bertrix. After the war his grave was moved to Musson-Baranzy .
Ironically, there is very little other information to be found online about Iwan von Stietencron. He came from what seems to be a cadet branch of a noble family in Schötmar. "Iwan" was a family name and he had many ancestors, as well as a cousin who died in Liège on 10 August 1914, with whom he shared it. An 1882 marriage certificate for his parents Hartwig von Stietencron and Rosa Marianne Dapples lists her father's occupation as "Rentner zu Nizza" (pensioner in Nice), giving credence to the claim that his mother was French.
Iwan married late in life and died in 1952, apparently with no children.
 "Erkundigungsflug bei Sedan." Iwan's letter to his father reprinted in Erzähler vom Westerwald, 5 September 1914.
 "Mit dem nassauischen Regimenten nach Frankreich. IV." Wiesbadener Tagblatt, 24 September 1914.
 "Die erste Schlacht unserer 63er." Mitteldeutsche Rundschau, 7 November 1914
 "Der Krieg im Westen." Fuldaer Zeitung, 7 September 1914.
 "Les Rescapés de Bertrix." La Dépêche, 21 March 1915.
Edited by knittinganddeath