killing all the crew: Sergeants J. Porter, P. B. J. Butcher, R Carson, Pilot Officers R. Whitelock, A. G. Richards, B. C. H. King and Flight Sergeant B. J. Hardesty (RCAF seconded). Greiner was ordered to report to the Nachtjagdschule at Echterdingen as an instructor. Greiner did not want to serve in a teaching role but as a pilot with only two victories in a year of operations, he had little choice. His Staffelkapitän (squadron leader) Hauptmann (Captain) Helmut Lent promised him a return to the Gruppe after six months. Greiner's stint as instructor began on 23 November 1942. He looked for an opportunity to fly on patrol but the school did not have the required technical equipment. One night in early March 1943, Greiner took off and claimed a Short Stirling shot down near Rastatt. The date and location were not recorded but it was officially granted as his third victory. On 18 May 1943 Greiner returned to II./NJG 1 which from the 1 October 1942 had been re-designated IV./NJG 1. In March the British applied their area bombing and bomber stream tactics to the Ruhr. The Bomber Command attacks, which lasted until July 1943, were dubbed the Battle of the Ruhr. On the night of the 25/26 May Greiner claimed two Lancaster bombers that were part of a force attacking Düsseldorf. The first was accounted for west of Den Helder at 03:05 and the second north of Bergen aan Zee at 03:25 over North Holland. The claims were for Greiner's fourth and fifth victories which qualified him as a night fighter ace. One of the bombers was Lancaster I ED695, P-OJ, from No. 467 Squadron RAF. Flying Officer R. S. Giddey, Sergeant S. G. Keirs, W. V. Morris, R. V. Avaan, A.F Birkbeck, Pilot Officers K. R. Langhorne and P. R. Collins were reported missing in action. It transpired Langhorne and Birkbeck has been killed in action, the remainder of the crew were taken prisoner of war. On 1 June 1943 Greiner was awarded the Iron Cross 2nd Class (). Greiner achieved the last of his successes in the Ruhr campaign the following month. On the night of the 5/6 July 1943, he intercepted a Short Stirling on a "Gardening" sortie. Northwest of Leeuwarden he claimed the only night victory of the Luftwaffe at 02:31. The bomber was Short Stirling III, EF436, AA-A, of No. 75 Squadron RNZAF.