German radar kept losses of aircraft low. While some 40,000 firemen were available to tackle fires, control of their resources was damaged when the telephone exchange caught fire and rubble blocked the passage of fire engines through the city streets; fires were still burning three days later. A second, daylight raid, by the USAAF was conducted at 16:40. It had been intended for 300 aircraft to attack Hamburg and Hanover but problems with assembling the force in the air meant that only 90 Boeing B-17 Flying Fortresses reached Hamburg. The bombers attacked the Blohm and Voss shipyard and an aero-engine factory, with German flak damaging 78 aircraft. However the shipyard was not badly damaged and the aero-engine manufacturer could not be seen for smoke (a generating station was attacked instead). De Havilland Mosquitos of the RAF Light Night Striking Force (LNSF) carried out nuisance raids to keep the city on a state of alert and delayed-action bombs from the night's raid exploded at intervals. Extra firemen were brought in from other cities including Hanover; as a result when the US bombers attacked, these firemen were in Hamburg and fires in Hanover burned unchecked. Another attack by the RAF on Hamburg for that night was cancelled due to the problems the smoke would cause and 700 bombers raided Essen instead. Mosquitos carried out another nuisance raid. A third raid was conducted on the morning of the 26th. The RAF night attack of 26 July at 00:20 was extremely light because of severe thunderstorms and high winds over the North Sea, during which a considerable number of bombers jettisoned the explosive part of their bomb loads (retaining just the incendiaries) with only two bomb drops reported. That attack is often not counted when the total number of Operation Gomorrah attacks is given. There was no day raid on the 27th. On the night of 27 July, shortly before midnight, 787 RAF aircraft74 Vickers Wellingtons, 116 Short Stirlings, 244 Handley Page Halifaxes and 353 Avro Lancastersbombed Hamburg. The aiming points were the dense housing of the working-class districts of Billwerder, Borgfelde, Hamm, Hammerbrook, Hohenfelde and Rothenburgsort. The unusually dry and warm weather, the concentration of the bombing in one area and firefighting limitations due to blockbuster bombs used in the early part of the raid – and the recall of Hanover's firecrews to their own city – culminated in a firestorm. The tornadic