The tail turret was the most important defensive position and carried the heaviest armament. Despite this, the turrets used, starting with the FN-20, were never entirely satisfactory and numerous designs were tried. The FN-20 was replaced by the very similar FN-120 which used an improved gyroscopic gun sight (GGS).[13] Gunners using both the FN-20 and 120 removed perspex and armour from the turret to improve visibility, but trials by the RAF showed that a Mosquito night fighter was still able to get within a very short distance of the tail gunner without being spotted, confirming what the Luftwaffe had already realised. The Rose turret attempted to improve on the FN turrets by being completely open to the rear (improving visibility and allowing easier emergency egress) and by being fitted with two .50 inch machine guns and was installed in a small number of Lancasters but never became common.[13] Ultimately radar, rather than improved visibility, made the turret more effective. The FN-121 was the Automatic Gun Laying Turret (AGLT), an FN-120 fitted with Village Inn gun-laying radar.[13] Aircraft fitted with Village Inn were used as bait, flying behind the main formations to confront the night fighters that followed the formations and shot down stragglers. This significantly reduced operational losses; and gun-laying radar was added to the last versions of the turret. Before the end of the war Lancasters built in the UK standardized on the FN-82 fitted with two .50 inch machine guns and fitted with gun-laying radar as production allowed, which was also used on early models of the Avro Lincoln. The disadvantage of all radar and radio transmitting systems is that attacking forces can locate aircraft by picking up transmissions.
Later in the war Freeman Dyson made a case for removing all the Lancaster's defensive armament, arguing it would reduce the loss rate by increasing the Lancaster's speed by up to 50 mph (assuming the bomb load was not increased at the same time), and thus make it harder to shoot down. This became even more important when Dyson and Mike O'Loughlin concluded that some of the German night fighters were using Schräge Musik upward firing guns, as the Lancaster had no ventral gun turret to defend itself, although any defence would depend on the crew detecting the attack from underneath.[17] Dyson considered that the modification would be justified even if the aircraft loss rate was unchanged, as two defensive air gunners would not be required, reducing human losses. The case for speed over defensive armament was supported by the Mosquito, whose loss rates were far lower than the Lancaster's. As an example, during the Battle of Berlin (18 November 1943 to 30 March 1944) the average loss rate of the heavy bombers (overwhelmingly Lancasters) was 5.1%, whereas for Mosquitoes it was 0.5%,[18] though a speed-optimised Lancaster would still be up to 50 mph slower than a Mosquito and unlikely to match its low loss rates.
