55 Squadron attacked Freiburg on that date. They flew from their base at Tantonville, their target was a munitions works and a barracks housing reserve troops. Raids of this type were quite long distance affairs for the period, often covering several hundred miles, and generally speaking, they were pretty much on target with their bombs as well, 55 being regarded as one of the better bombing squadrons. You might be interested to know that WE Johns, author of the Biggles novels also flew DH4s in 55 Squadron.
A 55 Squadron DH4, the white triangle on the fuselage is their unit marking:
Needless to say, with such a long flight behind enemy lines, the raids often had to endure numerous fighter attacks and they invariably had little or no fighter cover either. It was on just such a raid that WE Johns was shot down.
On the raid to Freiburg on the 13th March 1918, 55 squadron's bombs overshot the main aim point a little and hit some public buildings, since high level bombing was tricky in those days. It is likely that they were bombing from around 13,000 feet using the Negative Lens Sight which DH4 had at that time, and it was often tricky to predict winds over the target in those days, as it is still tricky to accurately predict even today.
To give you some idea of how tricky it is to bomb from that altitude with a fairly basic bombsight, below is a picture of a DH on a bombing raid in WW1, giving you an impression of how high up they would be, you can see a bomb falling from one of the DH4s in a higher section of the flight, which I have highlighted on the picture.
On the Freiburg raid of the 13th, 55 Squadron aircraft were in all probability dropping 112lb RL Mk1 bombs with a delayed action fuse. These bombs had either a TNT or Amatol charge, with a fulminate of mercury detonator and a tetryl exploder to set off the main charge. Each DH4 could carry four of these bombs, and they had an endurance of up to five hours flying time. Depending on the target, the bombs they carried could have a detonation delay of between a quarter of a second and 15 seconds, the variable delay causing differing damage depending on the length of time before the explosion and how deep it drilled into the ground prior to going off. The observers may also have had 'Baby Incendiaries' which they would drop by hand from the cockpit, these being intended to set the target alight after the main bombs had drilled down into the ground before detonating, the main explosions being intended to rip open gas mains so that the gas leaking from these would be ignited by the Baby Incendiaries, which had phosphorous cores.
Unfortunately for your late uncle, the successes of the fledgling IF bombers of which 55 Squadron was a part, were causing significant damage on daily raids, and outcries from a terrified German public along with very vocal politicians and newspapers resulted in better organised home defence squadrons over Germany than had previously been the case. The IF squadrons had been bombing fairly relentlessly and it is likely that your late uncle would have been on several similar raids in the preceding days or weeks, so his skill and success is also what led to the defences being beefed up, defenses which ultimately got him, so one might say that he was something of a victim of his own success. Some German home defence squadrons were not very high quality, often with new or 'burned out' pilots making up their numbers, but not all of them were bad, and some were very good indeed. You might also be interested to know that the official view in Britain was that observers and gunners were under more stress than pilots, since it was reckoned that not being in control of the aircraft added to the feelings of stress during combat, so it gives you an idea of what your uncle was up against.
It was one of the better German defence squadrons which 55 ran into on that raid on the 13th, that being Kest 4b led by Leutnant Paul Liem. However, 55 Squadron's formation was good on that day, and despite what many people think, a decent formation of DH4s was quite a formidable thing to attack, as the overlapping fields of fire could be quite intense and German pilots had a fairly healthy respect for them. Attacking German fighters were often shot down by DH4 formations, as it was a very different proposition from the typical fighter ace attacking a lone artillery observation two seater over the front, where he could stay under the tail and avoid return fire for much of the time; there was no way to do that with a tight formation of bombers, so fighter pilots had to brave a hail of fire from the DH4 gunners as they closed in for their attacks.
The DH4 formation on that day would probably have been either: The leader off to the port side of his wingmen, who flew slightly above him. Thirty yards behind these would be the deputy leader, his wingmen to the right of him and slightly below. Other aircraft would be to the left of the deputy leader and their job was to watch gaps in the sections. alternatively, it would have been simpler formations of three aircraft arranged in vees, staggered at differing heights within the individual vees. Both formations were pretty good defensively.
Typically a raid would be twelve DH4s, in the hope that numbers would make up for aiming difficulties, although sometimes one or two would have to turn back with engine trouble, but this was more often the case with the later DH9, which had a troublesome engine, the DH4 was by comparison very reliable. Twelve DH4s with twin guns mounted on the rear-firing mount means that there could be as many as 24 machine guns targeting a fighter which was a relatively stationary target as it tailchased the DH4 formation, so it is not surprising that the DH4s got quite a few kills. The DH4 was pretty fast for a bomber too, and could make upwards of 130 mph on a good day after it had unloaded its bombs, and it could make it up to over 16,000 feet too. That is a good deal faster than many fighters of the time could manage, so the fighters often had to climb over the suspected target, await the enemy and then dive on the formations to catch them.
We can see how effective the DH4 was defensively by the fact that 55 Squadron sent down three German fighters on that 13th March raid, but unfortunately for 55 Squadron, the Germans persisted in their attacks and eventually shot down two DH4s, one of which was presumably be carrying your late uncle. At least one of these aircraft was claimed by Vizefeldwebel Weber of 4b, it being his fourth confirmed victory, so he was no beginner, but then again, it's quite likely your late uncle got one of the fighters that was sent down prior to the DH4s themselves being shot down, as with the pace of 55 squadrons bombing operations, he was probably no beginner himself either.
You might also know that the DH4 had the rather unfortunate nickname of 'The Flaming Coffin', however, contrary to popular belief, this was not because it was any more likely than any other aircraft to catch fire in the air, but rather because the fuel tank (which was located between the pilot and the rear gunner) would sometimes break loose from its mountings in a crash, slide forward and then rupture, trapping the pilot between the now leaking fuel tank and the hot engine, which would then ignite and 'cremate' the trapped pilot!
Despite this, the DH4 was generally well liked by its crews and the grim nickname was more a case of gallows humour than a lack of confidence in the aircraft, DH4s got some impressive air to air victories, one actually managed to shoot down a Zeppelin amongst other things. There are many cases of DH4s catching fire in the air and being safely landed, including once by a very famous DH4 pilot - Merian Caldwell Cooper - Cooper went on to write, direct and appear in the classic 1933 monster movie, King Kong, which you will note has biplanes attacking King Kong at the climax!
If you want an interesting book about the operations of 55 Squadron and the Independent Force of which they were part on such bombing raids, you might want to get hold of 'The first of the Many' by Alan Morris. It's out of print, but you can find it second hand on Amazon fairly easily.
Hope that helps a bit.
Al