In their own words...

[bobbysocks]

"WE began to carry out low-level flights over France. These
operations were known by the code name Rhubarb. The idea
was to take full advantage of low cloud and poor visibility and
slip sections of Spitfires across the coast and then let-down
below the cloud to search for opportunity targets, rolling-stock,
locomotives, aircraft on the ground, staff cars, enemy troops and
the like. They were usually arranged on a voluntary basis and a
few pilots seemed to prefer this type of individual, low-level work
to the clean, exhilarating team work of the dog-fight. But the
great majority of fighter pilots thought privately that the dividends
yielded by the numerous Rhubarb operations fell far short
of the cost in valuable aircraft and trained pilots.
First of all we had to contend with the weather. Usually the
cloud base was less than 1000 feet when we slipped our two
Spitfires into its concealing vapour. During the next few minutes
all our thoughts were concentrated on the likely height of the
cloud base over France. Our let-downs from the cloud were
usually made over reasonably flat countryside, but here and there
small hills rose a few hundred feet and presented serious hazards.
If we weren't in the clear when the altimeter recorded 500 feet,
then we climbed back into the cloud and called the show off.
So it was difficult to be cool and calculating when making our
let-downs on Rhubarb flights. Perhaps two of us had flown in
cloud, in tight formation, for a distance of fifty miles at 2000 feet.
Time to descend, for we are over the target area—or should be
if we have steered an accurate course and the wind hasn't changed.
We ease the throttle back and put the Spitfires into a gentle dive.
The engine note changes, but it seems strangely loud in the cloud
and the stick trembles in your hand. You flash a grin of encouragement
at your wingman who is only a few feet away, his
eyes and hands attuned to every movement of your Spitfire,
for if he loses you in this bumpy, swirling greyness there is
not enough height for him to make the difficult transition to
instrument flight. You ease her down slowly. Are we slightly
off course? Will the ground be higher than where we planned to
break out? And the flak? 600 feet on the altimeter and you
catch a sudden glimpse of a wet sombre landscape of
hedged fields and copses. Then you are at the bottom of a
sort of inverted bowl, whose translucent sides of falling rain
seem dangerously confining.
Then there was the light flak. Gibbs told us that once beyond
the heavily defended coastal belt we should be lightly opposed
from the ground, but it always seemed as if the enemy gunners
were ready and waiting. Airfields were always extremely well
defended and it was a dangerous business to try and make more
than one fast, low-level attack. Straight in and out was the only
method on these occasions.
The Germans prepared unpleasant counter-measures against
these low-level attacks. Here and there decoy targets were
established, and these sometimes took the form of stationary
locomotives heavily armoured and surrounded by numerous,
well-camouflaged light flak guns, arranged to provide a deadly
concentration of fire against air attack. Many pilots received the
shock of their lives when they streaked down upon what they
imagined to be a sitting duck.
Usually our Rhubarb efforts yielded little more than a staff car
(or was it a French civilian vehicle?) or some target ineffectively
sprayed with the puny bullets of our machine guns. Whenever
we went after bigger game on the airfields we took some bad
knocks, and our first losses were from such operations. The
engines of our Spitfires were cooled by a liquid called glycol,
which was held in a small tank just below the spinner. This
glycol tank and radiator were always exposed to ground fire,
and one machine-gun bullet through either meant that the
engine caught fire or seized up within a matter of minutes.
I loathed those Rhubarbs with a deep, dark hatred. Apart
from the flak, the hazards of making a let-down over unknown
territory and with no accurate knowledge of the cloud base
seemed far too great a risk for the damage we inflicted. During
the following three summers hundreds of fighter pilots were lost
on either small or mass Rhubarb operations. Towards the end of
1943, when I finished this tour of ops. and held an
appointment of some authority at 11 Group, my strong views
on this subject were given a sympathetic hearing and Rhubarbs
were discontinued over France, except on very special
occasions."

from: Johnny Johnson, "Wing Leader", 1956