Almost all
pilots and aircraft geeks have seen astonishing pictures of a test aircraft
taking off with the tail scraping the runway with a lot of sparks coming from
the rear fuselage during the testing for development and certification. The
truth is that a specific tail bumper is added to protect the tail from any
damage.
The
provisions for tail bumper are already installed in the A350 XWB first aircraft
MSN001.
This test
allows to determine speeds which are called VMU (Velocity Minimum Unstick).
Airbus needs to know the VMU because the computed take off speeds incorporate
some margin above VMU, just as they also do for VS (Stall speed), VMCG (Minimum
control speed on the ground) and VMCA (Minimum control speed in the air). These
“V” speeds therefore form the basic building blocks of take-off performance.
The
optimization of take-off performance is complex. Firstly, the aircraft must be
able to get airborne safely, even in the case of failure of one engine. It may
also have to overfly obstacles, close or far from the runway end, with
sufficient margin, still with an engine failed. The optimization has to be
performed for all weights, altitudes and temperatures and obviously some
compromises have to be made, as no aircraft can be perfect for all conditions.
click the
image to watch a video of the A380 VMU test
Among all
development and certification tests, VMU are probably the most spectacular for
observers, with the small “firework” below the tail just before lift-off. For
crew members, they are also one of the most stressful, as the risk of damage to
the aircraft is rather high. Few pilots can say that they have performed VMU
tests on several programs without damaging anything!
In the case
of the A350 XWB, some structural reinforcements and metallic protection are
being made during the installation of the tail bumper so that it could sustain
a force up to more than 150 tons. They can be seen in orange paint.
The VMU tests
are difficult to carry out, mainly because it has to perform a soft touch down
of the tail bumper, as the structure is not designed for a strong impact.
Perfect
weather conditions are needed, with no turbulence and wind less than 5 kts, to
insure the precision of the measurements. For these tests, all the audio
warnings are “killed” by the crew prior to the test, otherwise the crew receive
a stream of continuous warnings: “Thrust not set”, then “Stall, stall” and
possibly some others.
On A380, a
total of 22 VMU tests were executed including both development and
certification.
The tests will be performed at Istres Air Force Base (South of France) where there is a 5 km runway and no houses or other obstacles on the runway axis for several kilometers.
A teamwork of
5 in the cabin:
The left hand
seat pilot is responsible for flying the pitch. His seat is in the lowest
position as he does not need to see the runway. He adjusts the attitude using
the horizon of the PFD, performing a smooth touch-down of the tail bumper,
keeping the tail on the ground until lift-off and maintaining the pitch
attitude after take-off until out of the ground effect (one wing span) or 400
ft.
The right
hand seat pilot has his seat in the upper position to be able to see the runway
even with a high pitch attitude. On the ground, he maintains the aircraft on
the runway. When in flight, he keeps the roll close to zero using very small
inputs on the rudder (induced roll), and not with ailerons and spoilers to
avoid a drag increase. Finally, he is responsible for safety, which means that
he can take over anytime, typically if the aircraft is not climbing in ground
effect.
The Test
Flight Engineer on the flight deck is in charge of setting very precisely the
thrust, which is important when they are performing tests at very low ratio
thrust over weight.
In the cabin,
in front of all their screens, 2 Flight Test Engineers are monitoring the test,
and thanks to the traces, they validate it (or not).
