The Boeing 747-400 requires a runway length of more
than 3000m right?
- V1: The speed at which an aircraft no longer has room to stop given the remaining length of runway
- VR: The speed at which the aircraft begins to 'rotate' (i.e. the nosewheel leaves the ground)
- V2: The speed at which the aircraft actually becomes airborne (the main gear leaves the ground)
- Vzf The speed at which the lift devices (flaps) are retracted
- Vmc The 'minimum control speed'. This is important - more on Vmc later
- Vs The 'clean stall speed' or the speed at which the aircraft will stall in normal configuration (gear up, zero flaps etc)
The important thing about all of these speeds is safety and
in normal operations an aircraft will often actually take off shortly
after it is able to. Consequently V1 is normally called early. Subsequent
Vrefs are called late.
It is impossible to give definitive figures for any given aircraft
without knowing the conditions. The Vref speeds on any aircraft
depend on wind conditions, barometric pressure, ground temperature,
precipitation, air temperature, altitude (and attitude, although
most are pretty flat) of runway and finally the take-off weight
of the aircraft.
Going back to our 747-400, the aircraft has a maximum take off weight
of about 395 metric tons. Vr would typically occur at about 170
kts and V2 at about 185 kts with this sort of weight. The empty
weight of the aircraft is about 180 metric tons. The total load
of passengers could be anything from 20 to 45 metric tons. Cargo
could be anything from 10 to 25 metric tons. All of this makes the
zero fuel weight somewhere between 210 and 250 tons.
Then there is fuel...
The 747-400 has tanks capable of containing more than 170 tons of
fuel. If our aircraft has a dry weight of 250 tons then the maximum
weight of fuel we will be able to load' is 145 tons. Obviously,
in this case our 747-400 will not be able to fly to it's maximum
range and it will need all of the runway it can get to ensure that
the V2 speed of 185 kts is met.
On the other hand, if our route is a 'modest' 5500 nm we may only
need 135 tons of fuel on board. Even in the worse case (250 tons
fully loaded) we are now 10 tons below maximum take off weight.
The aircraft can rotate earlier and leave the ground earlier. Because
it does not need to reach such a high speed (and it accelerates
slightly quicker) less runway is required....
If the aircraft has a fully loaded dry weight towards the lower
end of the scale (i.e. less passengers) things really start to change.
On a 'light' run we may have 210 tons of aircraft plus passengers
and cargo, and 135 tons of fuel to give a total of 345 tons, 50
tons below MTOW. Now our 747-400 will start rotation at about 160
If the route were 3500 nm (typical transatlantic) then our fuel
requirement is down to about 100 tons, out TOW could be as low as
310 tons and V2 could be as low as 165 kts.
Moving on quickly to Vmc, this is one of the most important Vref
speeds of all. When an aircraft banks it looses lift and drag increases.
The likelihood of a stall becomes greater in a turn as the aircraft
becomes closer to the lower end of the flight envelope. The Vmc
is the speed at which an aircraft is no longer limited to banking
at less than 25 degrees. With a 747-400 the Vmc is approximately
V2+100 kts (in other words, well over the 250kts minimum for operations
below 10000ft. Considering the fact that the most critical manoeuvring
is carried out during the climb out and on the approach Vmc needs
to be as low as possible in order for the aircraft to be operated
safely. Furthermore, with safety in mind ATC may allow aircrafts
to exceed the 250 kts maximum should this be safely carried out.
I guess what I am saying is that the weight carried on an aircraft
has a huge impact on the way an aircraft actually flies. If you
have one of those EOAC Boeing 747-400 aircrafts en route to Hong
Kong or Tokyo you will know what a heavy load actually feels like.
New York, despite high loads, is a lot closer and you need 80 tons
less fuel to get there. At 3hrs 25 the route between Singapore and
Chennai/Madras is another 'light' run for the 747-400, despite high
capacity. Thinner routes such as Rio de Janeiro - Charlotte exist
where although take off weights are still high due to fuel load
passenger (and more importantly cargo) weights are lower allowing
for easier landings.
Chairman of the Board of Directors
European Overseas Airways Company