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Friday, 14 July 2006 |
For those pilots flying modern airliners calculating departure speeds is as simple as imputing the departure runway, temperature, altimeter setting and take off weight. Ten seconds later the data link printer spits out all the necessary data.ther aircraft use V speed charts to calculate the applicable V1, V2 and Vr speeds to depart safely.
If these devices ever failed or went missing and you actually had to go to those thick aircraft flight manuals (that you rarely open) to calculate runway requirements there is no doubt that departure time would come and go and the airplane would be still be sitting at the gate.
Takeoff Field Length
Regulations dictate that no takeoff should be attempted unless the aircraft is light enough to complete the takeoff or reject within the allowable takeoff field length.
Simply put, the weight of the aircraft has to be such that the aircraft must be able to accomplish the following:
1.Reject a takeoff at V1 and stop on the remaining runway
2.Continue the takeoff with one engine inoperative and clear a 35 foot obstacle at the end of the runway
3.Cross that 35 foot obstacle at the runway end with all engines operating (distance required plus 15%).
All calculations allow 2 seconds for pilot reaction time.
All calculations are performed on a new aircraft under ideal conditions.
V Speed Review
V1 – The captain’s hands are poised over the throttles waiting for a pre-determined reason to stop his accelerating mass of aluminium and jet fuel. This is the decision speed where we must decide to stop or continue the takeoff. Rejecting a takeoff at MTOW approaching V1 involves precision handling.
Vr – At Vr the pilot flying initiates the rotation as required by aircraft type (Airbus 340-300 is approximately 3 degrees per second until 12 degrees nose up). Rotating at Vr ensures the ability to accelerate to V2 by that 35 obstacle at the end of the runway. Rotating to aggressively can lead to a tail strike (14 degrees nose up A340-300) or waiting too long can create excessive loading on the horizontal stabilizer and prevent the aircraft from leaving the ground.
Also remember that V1 and Vr speeds can differ by up to 15 knots on large transport aircraft. You can have an engine failure at a V1 speed of 140 knots and have to track the runway centerline until the aircraft accelerates to 155 knots before you can lift off. Throw in contaminated runways and reduced visibility (airlines can depart on properly equipped runways when visibility is as low as 600 feet) and this manoeuvre only gets more difficult.
V2 – Once the aircraft is rotated the speed accelerates to the V2 speed which must be flown for the initial climb out with an engine failure. This is called the “safety speed”. The aircraft must attain this speed by that 35 foot obstacle at the end of the runway. The V2 speed is required to equal or exceed 1.2 Vs and 1.1 Vmca.
Vmcg – Minimum Control Speed on the Ground – This is the speed at which following the failure of the critical engine it is still possible to control the aircraft with primary flight controls while still tracking the runway center line.
Vmca – Minimum Airborne Control Speed – When the aircraft is airborne this is the minimum speed at which control of the aircraft can still be maintained with the critical engine inoperative. Vr is greater than or equal to 1.05 Vmca.
Things never seemed easier. Make sure you have a 12,000 ft. runway and a lightly loaded aeroplane on a nice VFR day, roll down the runway till the V1 speed (stopping prior to V1 if you have an engine failure or any other failure as described by the particular aircraft manufacturer) rotating at the Vr speed and accelerating to the V2 speed or the recommended climb speed. Now let’s try it again with a 5000 foot contaminated runway, a strong crosswind in reduced visibility. Not sure if you have enough runway?
It’s a lot easier to run through those scenario’s and review the data when your reclining on your living room couch rather than the day when you are sitting in the cockpit with you boss breathing down your neck to get going. Know where the take off data charts are and reviewing their use will make these real life situations a lot less stressful.
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