Control your speed… at take-off

Our objective is to highlight the design and operational considerations underlying all recommendations Airbus has issued to flight crews regarding speed monitoring during take-off.

Take-off operating speeds V1, VR and V2 very precisely frame the aircraft take-off performance limits and the margins that exist in the event of a failure (fig.1).

For every aircraft type, V1, VR and V2 are computed by Airbus on the basis of design speeds and evidence collected during the certification testing of the airplane.

(fig.1)
V1: Decision speed
VR: Rotation speed
V2: Take-off safety speed

• Definition

V1 is the maximum speed at which a rejected take-off can be initiated in the event of an emergency.

V1 is also the minimum speed at which a pilot can continue take-off following an engine failure.

This speed is entered by the crew in the MCDU during flight preparation, and it is represented by a “1” on the speed scale of the PFD during take-off acceleration (fig.2).

(fig.2) V1 on the PFD speed scale

• How is V1 determined?

If take-off is aborted at V1, the aircraft must be able to be stopped before the end of the runway, without exceeding the maximum energy the brakes can absorb.

In addition, if an engine failure occurs after V1, then the aircraft must be able to achieve safely take-off with TOGA or derated power (enough lateral control).

These two conditions require identifying:

1 – The ground speed at which maximum energy is put into the brakes, when a RTO is performed at MTOW.

This limit speed is defined during Airbus flight tests and is called VMBE = Maximum Brake Energy speed. V1 must be lower than VMBE.

Video: A380 Tests – Maximum Energy RTO

2 – The minimum speed during take-off roll at which the aircraft can still be controlled after a sudden failure of one engine (be it a two or four-engine airplane).

In such a case, and if the take-off is continued, only the rudder will be able to counteract the yaw moment that is generated by asymmetric engine(s) thrust. Therefore if a failure occurs before reaching this minimum speed, the take-off must be interrupted to maintain control of the aircraft. This limit speed is determined during Airbus flight tests and is called VMCG = Minimum Control speed on the Ground. VMCG mainly depends on engine(s) thrust and pressure altitude. V1 must be greater than VMCG.

3 – The minimum aircraft speed at which the most critical engine can fail without compromising the safe completion of take-off after failure recognition. This design speed is called VEF = Engine Failure speed.

Considering that it is generally assumed humans have a reaction time to an unexpected event (such as a failure) of 1 second, V1 must be greater than VEF.

In addition, if an engine failure happens at VEF, then it must be possible to continue and achieve safely take-off with TOGA power. This means that VEF must be greater than VMCG.

Video: A380 Tests – Engine Failure at Takoff

2 – An RTO is initiated above V1.

Virtually, any take-off can be “successfully” rejected, on the proviso that the reject is initiated early enough and is conducted properly. In this respect, the crew must always be prepared to make a GO/ NO GO decision prior to the aircraft reaching V1.

Doing otherwise exposes the aircraft to an unsafe situation where there either may not be enough runway left to successfully stop the aircraft – therefore resulting in a longitudinal runway excursion-, or maximum brake energy is exceeded and brakes catch fire.

• Definition

VR is the speed at which rotation can be initiated at the appropriate rate of about 3° per second. VR ensures that V2 is reached at 35 feet above the runway surface at the latest, including in the event of an engine failure at VEF. Therefore at 35 feet, the actual speed is usually greater than V2.

• How is VR determined?

In principle, VR shall not be lower than V1.
In addition, whenever pilots initiate the rotation at VR, they must be assured that the aircraft will be controllable once airborne, including when the most adverse engine has failed after V_EF.

On the upper end, if the rotation of the aircraft is started at VR at maximum practicable rate, lift-off must be possible at the end of the maneuver.

These concepts involve understanding the following limit speeds:

• The minimum speed in the second segment (take-off) at which the pilot is still able to maintain lateral and directional control when the most adverse engine fails.
  This limit speed is demonstrated by Airbus flight tests and is called V_MCA = Minimum Control speed in the Air. VR shall not be lower than 1.04 or 1.05 V_MCA, the factors 1.04 and 1.05 being defined by Airworthiness Authorities to ensure a safety margin.
• The minimum speed at which the aircraft becomes able to lift off and escape ground effect.

This limit speed is based on evidence collected during certification tests and is called V_MU = Minimum Unstick speed. V_MU is achieved by pitching the aircraft up to the maximum (tail on the runway, for aircraft that are geometrically limited) during the take-off roll. The speed at which the aircraft first lifts off is V_MU; therefore lift-off is not possible prior to V_MU.

V_MU is different from the design lift-off speed V_LOF, which applies to general case scenarios and is necessarily greater than V_MU, according to the following criteria:

Video: A380 Tests – VMU test

In turn, V_LOF is limited by the design speed V_TIRE, which corresponds to the maximum tyre speed (tyre structural limit).

Coming back to VR, if we consider that when a rotation is initiated at VR at the maximum practicable rate, it has to result in a satisfactory lift off speed, then VR must be limited by V_LOF.

• What are the operational implications of not respecting VR?

One direct consequence of initiating a rotation before VR is a tail strike.

Second, if the rotation is done at VR but too slowly, or if the rotation is initiated after VR, then the aircraft intrinsic performance will very likely not allow it to reach 35 feet at the end of the runway, and/or not respect the clearway if the take-off speeds were limited by the runway length or obstacles.

• Definition

V2 is the minimum take-off speed that the aircraft must attain by 35 feet above the runway surface with one engine failed at VEF, and maintain during the second segment of the take-off.

This speed must be entered by the crew during flight preparation, and is represented by a magenta triangle on the PFD speed scale (fig.3).

(fig.3) V2 on the PFD speed scale

• How is V2 determined?

V2 is always greater than VMCA and facilitates control of the aircraft in flight.

On the upper end, Airworthiness Authorities have agreed that all operating speeds must be referenced to a stall speed that can be demonstrated by flight tests. This speed is designated VS1g. V2 must obviously be greater than this stall speed.

• What are the operational implications of not respecting V2?

Supposedly, there are two different ways of “disrespecting” the V2 speed criteria:

1 – Flying below V2 in case of an engine failure.
The drag increase below V2 may lead to a situation where the only way to recover speed is to descend.

If the speed further decreases and V2 is not recovered, then the high angle of attack protection may be reached, and the aircraft may ultimately enter into an unrecoverable descend trend. In particular, if the speed decreases below VMCA, the aircraft might not be recoverable due to lack of lateral control.

2 – Flying above V2 in case of an engine failure.
In case of excessive speed, the required climb performance may not be reached, thus increasing the chance to trespass the obstacle clearance.

Clearly flight crews are expected to be able to rapidly scan the essential and relevant parameters that support key decisions, such as continue or abort a take-off essentially. Doing so, the PM must be able to differentiate between situations that are detrimental to operational safety, and those that are not.

In this respect, he/she must be prepared to adapt his/her monitoring to the level of the threat and reach out in a communication sense to the PF to encourage action if necessary, by making callouts as per SOP. Callouts
coupled to responses are a very effective means indeed to cope with demanding situations, and allow the crew to act as a well coordinated team.

Second, he/she must be aware of the primary threats to the safe completion of take-off in order to actively help to prevent take-off speed errors. Take-off speed calculation errors are often due to a combination of two factors:

• Error in parameter entry
• Poor crosschecks by other crewmember.

Prevention strategies should therefore be developed to ensure efficient crosschecks, particularly after last-minute changes (runway change, loadsheet modification, etc).

For this purpose, we want to highlight the main factors often observed when analysing take-offs in which speeds were not respected:

1 – Errors in take-off speed computation

• Data issued from a computerized system is rarely challenged. However, incorrect inputs may occur, thus resulting in inadequate take-off speed values computation.
• In take-off speed calculations, Zero Fuel Weight (ZFW) is sometimes mistaken for Gross Weight (GW). This is particularly true when a last minute change occurs in cargo loading, or when time pressure and workload are high. Therefore calculated speeds will be much lower than expected, and will potentially lead to tailstrikes, “heavy aircraft” sensation, and high-speed rejected take-offs.
• Take-off speeds calculations are based on specific configurations. Any change in the parameters of these configurations will invalidate take-off speeds. Examples of such parameters include a runway change, a wet runway that becomes contaminated, or a take-off from an intersection.

2 – Errors in take-off speed utilization

• When a last minute change occurs, take-off speeds are sometimes modified and crosschecked during pushback or taxi. During such phases of flight, the PF workload is high. As a result, the PF may not have sufficient time or resources to perform efficient crosschecks.
• If an incident occurs before V1, the PM’s attention may be focused on trying to assess the situation and may forget the V1 announcement.
• In the event of an engine failure after take-off, and in an attempt to climb faster, there may be a tendency to set a pitch attitude too high if FD bars are not followed. The aircraft is then flown below V2, and climb performance cannot be maintained.