“DISRESPECTING” THE V1 SPEED - IMPLICATIONS OF NOT RESPECTING V1

 

Sources:

AIRBUS 
Getting to grips with aircraft performance. Jan 2002.
Michel PALOMEQUE
A320 Flight Safety Director & Chief Engineer Advisor A320 Program
Safety first #06 July 2008
LORRAINE DE BAUDUS
Flight Operations Standards & Safety management
PHILIPPE CASTAIGNS
Experimental Test Pilot
Stéphane PUIG
Project Leader, Safety Initiatives
Engineering
 24th Flight Safety Conference
19-22 March 2018
Vienna, Republic of Austria
Safety first - Special Edition - February 2018

“disrespecting” the V1 speed - implications of not respecting V1

 

a)     The crew decides to continue take-off while an engine failure occurred before V1.

The aircraft can potentially exit the runway laterally, or be unable to take-off before the end of the runway.

b)     An RTO is initiated above V1.

GO/ NO GO decision prior to the aircraft reaching V1.

 After V1, the crew must continue take-off and consider using TOGA thrust except if a derated take-off was performed.

 What speeds exactly should be monitored?

What do these speeds mean and where do they come from?

What happens if such speeds are exceeded?

V1: Decision speed

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.

VMBE = Maximum Brake Energy speed.

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

V1 must be lower than VMBE.

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.

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

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

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).

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.

VMCG = Minimum Control Speed on the Ground

It is the limit speed determined during Airbus flight tests.

If a failure occurs before reaching this minimum speed, the takeoff must be interrupted to maintain control of the aircraft.

V1 must be greater than VMCG.

VEF = Engine Failure Speed

The maximum aircraft speed at which the most critical engine can fail without compromising the safe completion of take-off after failure recognition.

V1 must be greater than VEF.

Considering that it is generally assumed humans have a reaction time to an unexpected event (such as a failure) of 1 second.

VEF must be greater than VMCG.

If an engine failure happens at VEF, then it must be possible to continue and achieve the safe take-off speed with TOGA power triggered.

Minimum Control Speed on the Ground: VMCG

In the determination of VMCG, assuming that the path of the airplane accelerating

with all engines operating is along the centerline of the runway, its path from the point

at which the critical engine is made inoperative to the point at which recovery to a

direction parallel to the centerline is completed, may not deviate more than 30 ft

laterally from the centerline at any point.”

V2: Take-off safety speed

 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.

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

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.

 Minimum Control Speed in the Air: VMCA

VMC[A] may not exceed 1.2 VS with

• Maximum available take-off power or thrust on the engines;

• The most unfavorable center of gravity;

• The airplane trimmed for take-off;

• The maximum sea-level take-off weight

• The airplane in the most critical take-off configuration existing along the flight path after the airplane becomes airborne, except with the landing gear retracted; and

• The airplane airborne and the ground effect negligible

 Minimum Control Speed during Approach and Landing: VMCL

The minimum control speed during approach and landing with all engines operating, is the calibrated airspeed at which, when the critical engine is suddenly made inoperative, it is possible to maintain control of the airplane with that engine still inoperative, and maintain straight flight with an angle of bank of not more than 5º.

VMCL must be established with:

• The airplane in the most critical configuration (or, at the option of the applicant, each configuration) for approach and landing with all engines operating;

• The most unfavorable center of gravity;

• The airplane trimmed for approach with all engines operating;

• The most unfavorable weight, or, at the option of the applicant, as a function of weight.

• Go-around thrust setting on the operating engines

Minimum Unstick Speed: VMU

It is the calibrated airspeed at and above which the airplane can safely lift off

the ground, and continue the take-off…”

During the flight test demonstration, at a low speed (80 - 100 kt), the pilot pulls

the control stick to the limit of the aerodynamic efficiency of the control surfaces. The

aircraft accomplishes a slow rotation to an angle of attack at which the maximum lift

coefficient is reached, or, for geometrically-limited aircraft, until the tail strikes the

runway (the tail is protected by a dragging device). Afterwards, the pitch is

maintained until lift-off.

Two minimum unstick speeds must be determined and validated by flight tests:

- with all engines operatives : VMU (N)

- with one engine inoperative : VMU (N-1)

In the one-engine inoperative case, VMU (N-1) must ensure a safe lateral control

to prevent the engine from striking the ground.

 Typical tailstrike scenario

Most of the tailstrikes on A320 family aircraft occur during landing in manual mode (Auto Pilot OFF), when the sidestick is maintained in the aft position after touch down.

Additional alerts to impeding tailstrike

• A pitch limit indicator on the Primary Flight Display, which is displayed at landing (below 400 feet AGL in both manual and automatic modes) when the thrust levers are below the FLEX/MCT setting.

• A “PITCH, PITCH” call out, activated when the pitch is greater than a certain threshold and if TOGA is not selected.

(The call out is available on the following standards : FWC H2F3 or H2F3P and FAC 618 or 619).

Managed Descent 

The managed descent mode guides the aircraft along the FMS computed vertical flight path. The  mode is preferred when conditions permit since it ensures the management of altitude constraints and reduces the operating cost when flying at ECON DES speed. The  mode is only available when the aircraft flies on the FMS lateral flight plan, i.e. when the aircraft uses the  horizontal guidance mode.

MANAGING SPEED DURING APPROACH AND LANDING

In a decelerated approach, the aircraft is decelerating during its final approach segment to be stabilized at VAPP at 1000ft above the airport elevation. In most cases, it reaches the Final Descent Point

(FDP) in CONF1 at S speed. However, in some cases, when the deceleration capabilities are low (e.g. heavy aircraft, a high elevation airport or tailwind), or for particular approaches with a deceleration segment located at low height, the flight crew should select CONF 2 before the FDP. The FCOM recommends selecting CONF 2 before the FDP when the interception of the final approach segment is below 2000ft AGL (A320) or 2500ft AGL (A330/A340, A350 and A380). In this case, for ILS, MLS or GLS approaches, or when using FLS guidance, it is good practice to select FLAPS 2 when one dot below the glideslope on the PFD deviation scale.

The take-off preparation by the pilots entails the computation of the aircraft weights (Zero Fuel Weight, Take-Off Weight) and respective CG positions, as well as the calculation of the different Take-Off speeds (V1, VR, V2) and thrust rating.

These data may be obtained either by using load sheets and take-off charts, or by means of non-aircraft software applications (i.e. flight operations laptops).

 Three types of errors may be performed during this process:

• Parameters entered into the tables or into the programs may be wrong (carried load, outside temperature, runway length etc…)

• Computations may be inaccurate (wrong interpretation of charts, bug in the software etc…)

• The data entry process into the Flight Management System (FMS) may be incorrect (distraction, stress etc…).

 Each of these types of errors may have consequences on the Take-Off speeds:

• A too low VR inserted through the Multipurpose Control & Display Unit (MCDU), may lead to a tail strike.

• A too low V2 may lead to the flight path not clearing the obstacles in an one engine out condition.

• A set of too high Take-Off speeds may lead to a runway overrun or too high energy rejected take-off (RTO).

 Other possible consequences:

• An error on the A/C configuration at take-off (CONF/TRIM setting) may lead to an “auto rotation” or a nose heavy condition

• A take-off from a different runway from the intended one, or even from a taxiway, may lead to:

- A collision on ground with another aircraft, vehicle or obstacle

- A collision in the air with an obstacle

- An overrun if no lift-off before the end of the runway (even more so if combined with a high temperature FLEX take-off)

- A low or high energy runaway overrun (in case of RTO)

• A wrong thrust rating may result in a tailstrike, a runway overrun or a shift of the climb path.

Take-Off Securing function (TOS)

The TOS has been developed to detect, to the best extend possible, wrong data entered into the

FMS.

The Thales system checks:

• The Zero Fuel Weight (ZFW) range

• The Take-Off speeds consistency.

 The Honeywell system checks:

• The Zero Fuel Weight (ZFW) range

• The Take-Off speeds consistency

• The Take-Off speeds limitations.