“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.

WAS IT A FLARE ACCIDENT OR LANDING ACCIDENT?

 

INVESTIGATION MUST REPORT IF THE ACCIDENT WAS A LANDING ACCIDENT OR A FLARE ACCIDENT

Mastering the Landing Flare/Round out

Pilot Perception

Analysis of 6,676 aircraft accident reports published by the National Transportation Safety Board

134 pilots with varying experience levels

Sources:

•           THE INTERNATIONAL JOURNAL OF AVIATION PSYCHOLOGY, 12(2), 137–152
•      Oklahoma State University
•      Department of Psychology
•      Laboratory of Comparative and  Behavioral Biology

           Danny Benbassat and Charles I. Abramson

•  Razia Rashid - founder of Psychology To Safety
•  ATSB RESEARCH AND ANALYSIS REPORT
• Aviation Safety Research Grant – B2005/0119

Aerial perspective is a monocular cue which is used for depth perception [the atmosphere causes distant objects to look hazy or blurry], which is used to judge how far away objects are. Monocular cues are named because they can occur only using one eye (as opposed to binocular cues which only occur with the use of both eyes).

Critical perceived time to contact (TTC)

Time remaining until the aircraft’s wheels make contact with the runway if no further action is taken (Mulder et al 2000).

TTC must be estimated directly, without first estimating speed and distance, based on the following ratio (known as tau):

TTC ≈ θ/(dθ/dt)

Where θ is the visual angle between the aimpoint and any other point on the ground plane at time one (see Figure above); and dθ/dt is the rate of change of this angle over time (Hoyle F 1957; Kaiser & Mowafy 1993; Lee, DN 1976).

Tau is a monocular cue to time-to-contact. During a landing, tau can be defined as the ratio of the angular distance between any two points on the ground (which happen to lie along the aiming line) divided by how fast this angular distance is increasing. Several other versions of tau have been proposed (see Regan & Gray 2000).

In more recent aviation research, Mulder and colleagues (2000) found mixed support for the proposals that flare timing is based on perceived runway angle (ψ) and perceived TTC (based on tau).

Study

The effects of pilot’s critical flare operation on long and hard landing events based on real flight Quick Access Recorder (QAR) data.

Flare accident rates

Causes for improper flares

The flare is the transition from a controlled descent to actual contact with the landing surface (Federal Aviation Administration [FAA], 1999; Grosz et al., 1995) and is also known as the flare out, round out, or level off (Jeppesen, 1985).

The task of determining the aircraft altitude above ground is crucial to a successful flare (Green, Muir, James, Gradwell, & Green, 1996) and is accomplished by the use of vision more than any other sense (FAA, 1999; Jeppesen, 1985; Menon, 1996; Nagel, 1988; Thom, 1992).

Depth Perception

Depth perception deals with the ability to see the environment in three dimensions and estimate distances of objects from us and from each other.

Aerial Perspective, objects at larger distances from us are affected by natural scattering of light and form less of a contrast with their background; making it harder to gauge a distance between the two and us.

Monocular cues operate when a person is looking with only one eye

 Pilots rely on monocular depth perception cues rather than binocular depth perception cues (Benson, 1999; Bond, Bryan, Rigney, & Warren, 1962; Langewiesche, 1972; Nagel, 1988).

Monocular depth perception is learned or dependent on experience (Benson, 1999; Bramson, 1982; Langewiesche, 1972; Love, 1995; Marieb, 1995; Tredici, 1996).

-          It is used by artists to induce depth in their two dimensional paintings. Thus, they are also called as the pictorial cues.

The monocular cues that help us in judging the depth and distance in two-dimensional surfaces:

·         Linear perspective: Imagine you are standing between the rail tracks and looking in the distance. You would see that the rail track become smaller and smaller, until there is a point where they meet each other. Linear perspective reflects this phenomena. It says that the distant between two objects far away appear to be smaller than what the distance actually is.

·         Interposition or overlapping:  This monocular cue occurs when one object covers the other. The object that is completely visible seems to be nearer and the object that is partially visible seems to be farther away.

·         Clearness: The more clear the object the nearer it seems. For example, on a hazy day a distant mountain would appear far away than on a clear day, because haze blurs the fine details. Therefore, if we see details of an object we perceive it closer.

·         Relative height: We perceive large objects to be closer to us and smaller objects to be farther away.

·         Motion parallax/ Movement: This cue occurs when the objects are in motion. The distant object appears to move slowly than the objects that are close.

Binocular cues operate when both our eyes are working together

Binocular depth perception is innate or acquired very early in life (Fox, Aslin, Shea, & Dumais, 1980; Kalat, 1998; Reading, 1983; Reinecke & Simons, 1974).

The cues that are provided by both the eyes working together:

·         Retinal or binocular disparity: Humans have two eyes which are horizontally separated by a distance of 6.5 centimeters. Because of this distance between two eyes the images formed on retina of a same object is slightly different. This difference in the images of two eyes is retinal disparity.

·         Convergence: when we see a closer object our eyes turn inward or converge, so that the image is formed on the fovea. Some muscles in our eyes send signal to the brain regarding the degree of convergence and our brain interprets it as a cue for depth perception. The more your eyes turn inward or converge the nearer the objects appear in the space.

·         Accommodation: The process by which ciliary muscles change the focal length of the eyes so that the image is clearly formed on the retina is called the accommodation of the eye. The accommodation varies for near and distant objects and also for objects moving away or towards the eye.

Failure to accurately determine aircraft altitude may result in flaring the aircraft too high (Gleim, 1998; King, 1999; Quinlan, 1999) or too low above the runway (Christy, 1991; Kershner, 1981; Love, 1995).

Improper flares also increase brake, nosewheel tire, and nosewheel shimmy dampener wear (on Cessnas; Christy, 1991; Jorgensen & Schley, 1990).

Improper flares may affect pilot self-esteem and selfefficacy.

The flare maneuver was defined as the ability to determine 10 to 20 ft from the ground and initiate the leveloff.

Three groups of pilots (novice, intermediate, and expert) were surveyed with purposive sampling.

Participants were 134 pilots (novice = 55, intermediate = 45, expert = 34) from three Part 141-approved flight schools in the state of Oklahoma.

Pilot perceptions were assessed with a 21-item questionnaire.

Pilots were asked to rate the flare maneuver and nine other randomly selected standard flight maneuvers for the level of difficulty on a scale ranging from 1 (extremely easy) to 7 (extremely difficult) under optimal conditions (i.e., no wind, 10 miles visibility).

After rating the 10 items, pilots turned the page and learned that the study was specific to the landing flare.

In Item 11

Pilots were provided with the number of total annual U.S. landing accidents and were asked to estimate the number of annual flare accident frequencies.

In Item 12

Pilots were asked to indicate how confident they were in their estimates of annual flare accident frequencies on a scale ranging from 1 (low confident) to 7 (high confidence).

The next items were not only specific to the landing flare but also to their ability to determine when to initiate the flare, that is, estimate 10 to 20 ft from the ground.

In Item 13

Pilots imagined that they were transitioning from descent attitude to flare attitude and indicated how confident they were that their aircraft was 10 to 20 ft from the ground on a scale ranging from 1 (low confidence) to 7 (high confidence).

In Item 14

Pilots recalled their first solo flare attempts and rated factors that assisted them in determining the aircraft altitude before initiating the flare (CFI instruction, instrument readings, practice, pilot manual, ground-school training, other) on a 7-point-scale ranging from 1 (not at all) to 7 (to great extent).

In Item 15

After a reminder that pilots flare the aircraft 10 to 20 ft from the ground, ascertained how pilots rated the task of judging altitude when initiating the flare on a scale ranging from 1 (very easy) to 7 (very difficult).

In Item 16

Pilots imagined that they were on approach for landing and were asked to choose how they determine when to initiate the flare. how did they know they were 10 to 20 ft from the ground (instrument readings, gut reaction, I don’t, sense of sight, sense of balance, other).

In Item 17

Pilots were asked to indicate if there was a need for improved flare-training methods, on a scale ranging from 1 (definitely yes) to 7 (definitely no).

In Item 18

To what factors (pattern practice, natural ability, sheer luck, aviation books, my instructor, other) did they attribute their current successful landing flares, on a scale ranging from 1 (not at all) to 7 (to great extent).

Items 19 and 20 (reiterated Items 16 and 18)

Required pilots to elaborate and explain their responses. Pilots were instructed to think carefully before they answered and be as specific as possible.

In Item 21

Pilots were asked to indicate what type of visual information assisted them in determining when to initiate the flare.

CONCLUSION

 Overall, 6,676 accident reports produced by the NTSB were analyzed for flare accident rates.

Flare accidents

Proper flares depend on monocular cues, and monocular cues depend on experience (Hawkins, 1993; Rinalducci, Patterson, Forren, & Andes, 1985).

On average, the flare only lasts approximately 6 sec and a pilot with a total time of 5,000 hr only has approximately 8 hr of flare time (King, 1998).

In more recent aviation research, Mulder and colleagues (2000) found mixed support for the proposals that flare timing is based on perceived runway angle (ψ) and perceived TTC (based on tau).

Critical perceived runway angle (ψ)

Pilots could initiate the landing flare when the visual angle (ψ) formed between the left and right edges of the runway at the aiming line reaches a critical value (Mulder et al, 2000).

In fact, pilots use different cues or combinations of monocular cues. For example, overall, the horizon and end of runway, shape of runway or runway markings, and familiar objects were the most frequent visual cues that pilots used to estimate their altitude during the flare.

However, University of Oklahoma pilots most frequently used the horizon or end of runway, whereas Oklahoma State University pilots used the shape of runway or runway markings.

Softer throttle reduction is helpful for a better flare performance. Not immediately IDLE. 

The force of the engines and of gravity are driving the airplane down and forward. When you rotate the nose up for the flare, some of the energy that was used to propel the airplane forward is now used to further arrest the force of gravity. You have less force available for forward motion, your speed necessarily decreases.

Flare energy

The rotation-to-flare requires energy to arrest downward momentum; the amount of energy required depends on the glide path angle, any differences in aircraft speed from target speed, and any acceleration/deceleration. It will be to your advantage to make the angle and speed differences the same for every landing.

The rotation-to-flare may or may not bleed airspeed, depending on aircraft flight idle and ground effect characteristics. Here are three examples:

 B747 — The combination of a very large wing span induced ground effect perfectly compensates for energy needed to arrest the descent. Once the descent has been arrested, the flight idle and ground effect result in no airspeed loss at all under most conditions. The airplane has to be flown onto the runway, it will not run out of speed and sit itself down.

GV — The GV also has a very large wing span and high flight idle engines. The descent can be arrested with very little loss of speed, but the airplane does lose speed gradually if held inches off the runway. But, once again, it should be flown onto the runway to avoid a long landing.

G450 — The G450 will lose about 5 knots in the rotation to flare, which is precisely the minimum speed increment to VREF. Once the rotation to flare is made, speed decay continues as flight idle and ground effect are not enough to maintain speed. Any exaggerated flare for the sake of touchdown will result in a significant loss of speed. Once again, the airplane should be flown onto the runway.

VERY SOFT LANDING

The FLARE curve it has two breaking points: The first, it is on (Xf, Hf) [Glide Slope breaking point] and the second one it is on Xtd flare breaking point [Flare breaking point it is at the touchdown point]. Ideally, from this point on the flare curve must be tangent to runway surface to make a soft landing.

Very soft landing, it is that the Pilot Flying does not let the green curve flare segment [beyond Xtd point] to descend below X axis.

That green curve segment after the touchdown point it can NOT descend below the X axis. 

It does tangent on the runway surface plane for smoothing landing.

When to flare

What "the book" says:

"Very few manufacturers specify a flare height. Gulfstream, for example, leaves you off at 50 feet and the next thing you know, you are in the touchdown zone. Some Bombardier manuals say, "at or below 50 feet." About the only manufacturer that does print a height is Boeing. In their Boeing 777 Flight Crew Training Manual, they say this: "Initiate the flare when the main gear is approximately 20 to 30 feet above the runway by increasing pitch attitude approximately 2° - 3°. This slows the rate of descent." That pretty much agrees with what we did in the Boeing 747." 

You should consult your manufacturer's books.

HINT

Pilot after crossing over threshold at the height 50 ft for landing MUST NOT LET his/her eye corners to capture any lateral vision, mainly Pilot Flying, because of lateral vision illusion which induces pilot to see a relative movement speeding up the plane. That false relative speed up produces a sensation of runway length it would be shortening very quickly. Runway excursion.