Understanding Vref and Approach Speeds

UNDERSTANDING UNDERSTANDING VREF A ND APPROA A PPROACH CH SPEEDS EMPRESA BRASILEIRA DE AERONÁUTICA S.A.

THIS THIS DOCUMENT INCLUDES INCLUDES ALL REQUIRED RE QUIRED INFORMATIO INFORMATION N TO CLARIFY EMBRAER’S OPERATIONAL PHILOSOPHY REGARDING APPROACH SPEEDS. THIS THIS MANUA MANUAL L IS APPLICABL APP LICABLE E TO EMB-1 EMB-145, EMBRAER 17 170 AND EMBRAER 190 FAMILIES OF AIRPLANES. AIRP LANES.

GP–1971 APRIL A PRIL 26, 2004 REVISION 6 – SEPTEMBER 20, 2010 Copyright 2010 by EMBRAER - Empresa Brasileira de Aeronáutica S.A.. All rights reserved. This document shall not be copied or reproduced, whether in whole or in part, in any form or by any means without the express written authorization of Embraer. The information, technical data, designs and drawings disclosed in this document are

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UNDERSTANDING VREF AND APPROACH SPEEDS

LIST OF EFFECTIVE PAGES ORIGINAL ............0 ...........APR 26, 2004 REVISION ............1 ........... SEP 20, 2005 REVISION ............2 ...........OCT 10, 2005 REVISION ............3 ...........AUG 11, 2006 REVISION ............4 ........... SEP 21, 2007 REVISION ............5 ............J UL 01, 2008 REVISION ............6 ........... SEP 20, 2010

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UNDERSTANDING VREF AND APPROACH SPEEDS TABLE OF CONTENTS

INTRODUCTION .................................................................................. 1 SECTION I - LANDING REGULATIONS ............................................. 1 FAR/JAR 25.125 - LANDINGS........................................................ 1 FAR 121.195/JAR OPS 1.515 - FIELD LENGTH LIMIT WEIGHT........................................................................... 3 LANDING DATA PRESENTATION................................................. 4 SECTION II - WIND EFFECTS ............................................................ 5 UNIFORM WIND MODEL DISTRIBUTION..................................... 5 VARIABLE WIND MODEL DISTRIBUTION.................................... 7 VERTICAL WIND DISTRIBUTION IN THE REAL WORLD .......... 10 SPEED MARGIN TO V REF DURING THE APPROACH................11 SECTION III - APPROACH SPEEDS................................................ . 12 SECTION IV - V APP CONSIDERATIONS ........................................... 13 V APP MINIMUM VALUE .................................................................13 V APP MAXIMUM VALUE ................................................................ 14 ABNORMAL/EMERGENCY PROCEDURES ............................... 15 SECTION V - CONCLUSIONS........................................................... 16

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INTRODUCTION The main purpose of this publication is to clarify Embraer’s operational philosophy regarding approach speeds by providing sufficient background theory to aid operators with their own operational policy related to this topic. This publication provides guidelines only and does not annul, amend or complement instructions recommended in the Airplane Flight Manual.

SECTION I − LANDING REGULATIONS Before establishing any definition about approach speeds, we must first understand how a landing is defined under current civil aviation regulations, especially factors that must be accounted for.

FAR/JAR 25.125 - LANDINGS (a) The horizontal distance necessary to land and to come to a complete stop from a point 50 feet above the landing surface must be determined (for standard temperatures, at each weight, altitude, and wind within the operational limits established by the applicant for the airplane) as follows: (1) The airplane must be in the landing configuration. (2) A stabilized approach, with a calibrated airspeed of V REF , must be maintained down to the 50 feet height. V REF may not be less than: (1) 1.23 V S1g . (2) V MCL (Minimum configuration).

Control

Speed

in

Air

on

landing

(3) A speed that provides maneuvering capability on approach and landing. As per the text above, the first statement that validates all the certified landing data is CROSSING THE THRESHOLD WITH V REF at a height of 50 ft.

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During certification, the actual landing distance is demonstrated as follows:

•

Standard temperature.

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Landing configuration: landing gear and flaps set for landing.

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Stabilized approach at V REF.

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Changes in configuration, power or thrust, and speed, must be made in accordance with the established procedures for service operation.

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Determination on a level, smooth, DRY and hard-surfaced runway.

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The landing must be made without excessive vertical acceleration, tendency to bounce, nose over, ground loop, porpoise, or water loop.

•

If any device is used that depends on the operation of any engine (such as thrust reversers), and if the landing distance would be noticeably increased when a landing is made with that engine inoperative, the landing distance must be determined with that engine inoperative unless the use of compensating means will result in a landing distance not more than that with each engine operating. The reverse thrust effect is not accounted for during Embraer airplane landing certification.

•

The landing may not require exceptional piloting skill or alertness.

•

The pressure on the wheel braking systems may not exceed those specified by the brake manufacturer (maximum braking capability) and may not be used so as to cause excessive wear of brakes or tires.

•

Means other than wheel brakes may be used if that means:

− Operation is reliable and safe; − Operation is such that consistent results can be expected in service; and is such that exceptional skill is not required to control the airplane. In regards to Embraer airplanes, other braking resources means the use of spoilers on ground.

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FAR 121.195/JAR OPS 1.515 - FIELD LENGTH LIMIT WEIGHT The LANDING FIELD LENGTH regulations require that the landing distance on a DRY RUNWAY, based on a landing weight assuming normal fuel consumption, must not exceed 60% of the available landing distance. Thus, there is a 40% stopping margin to the end of the runway. In other words, the LDA (landing distance available) published for a specific runway must be 1.67 (or 1/0.60) longer than the actual landing distance. For WET runways, the minimum required length must be the dry runway required length increased by 15%. As the dry runway required length is the dry actual landing distance multiplied by 1.67, the WET runway required length is the dry actual landing distance multiplied by 1.92 (=1.67x1.15). The wet runway landing condition demonstration is not required during certification flight tests. DRY RUNWAY

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V=0 50 ft

ACTUAL DRY DISTANCE DRY FIELD LENGTH = 1.67 X ACTUAL DRY DISTANCE

FIGURE 1: DRY RUNWAY CERTIFICATION WET RUNWAY

V=0 50 ft

ACTUAL DRY DISTANCE DRY FIELD LENGTH = 1.67 X ACTUAL DRY DISTANCE WET FIELD LENGTH = 1.92 X ACTUAL DRY DISTANCE

FIGURE 2: WET RUNWAY CERTIFICATION 1 7 9 1 P G

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LANDING DATA PRESENTATION The LANDING FIELD LENGTH LIMITED WEIGHT presented in the AFM or Airport Analysis Software is the maximum weight at which the airplane is capable of landing in 60% of the available runway length under DRY conditions. It is important to know that flight-testing is performed on DRY runways only. The WET runway condition is not evaluated during flight-testing and the values contained in the AFM are calculated by adjusting the DRY condition parameters. Special certifications, such as slippery and contaminated runways, require additional flight tests and may present reverse thrust considerations, depending on certification parameters. The landing data shown in the AFM must include correction factors for winds. They cannot be more than 50% of the nominal wind components along the landing path opposite to the direction of landing, and not less than 150% of the nominal wind components along the landing path in the direction of landing. For FAA and EASA certification purposes, ice accretion corrections must be applied to VREF should icing conditions on landing be encountered or predicted and, as a result, all the landing data presented in the AFM should be adjusted.

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SECTION II − WIND EFFECTS Now, let’s take a close look at wind behavior from the threshold to the ground and how it may affect overall landing performance. In order to perform our study, it is necessary to assume a theoretical wind distribution for the vertical headwind profile to the ground, using as a reference the headwind value encountered over the threshold at 50 ft height. We also assume that there are no air disturbances over the runway caused by nearby obstacles or other airplane.

UNIFORM WIND MODEL DISTRIBUTION Assume that headwind intensity is constant from 50 ft above the threshold to the runway surface, along all the runway’s extension. Consider that the runway is at sea level. Also assume that the airplane is dispatched for zero wind on landing and the landing weight is exactly the field length limit. Due to the headwind component constant from 50 ft height to the ground, crossing the threshold at V REF will produce a shorter field length than expected on zero wind. Put another way, if the threshold is crossed at V REF plus an additional speed margin that is exactly equal to the headwind’s intensity value, the effective ground speed would be exactly equal to the V REF value. In this model of wind distribution, the effective field length should be exactly the same value when crossing the threshold with V REF and calm wind.

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+ VW

VW

VW V=0

50 ft

SHOULD BE EQUIVALENT TO ...

W V=0 50 ft

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FIGURE 3: UNIFORM WIND MODEL We can conclude that, using the uniform distribution model, adding the same headwind value to V REF, will produce the same performance as a calm wind approach on VREF. Since conditions in nature are not constant, the model must be adjusted for changes to be more accurate.

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VARIABLE WIND MODEL DISTRIBUTION Due to airflow effects next to the ground, we can assume mathematically that the headwind value is zero on the surface. This includes the boundary layer effect on the vertical profile of the wind distribution. In the real world, zero headwind is very difficult to achieve next to surface, due to the presence of other vortex systems or disturbed air flows. These kinds of disturbances may be generated by the presence of other aircraft, nearby obstacles or random wind patterns. The turbulent boundary layer model can be used to predict headwind distribution from 50 ft from the ground, if the disturbing airflow pattern next to the ground is not considered. The following formula is commonly used in order to estimate this profile is:

 h  V = VW ×    50 

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Where VW is the headwind component at 50 ft above the threshold and h is the height above ground. Consider again the runway at sea level. If the threshold is crossed with VREF plus the same value of the headwind at 50 ft, the resultant effect will be the same as an increasing tailwind acting upon the airplane, reaching the maximum value of exactly V W at h=0 ft. This leads to an increasing ground speed that would produce a longer effective field length when compared with crossing the threshold on VREF at 50 ft with calm wind.

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+ VW

VW

VW V=0

50 ft

IS EQUIVALENT TO ...

V W = 0 AT h = 50 ft V=0 50 ft VW MAX AT h = 0 ft

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FIGURE 4: VARIABLE WIND MODEL How can you eliminate this performance penalty? It can be verified that if the threshold is crossed with V REF plus half the value of VW, the performance effect is approximately equivalent to a uniform vertical headwind distribution with half the intensity of the wind estimated at 50 ft over the threshold. That is the reason to assume half the headwind value of AFM data. In this manner, the boundary layer performance effect is traded to a constant wind model, as shown on the following figure.

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V


+ VW

VW

VW V=0

50 ft

IS EQUIVALENT TO ...

V

+ VW V

W

/2

V

W

/2 V=0

50 ft

THAT IS EQUIVALENT TO ...

W V=0 50 ft

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FIGURE 5: WIND EQUIVALENCE This can be verified in the numerical example below: Consider an EMBRAER 170 landing at a sea level airport with o OAT=15 C. At maximum landing weight (32800 kg) and VREF = 128 KIAS, the following results are verified with the AFM data: For Calm wind and VREF at threshold: Unfactored Landing Distance = 810 m. For 10 KIAS Headwind and VREF+5 kt at threshold: Unfactored Landing Distance = 805 m. For 10 KIAS Headwind and VREF+10 kt at threshold: Unfactored Landing Distance = 850 m. If using full headwind value added to V REF, the Unfactored Landing Distance is 4.94% greater than when using V REF with calm wind.

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If using half the headwind value added to V REF, the Unfactored Landing Distance is –0.62% less than when using V REF with a calm wind. Within CAFM performance software, the wind model is considered until the height that the Mean Aerodynamic Chord stands above the ground, when the airplane is completely landed (with all nose and main landing gear wheels on ground). But, in the real world, numerical models are based on assumptions that may not be so simple to put into equations.

VERTICAL WIND DISTRIBUTION IN THE REAL WORLD Headwind distribution is similar to the turbulent boundary layer model shown above only if there are no significant flow disturbances near the surface. In fact, the real world is different since there may be obstacles near the runway that can produce nearby vortices and different flow patterns. Even distant obstacles or hills can produce vortex effects that may induce different wind flows or loss of flow energy. The recent passage of other airplanes on the runway (either departing or arriving), may induce or disturb the wind pattern since wingtip vortices may remain over runway surface for several minutes. The wingtip vortex system of one’s own airplane next to the ground may generate disturbances, and may be an important factor to consider. The ground effect disturbs the boundary layer. The wind reported by the tower is measured at a height of 33 ft. (10 m), and not at 50 ft. Theoretically, that differs 6% from the real headwind on the threshold. Also, the location of the anemometer may be a factor and its readings may be significantly different when compared to real wind conditions at 50 ft over the threshold. Therefore, headwind distribution in the real world may be different from the model above, depending on the geometry (or geography) of the landing. In other words, there are no guaranties that the last 50 ft of a landing will be strictly influenced by the theorical model, but it can be used as an initial guideline.

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SPEED MARGIN TO VREF DURING THE APPROACH The VREF is defined by 1.23 VS-1g. Typical VREF values for the EMBRAER 170 are from 100 to 130 KIAS up to maximum landing weights, which translates into a 20 KIAS to 30 KIAS margin to Stall. During a moderately turbulent approach, a variation up to 20 KIAS can be expected, which could bring the airplane unacceptably close to a stall should it be flying at VREF . Moreover, variations of up to 5 KIAS above VREF are acceptable and have been proven to exist when considering the average pilot’s hand flying skills and technique on a stabilized final approach, even under calm wind conditions. Therefore, from an operational point of view, it is acceptable to maintain speeds higher than VREF on final approach in order to improve the airplane’s flying qualities while subjected to turbulent air, thus providing an adequate maneuver margin above stall speeds.

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SECTION III APPROACH SPEEDS Many airlines have policies for applying wind and gust corrections (additives) to the landing reference speed (VREF ) on final approach. These additives are initially intended to compensate for sudden unexpected wind changes, especially on the final 500 ft, where the boundary layer is disturbed by local elevations, wake turbulence vortex systems and even low intensity windshears. The policy adopted by EMBRAER can found at the SOPM of each airplane model. It must be noted that AFM landing performance data does not take into consideration these VREF additives. Indeed, the AFM considers that the airplane is always landing at a speed exactly equal to VREF . The touch down speed will decrease naturally by 2 kt to 3 kt below VREF if the threshold is crossed with VREF at 50 ft and the normal flare maneuver is performed.

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SECTION IV V APP CONSIDERATIONS If, for safety reasons (or any other reason established by the Captain), the airplane crosses the threshold with VAPP instead of VREF , the Captain must keep in mind that the results shown in the theoretical model may not be applicable. This is attributable to real world factors that cannot be easily predicted for the last 50 ft of altitude above the ground. If the threshold is crossed at VAPP , landing performance might not be equivalent to the predicted uniform wind distribution model (zero wind with VREF on threshold). There are no assurances whether the regulatory 40% runway margin will be exceeded or not in this scenario.

Crossing the threshold with VREF, with whatever wind is present, assures that, at least, the performance values presented in the AFM are achieved.

V APP MINIMUM VALUE Small disturbances to a stabilized approach are expected even when calm winds are reported. As the airplane approaches the ground, the boundary layer may be affected by local elevations or vortex systems. Approaching at VREF may lead the airplane to a marginal performance situation if some air instability is encountered, bringing it near to stalling speed values. It has also been ascertained that an average pilot’s hand flying technique may show 5 kt target speed variations, which is acceptable. For minimum VAPP values, refer to SOPM.

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V APP MAXIMUM VALUE The recommended upper VAPP value is related to the structural limits of the flaps. A complete analysis of ABNORMAL VREF adjustments and their margin to maximum flap placard speed was performed in all Embraer airplanes. The higher VREF additive found under ABNORMAL conditions, considers maximum landing weight and the structural limit of the flap used on the approach. This defines the maximum recommended V REF additive.

The maximum flap placard speed (VFE) must always be obs erved since this is the official AFM limitation. If V APP is higher than maximum flap placard speed (VFE), it may not be necessary to use lower flap settings in order to get higher VFEs. If flaps placard speed is exceeded by V APP for a given flap setting, reducing V APP down to VFE should be considered. Embraer analyzed all combinations of flight conditions, malfunctions and wind corrections and concluded that a slight reduction on VAPP (up to 5 kt) is necessary to respect VFE in cases of high landing weights. This reduction does not represent a significant change on flight qualities during the approaches. Example for EMB-145LR: Consider a normal landing with FLAPS 45° after ice encounter, with a landing weight of 19000 kg. •

VREF FLAPS 45° = 128 KIAS

•

Ice encounter correction on VREF = 6 KIAS

•

Wind correction = 15 KIAS

•

VFE 45 = 145 KIAS

The theoretical VAPP would be 149 KIAS. Considering that VFE for flaps 45° is 145 KIAS, VAPP is set to 145 KIAS and VREF is set to 134 KIAS. Nevertheless, the pilot in command is responsible for electing the best flap setting for a given approach and landing considering all related operational factors.

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This recommended limit must also be chosen in such way that it is possible to gradually remove it during the approach in order to reach VREF at threshold. More than 20 KIAS over VREF may lead to difficulties in decelerating during the last seconds of the approach. For maximum VAPP values, refer to the AOM Performance/Approach section.

ABNORMAL/EMERGENCY PROCEDURES Some abnormal/emergency procedures in the QRH require a VREF adjustment that considers an adequate speed margin in order to meet the following criteria: •

30° bank margin to shaker activation;

•

Tail strike avoidance;

•

Rate of descent on touchdown not higher than 600 ft/min and;

•

Acceptable flight qualities.

In all Embraer airplanes the baseline VREF , which is applied to the ABNORMAL correction, is related to the highest flap position even if the landing is performed with lower flap positions. This means that VREF FULL for EMBRAER 170/175/190/195 and VREF45 for ERJ 135/140/145/Legacy are used for baseline. General Guidelines for ABNORMAL procedures: •

•

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THE MAXIMUM STRUCTURAL FLAP LIMIT SPEED MUST ALWAYS BE RESPECTED AND IS THE UPPER V APP LIMIT FOR ABNORMAL PROCEDURES. IN CASE OF MULTIPLE MALFUNCTIONS THAT REQUIRE SPEED ADJUSTMENT ON VREF, THE GREATEST VALUE OF “ NEW” VREFS MUST BE ADOPTED.

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CONCLUSIONS

From the certification point of view, landing performance data presented in AFM/AOM is generated considering that the airplane will be at VREF once it has reached the threshold and is at a height of 50 ft above the ground. From a practical point of view, every pilot knows that almost every approach is better if the airplane maintains a speed higher than VREF on final approach in order to assure a speed margin above stall should turbulent air or variable wind conditions be encountered along the flight path. Due to these reasons (or at the Captain’s discretion for safety reasons), sometimes it is not possible to cross the threshold exactly at VREF . It must be clear that VAPP must be the upper limit in this case. In this scenario, the pilot must always keep in mind that the performance achieved cannot be determined exactly but may be close or better than the calm wind performance data. Crossing the threshold with VREF at 50 ft height will always produce, at least, the predicted performance in the AFM.

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