Approach speed considerations Feb 08, 10 Origin: Status: Closed Classification: Operation REASON The aim of this article is to give operational recommendations for computing the approach speed V APP while factoring in the effect of prevailing winds. INFORMATION PERFORMANCE DEFINITIONS The Airplane Flight Manual (AFM, Section 5: Performances - definitions) gives the following definitions: Landing distance (LD): horizontal distance required to land and come to a complete stop from a point at a height of 50 ft above the landing surface. Reference speed (V REF): minimum speed at the height of 50 ft during a normal landing. V REF should not be less than: o 1.3 V S (stalling speed) in the 40 FLAPS +SLATS configuration and o V MCL (minimum control speed during landing approach with all engines operating) Therefore LD as published in the AFM charts 5-75-2 (5-750-2 for F900C) is computed with speed equal to V REF when passing 50 ft. If any overspeed is encountered at the threshold, "Each 10 kt overspeed at the threshold increases the landing distance by 15%.". This statement will be introduced in the AFM. In the general case V REF given by AFM chart 5-75-3 (5-750-3 for F900C) takes into account the landing configuration for slats / flaps and all engines operating. COMPUTING APPROACH SPEED (V APP ) - GENERAL CASE The final Approach Speed (V APP ) is the airspeed to be maintained down to 50 ft over the runway threshold. It is also called the "target threshold speed". 450B 1/6
The objective of V APP is to provide a sufficient controllability / maneuverability margin while taking into account the wind gradient effect while flying on a constant ground slope. Flying over the runway threshold at a speed higher than V REF remains the PIC's option, as no airworthiness requirements mandate this criteria. However, it is considered a common practice to compute V APP not only considering V REF, but also the effective headwind (V HEADWIND ) component and gust (V GUST ), with the following formula: V APP = V REF + (1/2 V HEADWIND + V GUST ) The wind correction (e.g. 1/2 V HEADWIND + V GUST ) must not exceed 20 kts. This V APP should be maintained down to 50 ft over the threshold. The term V HEADWIND is self explanatory: it means that for the average value of the wind, only the headwind component is taken into account in that term; crosswind or tailwind components do not lead to a correction, meaning: If a tailwind plus a gust is present, only the gust factor is taken into account if a direct crosswind of 10 kt is encountered with no gust, no correction is to be made. The gust factor (V GUST ) is taken into account regardless of the wind direction is (headwind, tailwind, crosswind ). EXAMPLE Wind correction Runway 36 in use, ATC broadcasts Wind 330/30G40: in that case steady state headwind component is 26 kts and the gust is 10 kts so that recommended wind correction is 13 kts + 10 kts = 23 kts, limited to 20 kts. The recommended correction is V APP = V REF + 20 kts. ADVANTAGES AND DRAWBACKS CONTROLLABILITY / MANEUVERABILITY The wind value given by the ATC is the average wind during the last two minutes; a gust is announced if its value is 10 kts or more above the average value. The variation of the instantaneous wind value during those two minutes commonly reaches half of the average value: for example for an average wind of 10 kts, the actual instantaneous wind can vary from 5 kts to 15 kts. 450B 2/6
Let us consider a 10 kts average headwind. Should the captain decide to fly the approach at V APP = V REF + 5 kts, if the instantaneous wind drops down to 5 kts, the airspeed will then become V REF, still providing the same margin above stall speed (i.e. 1.3 V S ). LANDING DISTANCE This wind (Headwind & Gust) correction provides an additional stall margin for airspeed excursions caused by turbulence and windshear, but it also affects the Landing Distance (LD). Consequently, a particular sequence of when computing LD must be followed. EXAMPLE F900, landing at sea level, QNH = 1,013 HPa and LW = 32,000 lb and steady headwind component of 20 kts (no gust): Step 1: determine LD from AFM chart 5-75-2, here below (5-750-2 for F900C). This LD is computed with speed equal to V REF when passing 50 ft and takes into account the effect of the wind (if any) during the flare and the landing ground roll: LD = 2,300 ft LW = 32,000 lb QNH = 1,013 HPa and Landing at sea level Field pressure altitude = 0 ft 20 kt headwind With 20 kt headwind LD = 2,300 ft With no wind LD = 2,600 ft 450B 3/6
Step 2: determine the increase in LD in % due to the wind correction (15% by 10 kts): for 20 kts steady headwind, the recommended wind correction is 10 kts so that LD must be increased by 15%. Step 3: apply this correction to the LD computed under Step 1. LD = 2,300 ft x 1.15 = 2,645 ft. o Note that this applicable LD (2,645 ft) is very close to the "zero wind" result (2,600 ft): this example shows that increasing the approach speed for headwind component correction has very little impact on the LD if we compare it with a "zero wind" LD, whereas it will provide an additional safety margin against stall as shown here above. Step 4: compute the Landing Field Length (LFL) as applicable and compare with the Landing Distance Available (LDA) Step 5: find V REF using chart 5-75-3 (5-750-3 for F900C): V REF = 116 kts V REF = 116 kts Step 6: compute V APP = V REF + 10 kts = 126 kts, to be maintained down to 50 ft. 450B 4/6
IN CASE OF FAILURE When a system malfunction results in a configuration correction to V REF the final approach speed becomes: V APP = V REF + (1/2 V HEADWIND + V GUST ) + configuration correction Wind correction The configuration correction is the one stated in the abnormal / emergency procedure (refer to AFM / OM). The wind correction is only recommended. EXAMPLE F900, landing at sea level, QNH = 1,013 HPa and LW = 32,000 lb and steady headwind component of 20 kts (no gust): SLATS SYSTEM FAILURE Step 1: determine LD with no failure from AFM chart 5-75-2 (5-750-2 for F900C) as explained here above: LD = 2,300 ft Step 2: determine the increase in LD in % due to the wind correction (15% by 10 kts): for 20 kts steady headwind, the recommended wind correction is 10 kts so that LD must be increased by 15%. Step 3: apply this correction to the LD computed under Step 1. LD = 2,300 ft x 1.15 = 2,645 ft. Step 4: check failure penalty. In our case AFM 3-15-4 (3-143-1 for F900C) states that the approach speed (zero wind) must be V REF + 5 kts so that LD must be increased by 200 ft Step 5: this penalty will have to be applied to the LD computed in the normal case as explained here above. In the example above, the normal LD (without failure) including the wind correction is 2,645 ft. When the 200 ft penalty due to the slats failure is applied, it becomes 2,645 + 200= 2,845 ft. Step 6: compute the Landing Field Length (LFL) as applicable and compare with the Landing Distance Available (LDA) Step 7: compute V REF using chart 5-75-3 (5-750-3 for F900C) : V REF = 116 kts Step 8: compute V APP with the here above rule: 450B 5/6
o V APP = V REF + 10 kts (wind) + 5 kts (failure) = 131 kts, to be maintained down to 50 ft. CAUTION In every cases, especially in the case of strong winds and/or gusts and a very degraded aircraft status leading to a significant LD penalty (example: loss of both hydraulic systems), the PIC must find the best compromise between handling qualities (controllability / manoeuvrability) and performance and use the recommended wind correction in addition to the failure correction taking into account: Landing performance at destination, Landing performance at diversion airfield, Weather considerations, etc. CONCLUSION In the general case (with no failure) the recommended computation of VAPP has little impact on the landing distance when there is no gust. When there is some gust, there is an increase of the LD but a positive benefit against stall. This recommended approach speed provides the best compromise between handling qualities (stall margin or controllability / maneuverability) and landing distance. Whenever landing distance becomes an issue, the Pilot in Command might decide to tip the compromise in favor of landing distance performance by disregarding the gust correction, and to manage the approach speed to meet V REF + 1/2 V HEADWIND + configuration correction at 50 ft, under his/her full responsibility. Operating manual will be updated to take into account these recommendations. General information on this subject can be found in a Flight Safety Foundation tool kit by following this link: http://www.flightsafety.org/current-safety-initiatives/approach-and-landingaccident-reduction-alar/alar-briefing-notes-english 450B 6/6