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AP4ATCO - Factors Affecting Aircraft Performance During Takeoff and Climb
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24. Factors affecting aircraft performance during takeoff and climb
a) article description Takeoff calculations and conditions affecting performance during take-off. Factors affecting performance during climb (rate of climb, gradient). Engine failure after takeoff.
b) source (IANS) 6.1 6.2 1.2 (examples)
c) additional sources FAA Airplane Flying Handbook (FAA-H-8083-3A) – chapters 12, 16 FAA The Pilot's Encyclopedia of Aeronautical Knowledge - chapter 9
- FAA Pilot’s Handbook of Aeronautical Knowledge – chapter 10
Airbus - Takeoff and Departure Operations - Response to Stall Warning Activation at Takeoff Airbus - Takeoff and Departure Operations - Understanding Takeoff Speeds Australian Transport Safety Bureau - Take-off performance calculation and entry errors AAIB Bulletin: 7/2010 ref EW/G2009/12 /04, A346 Erroneous parameters at takeoff, London Heathrow (http://www.skybrary.aero/bookshelf/books/1315.pdf)
FACTORS AFFECTING AIRCRAFT PERFORMANCE DURING TAKEOFF
The takeoff part of a flight is the distance from the brake release point to the point at which the aircraft reaches a defined height over the surface. For any particular takeoff it must be shown that the distance required for takeoff in the prevailing conditions does not exceed the takeoff distance available at the aerodrome.
During the takeoff roll, lift is created on the wings to overcome the aircraft weight. This is done by forward acceleration of the aircraft produced by greater thrust force then drag.
The takeoff distance required depends on the interaction of forces:
• The thrust varies during takeoff, in general it decreases as aircraft speeds up; • The total drag of the aircraft during takeoff results from aerodynamic drag and wheel drag. As the aircraft speeds up the aerodynamic drag will increase. The wheel drag depends on the load and the runway surface resistance. But as the aircraft speeds up the lift force increases, which reduces the load on the wheels and therefore reduces the wheel drag (eventually to zero); • The lift force increases, as aircraft speeds up; • The aircraft weight remains constant;
The factors that affect these forces and their interaction are the factors that affect aircraft performance during takeoff:
• aircraft takeoff mass and balance; • temperature; • air density; • wind; • runway conditions (runway surface, runway slope); • flap setting and airframe contamination;
Aircraft takeoff mass and balance
Aircraft takeoff mass and balance, determine the weight force. Limitations on mass are set to ensure adequate margins of strength and performance while limitations on the centre of gravity position (balance) are set to ensure adequate stability and control of the aircraft in flight.
The total takeoff mass of the aircraft consists of its empty mass, its passengers, cargo (payload), and the fuel. When loading the aircraft care must be taken not to exceed the limits mentioned.
The greater the takeoff mass the greater the aircraft weight. This means that greater lift force is required to overcome the weight, therefore greater speed is necessary for takeoff. Thus a longer takeoff distance is required in order to achieve this speed, because the rate of acceleration is reduced (inversely proportional to the mass) and the wheel drag will be greater due to increased load.
Let’s consider two identical aircraft in same flight conditions, but with different weight:
The efficiency of the jet engine depends on the temperature of the air surrounding it. The higher the air temperature, the less thrust can be produced by the engine. Because of that the difference between the thrust and the drag during takeoff is smaller. Therefore the rate of acceleration is smaller and the aircraft will need a longer takeoff distance.
In addition, a change in temperature affects the air density.
Air density affects the thrust, lift and drag forces in the following way: - low air density gives reduced thrust created by the engines; - low air density requires higher takeoff speed. The lift is proportional to the air density. So, higher speed is required to produce same lift when the air density is low; - low air density gives lower aerodynamic drag. However, the effect on the lift and thrust is more dominant since the aerodynamic drag is relatively small;
In general low density requires longer takeoff distance.
Air density is determined by the pressure (elevation), temperature and humidity: - low atmospheric pressure gives low air density; - higher the aerodrome is elevated, lower the atmospheric pressure hence lower the air density; - higher the temperature, lower the air density; - higher the humidity, lower the air density.
The lift and the drag during takeoff depend on air speed, but the distance required for takeoff depends on the ground speed. A headwind therefore reduces the ground speed at a required takeoff air speed and reduces the takeoff distance. On the other hand, a tailwind increases the ground speed, at a same required takeoff air speed, and increases the takeoff distance.
Crosswind component has no effect on the takeoff distance.
Pilots are permitted to use only 50% of the reported headwind component (or 150% of the reported tailwind component) when calculating the takeoff distance required. This is to allow for variations in the reported winds during takeoff. Runway conditions
If the runway is sloping, a component of the weight acts along the runway and increases or decreases the acceleration force. A downhill slope increases the accelerating force, and therefore reduces the takeoff distance required, whereas an uphill slope reduces the accelerating force and increases the takeoff distance.
The runway surface condition has effect on the wheel drag. If the runway is contaminated by snow, slush or standing water, the wheel drag will be greater. Thus the accelerating force decreases and the takeoff distance required increases. Further on, if the takeoff is abandoned in such conditions and breaking is required the stopping distance will greatly increase.
Flap setting and airframe contamination
Flap setting has an affect on the wing’s lift coefficient and on the aerodynamic drag. Increasing flap angle increases the lift coefficient, and therefore reduces stalling speed and the required takeoff speed (the same lift will be created at smaller air speed due to greater lift coefficient). This reduces the takeoff distance. In the same time increased flap angle increases drag, reduces acceleration, and increases the takeoff distance.
The net effect is that takeoff distance will decrease with increase of flap angle initially, but above a certain flap angle the takeoff distance will increase again. An optimum takeoff setting can be determined for each type of aircraft and any deviation from this setting will give an increase in the takeoff distance.
The flap setting also affects the climb gradient. Increasing the flap angle increases the drag, and so reduces the climb gradient for a given aircraft mass. If there are obstacles to be considered in the takeoff flight path, the flap setting that gives the shortest takeoff distance may not give the required climb gradient for obstacle clearance.
In addition if the airframe is contaminated by frost, ice or snow during takeoff the aircraft performance will be reduced, and the takeoff distance will be increased.
Takeoff calculation is performed before takeoff in order to confirm that the actual weight is below the maximum permissible takeoff weight at particular aerodrome in the conditions prevailing. On modern aircraft this value and the values for the various speeds can be obtained from the FMS, after feeding in the relevant data for the airfield and conditions.
Following basic conditions are considered in the calculations:
- Airfield elevation - Runway slope - Air temperature - Wind - Runway length and conditions - Flap configuration
Using either charts or computer software the maximum permissible takeoff weight is determined and when it is confirmed that the actual weight is within the limits, it is necessary to find the takeoff speeds and thrust setting corresponding to the actual weight.
Following speeds are determined or, on modern aircraft, obtained from the FMS:
• V1 – decision speed, at which in case of engine failure the continued takeoff distance required, will not exceed the takeoff distance available; • VR – rotation speed, at which aircraft nose is lifted from the ground (rotated) for takeoff; • V2 – takeoff safety speed with critical engine inoperative, at which the aircraft can take of safely with critical engine inoperative.
Typical takeoff calculation errors
Calculating and entering takeoff performance parameters into aircraft systems involves a number of steps that create potential opportunities for errors. The following list provides examples of the types of errors that have been identified from investigations into related accidents and incidents:
- the ZFW (Zero Fuel Weight) is inadvertently used instead of the TOW (Takeoff Weight) - an aircraft weight is incorrectly transcribed or transposed into an aircraft system or when referencing performance manuals; for example, a weight of 234.000 kg or 224.000 kg is used instead of 324.000 kg - V speeds are incorrectly transcribed or transposed when manually entered into an aircraft system - aircraft data from a previous flight is used to calculate the V speeds - takeoff performance parameters are not updated as a result of a change in flight conditions; for example, a change in the active runway, intersection departure or ambient temperature - selecting the incorrect value from the loadsheet or take-off data card - using the wrong performance charts for the aircraft type - inadvertently selecting the wrong table or column/row in the performance charts - using the incorrect value when referencing the performance charts - failing to convert values into the required unit of measurement (for example pounds to kilograms)
Typical takeoff calculation errors consequences
In the event the above errors are not detected and corrected prior to takeoff, the following adverse consequences may occur:
- tailstrike: when aircraft rotation is initiated at a speed below that required for the aircraft’s weight, lift-off may not be achieved. In response, the pilot may increase the nose-up attitude of the aircraft, which may result in the tail contacting the runway - reduced takeoff performance: during the takeoff, the crew may observe that the aircraft’s performance is not as expected; the aircraft may appear ‘sluggish’ or ‘heavy’ - degraded handling qualities: after takeoff, there may be a reduced margin between the aircraft’s actual speed and the stall speed until the aircraft accelerates up to the normal climb speed. If the V2 speed is also erroneous, this may not occur until after the aircraft passes through the acceleration height - rejected takeoff: if the aircraft fails to accelerate or lift-off as expected, the crew may reject the takeoff - runway overrun: if the aircraft fails to stop after a rejected takeoff or the aircraft fails to liftoff, the aircraft rollout may extend beyond the end of the runway resulting in an overrun - TO/GA (Takeoff/Go around) engine thrust: if the aircraft fails to accelerate or lift-off as expected, the crew may select take-off/go-around (TO/GA) engine thrust (the maximum thrust that the engines will supply) - increased runway length required: early rotation increases drag and significantly increases the distance from rotation to liftoff - overweight takeoff: this may occur if an erroneous TOW (Take Off Weight) is used to determine whether a runway is acceptable for the takeoff - reduced obstacle clearance: if the takeoff is commenced at low speed, the aircraft will not achieve the climb gradient required, and the clearance between any obstacles along the take-off path will be reduced
Here are some examples of occurrences involving take off calculations errors:
- on 14 October 2004, a Boeing 747-244SF aircraft, registered 9G-MKJ, attempted to takeoff from Halifax, Nova Scotia, but overshot the end of the runway, momentarily became airborne and then struck an earth bank. The TOW used to generate the take-off performance data in the laptop computer was from the previous flight (Transportation Safety Board of Canada, 2006).
- May 2002, Boeing 747, London, UK. During the take-off run, the aircraft’s rotation was initiated at a low speed. The rate of rotation was reduced to allow the speed to increase for climb out. A reduction to the V1 speed due to a wet runway resulted in an incorrect rotation speed (VR) being entered into the aircraft’s flight management computer (FMC).
- March 2003, Boeing 737, Darwin, NT. The crew calculated the take-off performance parameters for a full length runway departure and entered the corresponding V speeds into the FMC. The takeoff was then amended for an intersection departure. The crew briefed on the new V speeds and set their respective airspeed indicator speed bugs. The FMC was not updated with the new V speeds. The crew noticed the error during the take-off run. This resulted in a higher VR speed being used than that required for the reduced runway length.
- March 2007, Airbus A340, Paris, France. On 28 March 2007, an Airbus A340 aircraft, registered F-GLZP, was being prepared for takeoff from Charles de Gaulle International Airport, France. The crew had initially planned to conduct a reduced thrust takeoff. However, due to a 5 kt tail wind, this was changed and the take-off performance parameters were re-calculated. When entering the V speeds, an error was made, resulting in the VR speed being 20 kts lower than required. During the takeoff, the pilot flying delayed the aircraft’s rotation
FACTORS AFFECTING AIRCRAFT PERFORMANCE DURING CLIMB Factors affecting aircraft performance during climb
The climb phase of a flight starts after takeoff, when the aircraft reaches a certain height above the ground, and it ends when aircraft levels off at the cruising level.
For the first portion of the climb it is more convenient to consider the climb gradient rather then the rate of climb.
Climb gradient is the ratio of height gained to distance traveled, and it is expressed in percentage. All instrument departure procedures have the minimum climb gradient specified in the charts. This climb gradient is required to overfly the obstacles in the departure area at a safe altitude defined as minimum obstacle clearance.
When the obstacles are over flown it is more convenient to consider the rate of climb, since the aircraft would normally require to climb at the maximum rate of climb so as to reach the required altitude in the least possible time. Rate of climb is the vertical component of the aircraft’s velocity.
As a rule of thumb climb gradient may be converted to rate of climb by multiplying the gradient by airspeed in knots (example: climb gradient is 6 %, airspeed is 150 knots therefore rate of climb should be 900 feet per minute).
In steady climb, the weight has a component along the flight path, which adds to the drag force. To maintain a steady speed along the flight path, the opposite forces along the flight path must be equal.
Factors affecting the climb gradient
The climb gradient by definition is the ratio of height gained to distance traveled. If the angle of climb α is known then the climb gradient is equal to tan (α). For small angles tan (α) = sin (α). Now taking into the consideration the formulas from the drawing above:
Climb gradient = tan (α) = sin (α) = (Thrust – Drag) / Weight
This shows that the climb gradient depends on the difference between the thrust and drag (the excess thrust) and the mass of the aircraft. Factors that affect these forces will have affect on the climb gradient.
Speed and acceleration
For a given aircraft mass the maximum climb gradient will occur when the excess thrust is greatest. As both thrust and drag vary with speed, there will be a particular speed at which this occurs (the best angle of climb speed). If the aircraft is required to fly at different speed, the climb gradient will be reduced.
When the aircraft is accelerating during climb some portion of the excess thrust is required for the acceleration, so there will be less excess thrust and therefore the climb gradient will be reduced.
Increased mass gives higher drag which reduces the excess thrust (the difference between the thrust and drag), and therefore reduces the climb gradient for a given thrust.
Same as during the takeoff, higher the air temperature, less thrust can be produced by the engines. Because of that the difference between the thrust and the drag during climb is smaller. Therefore the climb gradient will be reduced.
Air density (altitude)
Increasing altitude (decreasing density) will reduce thrust and therefore reduce the climb gradient.
In wind conditions, headwind or tailwind will have affect on the aircraft’s ground speed. So, a headwind will reduce the ground speed and therefore reduce the horizontal distance that an aircraft travels in comparison to the no wind conditions. Therefore a headwind gives increased climb gradient, while a tailwind affects in opposite direction and gives reduced climb gradient. Crosswind component has no effect on the climb gradient.
The drag of an aircraft will depend on its configuration. After takeoff the configuration is with gear down and takeoff flaps. When the gears are retracted the drag is reduced, resulting with increased excess thrust (the difference between thrust and drag), therefore the climb gradient is increased. The same thing occurs when the flap is retracted.
If higher flap angle is used to reduce the take-off distance required, the climb gradient will be reduced.
When a lower flap angle is used the take-off distance required is increased, but the climb gradient is increased as well.
Factors affecting the rate of climb
The rate of climb is the vertical component of the speed, expressed in feet per minute. It depends on the airspeed (V) and the angle of climb. Using the same logic as for the climb gradient:
Rate of climb = V x sin (α) = V x Climb gradient = V x (Thrust – Drag) / Weight
Speed and acceleration
For a given aircraft mass the maximum rate of climb will occur when the product of the speed and the excess thrust is greatest. As both thrust and drag vary with speed, there will be a particular speed at which this occurs (the best rate of climb speed) that is different from the best angle of climb speed. If the speed is increased above that for the best angle of climb although the climb angle will decrease, the rate of climb will initially increase.
If the aircraft is required to fly at different speed, the rate of climb will be reduced.
Increased mass gives higher drag which reduces the excess thrust (the difference between thrust and drag), and therefore reduces the rate of climb.
The higher the air temperature, the less thrust can be produced by the engines. Thus the difference between the thrust and the drag during climb is smaller. Therefore the rate of climb will reduce.
Air density (altitude)
Increasing altitude (decreasing density) will reduce thrust and therefore reduce the rate of climb.
The rate of climb is independent from the wind speed, because it is always considered in reference to the air not the ground.
When the gears are retracted the drag is reduced, resulting in an increase in excess thrust, therefore the rate of climb is increased. Same thing occurs when the flap is retracted.
Modern aircraft operate at high altitudes and can achieve high rates of climb. In order to take advantage of these properties the interior of an aircraft flying at high altitude is pressurized to allow passengers and crew to function normally without any need for additional oxygen. Cabin pressurization systems are designed to produce conditions equivalent to those at approximately 8000 feet.
When the aircraft is climbing, the change of cabin pressure is proportional to the change of the ambient pressure, in order to control the structural stress on the fuselage from the inside. This is performed automatically by sophisticated control system. However, if the cabin pressure is manually controlled or in case of system degradation, care should be taken to ensure that the climb rates are safe and ensure that the structural stress is not exceeding the maximum limit. The maximum rate of climb and ceiling are affected. When exceeded the aircraft structure is overstressed from inside and structural failure (explosion) is possible.
Passengers comfort is also a factor. Usually the best comfort is achieved at rates of climb of 1500 feet per minute.
JET AIRPLANE TAKEOFF AND CLIMB
Jet airplane takeoff and climb
The following are speeds that will affect the jet airplane’s takeoff performance. The pilot must be thoroughly familiar with each of these speeds and how they are used in the planning of the takeoff:
- Vs – stall speed - V1 - Vr - VLO - V2
For more information about these speeds look above or refer to the article “Speed indication in aircraft”
Hyperlink to “SPEED TYPES AND PERFORMANCE SPEEDS” from the aricle “Speed indication in aircraft” (article 17 in the project)
Takeoff data, including V1/VR and V2 speeds, takeoff power settings, and required field length should be computed prior to each takeoff and recorded on a takeoff data card. These data will be based on variables presented above )airplane weight, runway length, temperature, etc.). Preferably both pilots should separately compute the takeoff data and cross-check in the cockpit with the takeoff data card.
A captain’s briefing is an essential part of cockpit resource management (CRM) procedures and it is accomplished just prior to takeoff. It is an opportunity to review crew coordination procedures for takeoff, which is always a critical portion of a flight. The takeoff and climb-out should be accomplished in accordance with a standard takeoff and departure profile developed for the particular make and model airplane (and with respect to local procedures if they apply, for example minimum gradient or maximum speed until given altitude).
Pilots may use the entire runway length for takeoff, especially if the pre-calculated takeoff performance shows the airplane to be limited by runway length or obstacles.
After taxing into position at the end of the runway, the airplane will be aligned in the center of the runway allowing equal distance on either side. The brakes will be held while the thrust levers are brought to a power setting and the engines allowed to stabilized. Pilots will check the engine instruments for proper operation before the brakes are released or the power increased further. The brakes will then be released and, during the start of the takeoff roll, the thrust levers smoothly advanced to the pre-computed takeoff power setting.
All final takeoff thrust adjustments should be made prior to reaching 60 knots. The final engine power adjustments are normally made by the pilot not flying. Once the thrust levers are set for takeoff power, they should not be readjusted after reaching 60 knots. Retarding a thrust lever would only be necessary in case an engine exceeds any limitation such as ITT (Inter Turbine Temperature), fan, or turbine RPM.
If sufficient runway length is available and traffic configuration allows it, a “rolling” takeoff may be made in which, as the airplane rolls onto the runway, the thrust levers will be smoothly advanced to the vertical position and the engines allowed to stabilize, and then proceed as in the static takeoff outlined above.
During the takeoff roll, the pilot flying will concentrate on directional control of the airplane. This is made somewhat easier in jet airplane because there is no torque produced yawing the plane as there is in a propeller driven airplane.
During the takeoff roll, the primary responsibility of the pilot not flying is to closely monitor the aircraft systems and to call out the proper V speeds as directed in the captain’s briefing. Slight forward pressure is held on the control column to keep the nosewheel rolling firmly on the runway. If nosewheel steering is being utilized, the pilot flying should monitor the nosewheel steering to about 80 knots (or VMCG for the particular airplane) while the pilot not flying applies the forward pressure. After reaching VMCG, the pilot flying brings his/her left hand up to the control wheel. The pilot’s other hand should be on the thrust levers until at least V1 speed is attained.
Although the pilot not flying maintains a check on the engine instruments throughout the takeoff roll, the pilot flying (pilot in command) makes the decision to continue or reject a takeoff for any reason. A decision to reject a takeoff will require immediate retarding of thrust levers.
The pilot not flying calls out V1. After passing V1 speed, as the airspeed approaches VR, the control column should be moved to a neutral position. As the pre-computed VR speed is attained, the pilot not flying will make the appropriate callout and the pilot flying will smoothly rotate the airplane to the appropriate takeoff pitch attitude.
Rotation, lift off and initial climb
The objective is to initiate the rotation to takeoff pitch attitude exactly at VR so that the airplane will attain V2 speed at around 35 feet.
Rotation to the proper takeoff attitude too soon may extend the takeoff roll or cause an early lift-off, which will result in a lower rate of climb, and the predicted flightpath will not be followed. A late rotation, on the other hand, will result in a longer takeoff roll, exceeding V2 speed, and a takeoff and climb path below the predicted path.
Each airplane has its own specific takeoff pitch attitude which remains constant regardless of weight. The takeoff pitch attitude in a jet airplane is normally between 10° and 15° nose up.
Once the proper pitch attitude is attained, it must be maintained. The initial climb after lift-off is done at this constant pitch attitude. Takeoff power is maintained and the airspeed allowed to accelerate. Landing gear retraction should be accomplished after a positive rate of climb has been established and confirmed. In some airplanes gear retraction may temporarily increase the airplane drag while landing gear doors open. Premature gear retraction may cause the airplane to settle back towards the runway surface.
The climb pitch attitude should continue to be held and the airplane allowed to accelerate to flap retraction speed. However, the flaps should not be retracted until obstruction clearance altitude or at least 400 feet AGL has been passed.
Ground effect and landing gear drag reduction results in rapid acceleration during this phase of the takeoff and climb. When the airplane settles down to a steady climb, longitudinal stick forces can be trimmed out. If a turn must be made during this phase of flight, no more than 15° to 20° of bank should be used.
If a power reduction must be made, pitch attitude should be reduced simultaneously and the airplane monitored carefully so as to preclude entry into an inadvertent descent.
When the airplane has attained a steady climb at the appropriate en route climb speed, it can be trimmed about all axes and the autopilot engaged.
STALL WARNING AT TAKEOFF
Stall warning at takeoff
At takeoff, stall warnings are the result of one, or a combination, of the following factors:
• Weather Factors: − Icing conditions − Windshear
• Human Factors: − Insufficient aircraft de-icing in cold weather operations − Incorrect loading (e.g. cargo not positioned in accordance with the load and trim sheet) − Incorrect slats/flaps configuration − Incorrect takeoff speed
• Flight Crew Techniques: − Early rotation below the specified speed, resulting in a higher peak AOA (Angle of Attack) − Maneuvering near the minimum speed at an excessive bank angle − Premature retraction of the flaps
• Aircraft Systems: − Engine failure and subsequent loss of energy − Malfunction of artificial stall warning, leading to spurious stall warnings, caused by a damaged AOA probe, by an AOA probe that is not correctly rigged, or by a computer failure.
A stall warning triggers when the aircraft’s Angle-Of-Attack (AOA) exceeds a predetermined value. This value depends on the slat configuration. The warning indicates the proximity of the aircraft’s AOA compared to the stall’s AOA.
The stall warning is inhibited on ground, until liftoff.
When an aircraft is airborne, stall warning activation can be catastrophic, if the flight crew does not respond correctly and effectively. Worldwide experience records events where flight crews have been misled by a spurious / untimely stall warning activation at liftoff. Some of them have resulted in fatal accidents (e.g.: rejected takeoff after rotation, CFIT).
If a stall warning triggers at a low altitude, the flight crew should consider that there is an immediate flight path threat, and a potential risk of ground contact. In other words, there is no time to differentiate between a real or spurious stall warning, and there is no altitude to convert to speed.
However, when a stall warning triggers (i.e. stick shaker activation), aircraft still have positive climb performance capability.
1. [Question type: multiple response, based on AirQuestions FACT-TO/012]
Q: The greater the take-off mass the longer the runway length required for take-off due to A1:greater lift force required to overcome weight, therefore greater speed is necessary for take-off A2: longer time required by the jet engines to produce the same thrust A3: faster acceleration (which is proportional to aircraft's mass) A4: lower speed required for creation of corresponding lift A5: greater thrust created by the engines A6: wheel drag is greater due to increased load
Correct answers: A1, A5, A6
2. [Question type: multiple response, based on AirQuestions FACT-TO/017]
Q: The lower air density the longer runway length required for take-off due to: A1: lower thrust created by the engines A2: wheel drag is greater due to lower aerodynamic drag A3: faster acceleration (which is proportional to aircraft's thrust) A4: lower temperature A5: greater speed required for creation of corresponding lift A6: greater lift force required to overcome weight
Correct answers: A1, A5, A6
3. [Question type: true or false, based on AirQuestions FACT-CL/049]
Q: Maximum climb gradient is not affected by temperature of the surrounding air. A1: True A2: False
Correct answer: A2
4. [Question type: true or false]
Q: Icing conditions and windshear are weather factors which may lead to stall after take off. A1: True A2: False
Correct answer: A1
25. Cabin pressurization