Every aircraft type has specific performance characteristics associated with it. These are the result of design goals which are determined before the aircraft is built and tested.
All aircraft performance however, is affected by certain conditions. These can be natural (wind/temperature) or man made (weight and balance). As a result, the performance characteristics will be affected either positively, or, more commonly, negatively. Air Traffic Controllers are required to understand these various conditions and their effect on aircraft operation. Effects may hinder separation and traffic flow, or, in some instances may be hazardous to the aircraft itself.
Factors Affecting Aircraft Performance
The following may influence aircraft operating performance:
1) wind
2)temperature and atmospheric pressure
3) rate of climb and descent
4) airspeed
5) aircraft load factors
6)company operating limitations
7) pilot proficiency
8) runway conditions and runway gradient
These factors will be examined individually.
Wind ‑ General Effects
Wind will have the following effects on aircraft operation:
i) cause changes in aircraft track
ii) create drift problems (takeoff/landing/enroute)
iii) cause groundspeed changes
i.e. ‑ a crosswind causes drift
‑ a headwind reduces groundspeed
‑ a tailwind increases groundspeed
Wind - Landing and Takeoff
Aircraft (including helicopters) normally take off and land into the wind. This is not always possible due to runway orientation, instrument approaches, noise abatement procedures etc.
Wind will affect airport operations in several ways;
1) Takeoff and landing into wind ‑ as wind speed increases, length of landing/takeoff roll is DECREASED.
2) Takeoff and landing with wind ‑ as wind speed increases, length of landing/takeoff roll is INCREASED.
3) Takeoff and landing with crosswind ‑ as wind speed increases, yaw increases on ground (weathervaning), drift increases once airborne.
All of the above can cause problems for aircraft operations. The level of influence will depend on the severity of wind, aircraft type and size, and pilot proficiency.
Crosswind landings and takeoffs can be difficult, especially for light tailwheel aircraft. Because there is no nosewheel, the weathervane effect can be very pronounced while taxiing, as well as on takeoff and landing rollout. This sometimes results in the aircraft performing a manoeuvre known as a “groungloop”.
During maximum crosswind situations (90 degrees), the general rule is to allow a maximum wind speed of 20 percent of the particular aircraft's stall speed. (i.e. Cl85 – stall/approx. 55kts x 20% = 11kts @ 90 degrees)
| Example #1 – Aircraft with a stall speed of 60k | ||
| Wind - Degree off Runway | | Permissible Wind Speeds |
| 90o | (0.2 x 60k stall speed) | 12k |
| 60o | Using graph | 14k |
| 30o | Using graph | 24k |
| 15o | Using graph | 48k |
| Example #2 – Aircraft with a stall speed of 50k | ||
| Wind - Degree off Runway | | Permissible Wind Speeds |
| 90o | (0.2 x 50k stall speed) | 10k |
| 60o | Using graph | 12k |
| 30o | Using graph | 20k |
| 15o | Using graph | 40k |
The above rule is important for aerodrome controllers to remember when planning which runway to use.
Temperature and Pressure
Temperature and pressure have a direct impact on aircraft performance. This is especially noticeable during takeoff and climb after departure. Specifications in the Pilot's Operating Handbook refer to ISA conditions (15°C./1013.2mb/1.98°C. lapse rate) when stating the performance capabilities of a specific aircraft. If the temperature and pressure values differ from ISA a correction must be applied by the pilot for the following reasons:
1) Higher temperature
i) more runway will be required to takeoff/landing.
ii) approach speeds will be higher (TAS >IAS).
iii) climb rate will be lower.
2) Lower pressure
i) more runway will be required for takeoff/landing.
ii) approach speeds will be higher (TAS > IAS).
iii) climb rate will be lower.
3) Higher temperature/Lower pressure
i) much more runway will be required to takeoff/landing.
ii) approach speeds will be much higher (TAS > IAS).
iii) climb rate will be much lower.
The above occurs because the DENSITY ALTITUDE is much higher under the above situations. This is a calculated figure that refers to the equivalent altitude for performance purposes at a particular airport, regardless of the actual aerodrome elevation. i.e. An airport located at 1500ft/450m AMSL could experience very high temperature and low pressure such that its density altitude at that time is 4000ft/1200m AMSL. This means that aircraft performance is equivalent to that occurring during departure from an airport located at 4000ft/1200m AMSL (ISA). This can have major implications for the pilot, especially if the aircraft is not capable of a high level of performance.
Remember that aircraft performance DECREASES with altitude
ATC implications
When atmospheric pressure is low and/or temperatures are high, the following can be expected.
- Takeoff performance will be sluggish
- Climb rates will be lower than usual
- Runway length requirements will be greater
- Aerodrome controllers may receive requests for pressure and temperature/dewpoint from pilots shortly before a request for taxi instructions prior to takeoff. This allows the aircrew to do a final check of takeoff reference speeds (V1, V2, Vr) and runway length requirements
- Aircraft on approach will be flying at higher airspeeds and require more runway to land
Climb and Descent Rate
1) Climb rate Vs climb speed ‑ climb rate increases as a function of airspeed due to the airfoil producing more lift as it moves through the air at a higher speed. Therefore, when an aircraft takes off, the aircraft first accelerates to takeoff speed, then accelerates again to climb speed. At this point most pilots have two choices, to ascend at the BEST RATE OF CLIMB for that particular aircraft, or, climb at a lesser rate but at a HIGHER airspeed. This choice is required because most aircraft produce less thrust than the weight of the aircraft. (Negative power to weight ratio) The ability of the wing to accelerate AND LIFT is therefore limited by the available thrust.
Only military combat aircraft have the ability to accelerate AND climb using thrust alone, (positive power to weight ratio) without regard for the lifting ability of the wing and engine together. This is done using an afterburner takeoff procedure with very high rates of climb available soon after rotation. (> 30,000fpm/9000mpm) The cost is extreme fuel consumption.
2) Descent rate Vs descent speed ‑ descent rate is determined by the safe operating speeds for each particular aircraft. These speeds vary depending on if landing gear/flaps/spoilers are extended or retracted etc. Every aircraft has specific descent profiles that are detailed in the Pilot’s Operating Handbook (POH). These rates of descent/airspeeds are designed to allow stabilised descent to occur. (Four forces under control) The goal of a stabilised descent is to not exceed the design parameters of the aircraft. The biggest danger of an uncontrolled descent is the overspeeding of the aircraft. Overspeeding can result in damage or even break-up of an aircraft in flight.
Pilots prefer to initiate speed and altitude changes separately. Thus when an aircraft changes from level flight to descent, the pilot starts descending and THEN adjusts airspeed as required. Both can be done simultaneously, but this increases pilot workload; depending on the aircraft type. Military pilots have the most leeway in this respect due to the aircraft types used.
ATC implications
The following should be considered when requesting pilots to make altitude changes:
- Remember the performance capability of the aircraft concerned. i.e. if a higher climb rate is requested, airspeed will usually decrease
- If high rate of climb is not required, many pilots prefer to maintain a higher airspeed at a lower climb rate (This also facilitates traffic flow)
- Military combat aircraft are capable of very high climb rates, but only if fuel is not a consideration (it usually is)
- High descent rates may cause unacceptable airspeed increases, especially for light aircraft
- Spoiler equipped aircraft (air transport/business jets) are capable of high rates of descent if required (this may cause difficulty for passengers)
- Military combat aircraft are capable of very high descent rates (this is not usually a problem)
- Allow the pilot to stabilise the aircraft on descent before requesting speed changes, if possible
Aircraft Stall Speed
All aircraft have a minimum speed where the wing is no longer producing usable lift. This is the stall speed. Below the stall speed controlled flight is no longer possible. The exact speed varies depending on the configuration and attitude of the aircraft. The figure normally associated with a stall condition is the straight and level speed at which the wing cannot produce sufficient lift due to a lack of relative wind. In reality, a stall can occur at much higher speeds.
The effects of a stall vary according to aircraft type. In a basic trainer the stall may produce a gentle nose down attitude as the aircraft attempts to gain airspeed. Altitude loss may be minimal. A higher performance aircraft may exhibit a drastic nose down break combined with a snap roll to the left or right. At lower altitudes this may result in an irrecoverable loss of control.
Factors affecting the onset of a stall are:
i) Airspeed ‑ when airspeed decreases to the point where an increase in Angle of Attack (AOA) can no longer maintain lift, the aircraft stalls.
ii) Weight ‑ at high aircraft weights, a greater AOA is required to lift the higher load. This results in earlier onset of the stall because the wing's critical angle is reached sooner.
iii) Turbulence ‑ large changes in the airflow over the wing can result in non-laminar air movement leading to a stall condition. Vertical gusts can also change the AOA to the point where a stall may occur.
iv) Wind shear ‑ wind shear may totally change the relative wind over the wing. This can lead to a stall condition as lift is destroyed. High airspeed is not usually a solution due to POH turbulent air penetration speed requirements, i.e. lower airspeeds are required to alleviate airframe stress in rough air.
v) Angle of bank ‑ as bank angle is increased, the wing rotates from straight and level to a steeper angle relative to the surface of the earth. This has two results; the wing's lift vector is aligned away from being directly opposite the pull of gravity, and, the centrifugal loading on the aircraft requires a higher AOA to achieve continued lifting of the wing. The consequences of these forces is an exponential increase in stall speed relative to bank angle.
vi) Flaps - the use of flaps will normally increase the upper wing camber (depending on flap type) to enhance lift. This will result in lower stall speeds. Spoilers however, will increase stall speed due to their disruption of laminar airflow.
vii) Slats ‑ leading edge slats on many air transport aircraft allow the wing to maintain a laminar airflow pattern at higher AOA. This decreases the stall speed and allows the wing to achieve a higher critical angle before stall onset is reached.
Critical Phases of Flight ‑ Performance Enhancement
To allow an improvement in takeoff and rate of climb, aircraft may use the following devices.
i) Flaps ‑ partial flap extension (lift type flaps) is routinely used on air transport aircraft to achieve proper takeoff performance. The improved lift characteristics result in less runway length being used before rotation and better climb performance.
ii) Afterburner ‑ The Concorde and many military combat aircraft will use afterburner to provide the necessary thrust for a successful takeoff.
iii) JATO ‑ Jet Assisted Take Off is used by certain military transport aircraft to achieve very short, high speed takeoff runs. This is useful in short field situations or when operating from rough, unimproved landing strips. (JATO is a solid fuel booster rocket system that can be attached to the aircraft fuselage)
Load Factors
In addition to climb rate, the aircraft weight affects other aspects of flight performance. A fully loaded aircraft cannot carry a capacity fuel load and successfully take off using the normal runway length available. Therefore, if long range is desired some of the load must be removed. If this is not an option then an intermediate fuel stop must be made and only partial fuel carried at departure.
The maximum weight which allows normal takeoff to occur is called the Maximum Gross Take Off Weight. (MGTOW) An aircraft at MGTOW will normally not be able to land immediately after departure because the stress loads on the landing gear and wings would be too great. The procedure in this case requires fuel dumping. If this is not possible then fuel must be burned off by flying a holding pattern until landing can be carried out.
An aircraft at MGTOW may also not be able to reach the final cruising altitude for that flight in a continuous climb. A step climb procedure is used to allow gradual climb based on best aircraft performance.
Company Limitations
Aircraft operators may apply company rules to the operation of their aircraft. This regulates airspeeds, climb/descent rates and other aspects of aircraft performance. Weather conditions for landing and departure may also be specified.
In this way similar aircraft may be flown and operated quite differently by various agencies and air carriers. Company limits are determined by senior pilots within each operation. They do not usually supersede national air regulations unless agreement is reached between the aircraft operator and government transportation officials (i.e. military aircraft operation may be very different than that specified in air regulations).
Pilot Proficiency and Technique
The skill and proficiency of individual pilots may determine the efficiency and smoothness of traffic flow into and out of an airport. Highly trained, experienced pilots such as air transport crews (ATR rating) can be expected to respond quickly and effectively to requests and instructions. Light aircraft pilots will display varying levels of technique and proficiency. Solo students and new holders of private pilot licenses (PPL rating) may be hesitant and respond incorrectly to requests and instructions due to lack of experience. These pilots may also attempt to comply with ATC requests that require a higher level of skill without being completely aware of the performance implications due to a lack of experience.
The following variables are dependant on pilot experience and technique:
i) Size of circuit flown
ii) Climb/descent rates
iii) Pilot reaction time for clearances/instructions
iv) Time spent on runway after landing/before takeoff
v) Approach airspeed
ATC implications
If an aerodrome controller suspects that a pilot is inexperienced, or is a solo student, the following requests and instructions should be used with caution:
i) immediate takeoff
ii) go around on final approach
iii) turns immediately after takeoff
iv) intersection takeoff with minimal TORA for that aircraft type
v) reduced spacing between landing aircraft ‑ watch for extended time on runway prior to taxiing off after landing
Runway Conditions
The aircraft POH performance figures usually assume that a bare and dry runway condition exists. Contaminants such as water, ice, slush, and loose snow all affect aircraft performance to certain degrees.
Water alone can cause increases in the takeoff run due to drag. Landing distances can be increased up to 700% due to hydroplaning. If a 10kt crosswind (90 degrees) is present a hydroplaning aircraft can drift off of a 200ft/75m runway in only 7 seconds.
Specific ATC procedures dealing with contaminated runway operations will be dealt with in later lessons.
Runway Gradient
Very few airports have completely level runways from one end to the other. Some runways will have a pronounced gradient. This will affect takeoff and landing performance in the same way as a hill may affect automobile performance.
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