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AP4ATCO - Factors Affecting Aircraft Performance During Descent and Initial Approach
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- The following SKYbrary Articles:
a) article description Factors affecting performance during descent and initial approach (descent rate and gradient).
b) source (IANS) 6.4
c) additional sources
d) SKYbrary sources FAA Airplane Flying Handbook (FAA-H-8083-3A) – chapter 12
ALAR Briefing Note 4.1 — Descent-and-approach Profile Management _____________________________________
FACTORS AFFECTING AIRCRAFT PERFORMANCE DURING DESCENT
The descent and initial approach phase of flight starts at the end of cruise phase, when descent is initiated by the crew, and it ends when aircraft will start the approach for landing. The point at which the descent is initiated from the cruising level is called top of descent point. The flight crew will have to calculate the top of descent point to ensure that they arrive at the correct level for the start of their approach. This is done in a way to facilitate a descent as close as possible to the optimum descent profile. The optimum descent profile includes thrust cut off (idle engines) from the top of descent and then long glide to the start of the approach phase. ATC endeavors to allow aircraft to use their optimum profile for the descent, but in high traffic environments this can be difficult.
When deciding on the top of descent point, the crew will have to consider the descent gradient and the rate of descent. Descent gradient is the ratio of height descended to distance travelled, and it expressed as a percentage. Rate of descent is the vertical component of the aircraft’s velocity, normally expressed in feet per minute.
Factors affecting the descent gradient
The optimum descent profile would have angle of descent that will give the maximum gliding distance given the height of the aircraft. If the angle of descent α is known then the descent gradient is equal to tan (α). For small angles tan (α) = sin (α). Now taking into the consideration the formulas from the drawing above:
Descent gradient = tan (α) = sin (α) = (Drag– Thrust) / Weight
This shows that the descent gradient depends on the difference between the drag and thrust (the excess drag).
Special case is when the thrust is equal to 0 (engines idle situation):
Descent gradient = tan (α) = Drag / Lift
So, the descent profile is closest to the optimum when the drag to lift ratio is minimum, and this occurs when the lift to drag ratio is a maximum. Following factors have affect on the lift to drag ratio:
For a given aircraft mass the lift to drag ratio maximum will occur at particular speed. This speed increases with aircraft mass. When heavier, the aircraft is expected to fly at higher speed during the descent and initial approach phase. However, the lift to drag ratio is not affected by the mass of the aircraft, so the descent gradient and the glide distance are not affected, they are same as for lighter conditions.
If the aircraft is required to fly at different speed during the descent, the descend gradient will be greater, and therefore the descent profile will not be optimal.
The descent angle relative to the ground will be affected by the wind. So, the descent gradient will be affected as well. A headwind will reduce the ground speed and therefore reduce the horizontal distance that aircraft travels in comparison to the no wind conditions. Therefore a headwind gives increased descent gradient (steeper descent in reference to the ground), while a tailwind affects in opposite direction and gives reduced descent gradient. Crosswind component has no effect on the descent gradient.
In headwind conditions it is to be expected that aircraft will start their descent later (the top of descent will be closer to the start of the approach); while in tailwind conditions they will start their descent earlier.
The total drag of an aircraft will depend on its configuration. When the flaps are lowered the drag is increased, resulting in an increase in excess drag, therefore the descent gradient is increased. Same thing happens when the landing gear is lowered; the descent gradient is increased. However, the usage of such devices is restricted by maximum speeds for their operation due to the excess stress they create on the aircraft structure.
Some aircraft types have spoilers in order to increase the drag more when it is necessary, and to increase the descent gradient. Depending on aircraft design, most of them can be used at high speeds and can act as speed brakes during the descent.
Factors affecting the rate of descent
The rate of descent is the vertical component of the speed, expressed in feet per minute. It depends on the true airspeed (V) and the descent gradient:
Rate of descent = V x sin (α) = V x Descent gradient = V x (Drag – Thrust) / Weight
Two aircraft that have same rate of descent but different horizontal speed will have different descent gradients.
The aircraft with the higher horizontal speed will have the lower descent gradient.
Using the same logic two aircraft that have the same descent gradient, but different horizontal speeds will have different rates of descent. The aircraft with the higher horizontal speed will have the higher rate of descent.
For a given aircraft mass the minimum rate of descent will occur when the product of the speed and the excess drag is minimal. It is important to note that in comparison to the minimum descent gradient, minimum rate of descent will occur not when the lift to drag ratio is maximum, but when the product of the speed and drag to lift ratio is minimum. This will happen at a speed that is lower then the speed that gives the best gliding distance (minimum descent gradient). The speed is called minimum rate of descent speed. This speed is increased as the aircraft mass is increased. So, the heavier the aircraft, the higher the minimum rate of descent speed and therefore the higher the minimum rate of descent.
On the other hand the maximum rate of descent occurs at the maximum permissible speed with the highest achievable drag. This maximum is individually set for every aircraft type since the usage of high drag devices at high speed is a design issue.
In general, rate of descent increases with increasing speed and increasing drag, and so air traffic controllers will not try to control both of these parameters during the descent phase.
It is only the speed that is controlled or it is only the rate of descent that is controlled, but never both of them (although in given conditions the desired can be achieved; it is considered as not a good technique).
An aircraft that is affected with speed control during descent should not be asked to maintain certain rate of descent; and vice versa, an aircraft that is affected by vertical speed control should not be asked to maintain certain speed.
The rate of descent is independent from the wind speed, because it is always considered in reference to the air not the ground.
When the flaps are lowered the drag is increased, resulting in an increase in excess drag, therefore the rate of descent is increased or, more important, the rate of descent can be maintained as the same while horizontal speed can be reduced. Most of the effect of aircraft configuration is used to reduce the speed during the descent while maintaining same rate of descent (steeper descent profile).
The cabin pressurization has a greater effect on the rate of descent in comparison to the rate of climb and ceiling.
When the aircraft is descending, 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 a sophisticated control system that is increasing the pressure inside the cabin by the use of compressors. It is important that the rate of descent is matched with a corresponding rate of cabin pressure increase (same structural stress). If the rate of descent is exceeding the corresponding rate of cabin pressure increase, the aircraft structure may have different structural stress or even stress in an opposite direction (ambient pressure greater than cabin pressure - implosion). Thus the maximum rate of descent would be limited by this factor. Special care has to be taken by the crew during descent and initial approach, when the cabin pressure is manually controlled or the system is running with degradation.
The best passenger comfort is achieved at rates of descent of 1500 feet per minute.
FACTORS AFFECTING AIRCRAFT PERFORMANCE DURING INITIAL APPROACH
Descent and approach – crew / ATC interaction
Incorrect management of the descent-and-approach profile and/or aircraft energy condition may result in: - a loss of situational awareness and/or - an unstabilized approach
Either situation increases the risk of approach-and-landing accident, including those involving controlled flight into terrain (CFIT).
To help prevent delaying initiation of the descent and to ensure optimum management of the descent-and-approach profile, the following procedures are recommended to the crews and ATC should expect crews to act as follows:
- descent preparation and the approach briefing will be completed typically 10 minutes before the beginning of descent point (or when within VHF communication range if automatic terminal information system (ATIS) information cannot be obtained 10 minutes before the beginning of descent point) - if standard arrival (STAR) is included in the flight management system (FMS) flight plan but is not expected to be flown because of radar vectors, the STAR will be checked (track, distance, altitude and airspeed restrictions) against the expected routing and the beginning of the descent point will be adjusted - if descent imitation is delayed by ATC, airspeed will be reduced (as appropriate to the aircraft type) to minimize the effect of the delay on the descent profile - if FMS navigation accuracy does not meet the applicable criteria for descent, terminal area navigation or approach, no descent will be made below the minimum en route altitude (MEA) or minimum safe altitude (MSA) without prior confirmation of the aircraft position
Also the flight crew, “staying ahead of the aircraft”, will concentrate on achieving desired flight parameters (e.g. aircraft configuration and position, energy condition, track, vertical speed, altitude, airspeed and attitude) during the descent and approach. Any indication that a desired flight parameter will not be achieved (also as an effect of ATC actions) will prompt immediate corrective actions or the decision to make a 360-degree turn / go around.
Crews will monitor and adjust the descent, based on a typical 3000 feet per 10 nautical miles (NM) descent gradient, corrected for the prevailing headwind or tailwind component, while adhering to the required altitude/airspeed restrictions. Below 10.000 feet, flying at 250 knots, a smooth transition between various approach phases are achieved with the following values:
- 9000 feet above airport elevation at 30 track nautical miles from touchdown, and - 3000 feet above airport elevation at 15 track nautical miles from touchdown (to allow for deceleration and slats/flaps extension)
If the flight path is significantly above the desired descent profile (for example because of ATC restrictions) then ATC may expect the crew to:
- maintain high airspeed (and a steep angle of descent) as long as practical, or - use speed brakes (significant and rapid speed reduction) - extend the landing gear earlier (as allowed by airspeed and configuration) if speed brakes are not sufficient, or - request a 360-degree turn
A hurried, last minute descent with power at or near idle is inefficient and can cause excessive engine cooling. It may also lead to passenger discomfort, particularly if the airplane is unpressurized.
In a descent, some airplanes require a minimum EGT, or may have a minimum power setting or cylinder head temperature to observe. In any case, combinations of very low manifold pressure and high RPM settings are strongly discouraged by engine manufacturers.
Descent and approach – operational “tips” for ATCOs
- for most jet aircraft, it is difficult to ‘go down and slow down’ at the same time unless one or both rates of change are modest. Simultaneous instructions to do both may not produce the response timing anticipated by ATC
- keeping aircraft high and close-in to the airport for noise abatement, or for separating from traffic below, can make a stabilised approach a challenge. This may lead to a missed approach or result in excess height and/or speed over the threshold.
- be realistic in tactical control and note that automatics, which will be in use for most approaches, both jet and turboprop, take time to respond to input commands.
- be aware that turboprop aircraft can often be made to ‘go down and slow down’ more easily than jets. This is because turboprops are more likely to be descending with significant power set in order to maintain the desired forward speed which can be removed to quickly increase the rate of descent.
- be aware that turboprop aircraft’s relatively lower cruise altitudes mean that their pressurisation systems are less flexible than those of jet aircraft and may not readily respond to sudden high rates of descent.
- recognise that during continuous descent when a previously-required speed will be reduced, ATC should anticipate a reduction in the rate of descent. Thus may or may not be transient depending on whether trailing edge flaps are deployed to provide additional drag. Descents at 190KIAS or less allow most jet aircraft types to deploy the initial stage of trailing edge flap as a means to maintain their rate of descent whilst slowing down, but speed reduction during descent at higher speeds will need the temporary deployment of speed / air brakes since thrust settings may already be quite low.
- anticipate the possibility of erratic speed variation as a delayed descent is initiated.
- high rates of descent to regain the desired vertical profile in busy terminal airspace are likely to lead to increased rates of both TCAS TAs and TCAS RAs with all the potential difficulty that may cause for ATC
- since high rates of descent may increase the prevalence of TCAS events, it will be helpful to minimise the crossing of arriving and departing traffic streams during strategic planning and tactically to maximize vertical separation where tracks do cross.
- offer increased track miles with any delayed descent clearance.
- non-standard or unexpected descent profile, due to ATC restrictions/decisions (delayed descent or significant shortcuts), may require crew’s additional concentration on calculations and may lead to “two heads down” situation. That may have a direct effect of reduced situational awareness of the crew and may even lead to problems with communication. It may also result in high energy unstabilized approaches.
For more information refer to: - Approach and Landing Accident Reduction (ALAR) toolkit - “Constant Descent Angle Approach” SKYbrary’s article - “Continuous Descent” SKYbrary’s article
1. [Question type: multiple choice, based on AirQuestions FACT-DE/126]
Q: A tailwind component A1: will increase the descent gradient A2: will reduce the descent gradient A3: has no effect on descent gradient
Correct answers: A2
2. [Question type: multiple choice]
Q: To allow a smooth transition between approach phases an airplane, flying below 10.000 feet at 250 knots, should be A1: at 4000 feet AAL (above airport elevation) at 30 NM from touchdown A2: at 6000 feet AAL (above airport elevation) at 30 NM from touchdown A3: at 9000 feet AAL (above airport elevation) at 30 NM from touchdown
Correct answers: A3
3. [Question type: true or false]
Q: Descents at 230KIAS or less allow most jet aircraft types to deploy the initial stage of trailing edge flap as a means to maintain their rate of descent whilst slowing down A1: True A2: False, the optimal speed is 190 KIAS
Correct answers: A2