If you wish to contribute or participate in the discussions about articles you are invited to join SKYbrary as a registered user
Unstabilised Approach: Inappropriate ATC Speed Instructions
From SKYbrary Wiki
| Article Information | ||
|---|---|---|
| Category: | Toolkit for ATC - Stabilised Approach | 
|
| Content source: | EUROCONTROL | 
|
| Content control: | EUROCONTROL |
|
The Flight Safety Foundation has found that a crew’s inability to control the aircraft to the desired flight parameters (airspeed, altitude, rate of descent) was a major factor in 45 % of 76 approach-and-landing accidents and serious incidents. Flight-handling difficulties have occurred in situations which included rushing approaches, attempts to comply with demanding ATC clearances, adverse weather conditions and improper use of automation.
Contents |
Definition
ATC speed control instructions that are incompatible with the distance to go and required vertical profile below FL100, taking account of any significant head or tail wind components evident at altitude.
Effect on Flight crew and Their Potential Response
An abnormally high workload is needed to achieve the required flight path, which effects a crew's ability to maintain situational awareness.
There is an increased risk of an unstabilised approach developing, which may then be difficult to correct, and culminate in either a go around or a poorly handled landing, with increased risk of a heavy landing or a runway excursion.
Description
Modern jet aircraft are designed to have the highly efficient aerodynamic characteristic of low drag. This helps in reducing fuel consumption but in rturn makes them need longer distances for descent and deceleration.
Modern jet aircraft need greater distance to descend and decelerate.
Speed instructions (e.g. "Maintain 280 knots518.56 km/h
143.92 m/s") are sometimes necessary to maintain traffic separation but remove some of the flight crew’s options for managing the descent. Descent planning is a matter of managing energy, which effectively is done by altering drag. As shown by the figure below altering an aircraft’s speed is a way of altering drag.
A jet aircraft descending at higher speeds (for example, above 250 knots463 km/h
128.5 m/s) will descend quicker if it accelerates as the Total Drag is higher. Going faster therefore increases the descend rate, which can be used as a means of keeping the aircraft on the vertical profile.
If no speed instruction is given to an aircraft it will probably descend at the optimum speed for the prevailing conditions as calculated by the FMS, and will probably descend at idle power. Should ATC instruct an aircraft to decelerate to a lower speed, the rate of descent will decrease and the aircraft will drift above the descent profile. At this point the only option is to use spoilers and/or to request more track miles. An instruction to maintain a higher than normal speed is usually not a problem as the flight crew can simply add power to keep the aircraft on the descent profile.
Instructing an aircraft to reduce speed during the upper parts of the descent will usually cause it to drift above its descent profile.
An aircraft that is simultaneously descending and decelerating is dissipating both its kinetic and gravitational energy, which obviously would require a longer distance compared to an aircraft only decelerating, or only descending. As the aircraft decelerates less drag is available to dissipate energy which increases the distance further.
Although deceleration characteristics largely depend on the aircraft type and gross-weight, the following typical values, based on FSF ALAR BN 4.2, can be considered for a quick assessment and management of the aircraft deceleration capability:
| With approach flaps extended | 10 - 15 kts per NM |
| With landing gear down and flaps full | 20 - 30 kts per NM |
| With landing flaps and gear down | 10 - 20 kt per NM |
When established on a typical 3 degree glide slope path with only slats extended (i.e., with no flaps), it takes approximately 3 NM (1000 ft304.8 m) to decelerate down to the target final approach speed and to establish the landing configuration. Usually, the use of speedbrakes is not recommended when below 1000 ft above airfield elevation and/or in the landing flaps configuration.
Example:
- The maximum deceleration achievable between the OM (typically 6.0 nm from the runway threshold) and the stabilisation point (1000 ft above airfield elevation / 3.0 nm): 10 kts per NM x (6.0 – 3. 0) nm = 30 kts.
- So in order to be stabilized at 130 kt at 1000 above airfield elevation, the maximum speed that can be accepted and maintained down to the OM is: 130 kt + 30 kt = 160 kts
160 knots is the maximum acceptable speed at the OM in order to get the approach stabilised at 130 kts and 1000 feet above the runway.
ATC Options to Avoid the Action
The objective is to allow a crew to readily manage the energy state of their aircraft in a way that will lead to a stabilised approach, and which is compatible with the various versions of ‘should’ and ‘must’ stabilised approach gates.
When one or more of the flight crew have previous experience of a particular airport, they will appreciate day-to-day consistency in the application of speed control so that they can take account of their expectations during the approach brief. No speed greater than 250K IAS should be required below FL100.
There are broadly two types of solution:
- Devise, attempt to keep to, and preferably publish standard guidance for approach speed control, which is applicable generically to all jet and/or all turboprop types. Both should end with a requested maximum speed between 7nm and 4nm from touchdown which does not exceed 160 KIAS after which speed control should cease, except for tactical requests in order to maintain traffic spacing.
- Provide controller training on what is practicable for each of the different aircraft types using the airport in respect of both simultaneous "go down and slow down" and deceleration in level flight.
The first of the above options is the only one that will work consistently well at very busy airports. However, it will need to take into account whether the landing runway is used continuously solely for landing traffic or at the same time for take offs, which presents additional challenges. The second option is more likely to be feasible at less busy airports used by only relatively few aircraft types.
Both should aim for a continuous descent off any holding pattern or below 7000ft above aerodrome level at not more than 180 KIAS. Radar vectors should be used so that the intercept heading to final can be flown with the aircraft in level flight if required. Once established on final, speed of 160KIAS to 6 nm or the OM is compatible with a stabilised approach for most aircraft types. Such standard speeds are achievable by both turboprop and jet transports.
ATC Options to Manage the Consequences
- Be able to recognise from the radar screen data that an aircraft is having difficulty achieving the combination of instructions issued in respect of speed and descent.
- Be prepared to respond promptly and constructively if a crew advise difficulty in complying with instructions issued.
- Be prepared for the declaration of a go around by the crew.
- Be prepared to instruct a go around if spacing or any other operational safety consideration appears to demand this.
Further Reading
- Stabilised Approach
- DGAC (France) Publications on Non-Stabilised Approaches
- CFIT Precursors and Defences
- Constant Descent Angle Approach
- Non-stabilised Approach After ATC-Requested Runway Change (OGHFA SE)
- Runway Overrun After Unstabilised Approach (OGHFA SE)
- CANSO Report - Unstable Approaches: ATC Considerations