Unstabilised Approach: Delayed Descent Instructions
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|Category:||Toolkit for ATC - Stabilised Approach|
Keeping an aircraft unduly high in relation to distance to go below FL100, taking account of any significant head or tail wind components evident at altitude.
Effect of Action on Flight Crew and Their Potential Response
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.
The necessity to progressively deploy trailing edge flaps and landing gear in a set sequence during the latter stages of an approach is controlled by a series of maximum speed limitations which will be a crew’s first priority. Strict compliance with any ATC speed control or minimum rate of descent instructions may take a lower priority than correctly configuring the aircraft for the approach.
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.
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.
Delayed descent instructions or significant shortcuts may result in pilots requesting additional distance or contribute to high energy unstabilised approaches.
In case of delayed descent instructions, according to the FSF ALAR Tool Kit, pilots are encouraged to reduce airspeed (as appropriate to the aircraft model) to minimize the effect of the delay on the descent profile.
Most FMS programs keep the aircraft at the maximum altitude for as long as possible, using a profile with thrust at or near idle. As a consequence, there is little or no additional descent capability for the aircraft when descent is delayed significantly beyond the computed “top of descent point”.
ATC options to avoid the Action
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.
ATC Options to Manage the Consequences
- Anticipate the possibility of erratic speed variation as a delayed descent is initiated.
- 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.
- Stabilised Approach
- DGAC (France) Publications on Non-Stabilised Approaches
- CFIT Precursors and Defences
- Continuous Descent Final Approach (CDFA)
- Non-stabilised Approach After ATC-Requested Runway Change (OGHFA SE)
- Runway Overrun After Unstabilised Approach (OGHFA SE)
- Runway Excursions - An ATC Perspective on Unstable Approaches
- Avoiding Unstable Approaches - Important Tips for ATCOs
- Unstable Approaches – ATC Considerations, January 2011