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Descent, Approach and Landing - A Guide for Controllers

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This article is an element of the NATS Flight Deck Procedures - A Guide for Controllers


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Category: Flight Technical Flight Technical
Content source: NATS NATS
Content control: EUROCONTROL EUROCONTROL

Descent, Approach and Landing

Finals
photo courtesy of ITL Video

General

During the descent and approach the flight once again becomes rather busy. Not only is there a lot to do but the aircraft often enters busy airspace, where the radio frequencies can become rather congested.

Calculating the Distance Required for Descent

The general formula used for calculating the distance required for descent is ‘3 times the height’, where the ‘height’ used is in multiples of thousands of feet. A more accurate figure is obtained using:

(3 x height) + (1nm per 10kts of speed loss) + (1nm per 10kts of Tailwind Component)

The ‘speed loss’ in the above formula is the difference between the maximum speed used during the descent and the minimum zero flap speed. For an average case of a maximum speed used during the descent of 300 knots and a minimum zero flap speed of 210 knots, this would therefore give a correction of 9 nm. Add to this occasions where a tailwind of 50 knots – 100 knots exists during the descent and it can be seen that corrections in the region of 15 nm – 20 nm may be required in order to give an accurate distance calculation rather than the general use of the ‘3 times the height’ rule. Even without a tailwind, corrections in the region of 7 nm – 10 nm are required for the speed loss.

Should the aircraft be above the ideal descent profile during the initial stages of descent (e.g. at 30,000 feet) then there should generally be sufficient time to take the appropriate action (e.g. using speed-brake) to recover and regain the ideal descent profile. However, in the later stages of descent it becomes much more difficult to recover from being high on the profile due to there being less distance and time available to accomplish this. It is therefore important to understand that flight crews must use the expanded formula during the later stages of descent as the distance required to reduce speed and the increased distance required due to a tailwind are a reality and cannot therefore be ignored. As an example, should the aircraft be at 15nm from touchdown at 5,000 feet and a speed of 250 knots, it may be thought, using the ‘3 times the height’ rule, that the aircraft is perfectly on profile. However, it may prove difficult or impossible in such a case to reduce the speed to that required for landing whilst maintaining, or attempting to regain, the required final approach profile (even more so if a tailwind exists). This may therefore mean that either the speed reduction should be commenced earlier in order to arrive at 15nm at 5,000 feet but this time already at the minimum zero flap speed or slower, or that, if possible, the aircraft should be at 4,000 feet at 15nm and a speed of 230 knots so that the aircraft can then be flown on a very shallow descent path in order to facilitate the speed reduction and will then arrive on the ideal final approach profile at 11nm at 3,600 feet at the correct speed with the initial stages of flap deployed.

When the aircraft is above the ideal descent profile the speed-brake is often used to increase the descent rate and descent angle in order to regain the ideal profile. Note that, due to aerodynamic equations, the effectiveness of the speed-brake is a function of speed and therefore its effectiveness becomes less as the speed is reduced. At speeds below 250 knots the effectiveness of using the speed-brake to recover from being above the ideal descent profile is often negligible. The mindset that the speed-brake can always be used to recover from being above the ideal descent profile should not therefore be encouraged, particularly when the situation in question is close to the final approach.

During or approaching the final approach, if the aircraft is above the ideal descent profile, often the undercarriage will be lowered earlier than it would otherwise be as this creates a great deal of drag and therefore significantly steepens the descent path. This is often the case when turning onto the final approach and being above the ideal final approach path (e.g. intercepting the ILS and being “above the glide”) or when flying downwind and changing to a ‘visual circuit’ from the originally planned (longer distance) ‘instrument pattern’.

FMS Descent

The FMS will produce a ‘Top Of Descent’ (TOD) point. This is typically displayed on the Navigation Display (ND) as a ‘TD’ symbol. The calculated TOD point will be such as to produce a continuous idle power descent from the cruising level until a point on the final approach where the undercarriage and landing flap are deployed and the thrust is increased as required to maintain the final approach profile. The exception to this being where descent restrictions are entered into the FMS, the FMS will therefore alter the descent profile accordingly in order to achieve the descent restrictions. The calculation of the TOD point will account for the distance required to reduce the speed to the final approach speed and will include other factors that affect the distance required for descent such as tail/head wind component and the descent speed (as given by the cost index).

Note that, due to the laws of physics, an increase in descent speed produces a decrease in the distance required for descent. Although with such an increase in descent speed this will therefore produce an increase in the distance required to reduce speed to the approach speed (or zero flap speed as used in the descent distance formula), an increase in descent speed will produce an overall decrease in the descent distance. Therefore, the cost index will have an affect on the TOD point.

The correct descent winds must be entered into the FMS by the flight crew in order for the FMS to calculate a realistic TOD point. If the descent winds are entered incorrectly, and therefore do not replicate the conditions that will be experienced in reality, any such predicted TOD point will therefore be inaccurate. If the inaccuracy is such that the actual tailwind experienced is greater than that entered into the FMS then this will mean that the FMS calculated TOD point occurs later than the required TOD point for the actual conditions. In such a case, if descent is commenced at the FMS calculated TOD point, the aircraft will be above the descent profile required for the actual conditions and may therefore be unable to achieve subsequent descent restrictions or be unable to become stabilised on the final approach (see later section with regards to the criteria for a ‘stabilised approach’). Depending on the severity of the inaccuracy, the use of other measures, such as using the speed-brake, may still render it impossible to achieve descent restrictions or a stabilised approach. It is therefore important that the flight crew check that the FMS descent speed is appropriate and that the FMS descent winds are correct in order for a successful and comfortable descent and final approach to be accomplished. With regards to achieving descent restrictions, should the flight crew forget or incorrectly input the descent restriction or the descent speed or descent winds as described above, then the aircraft may be unable to achieve the descent restriction. Descent restrictions are entered into the FMS by typing the required numbers in the FMS scratchpad, selecting the appropriate waypoint to which the restriction applies, then pressing the ‘Execute’ button on the FMS keypad. If the restriction is to be ‘above’ or ‘below’ a level then, typically, ‘A’ or ‘B’ is added to the numbers respectively to achieve this e.g. ‘230B’.

Although the process of entering descent restrictions into the FMS is, in general, simple, when such descent restrictions are given by ATC they must still be given in sufficient time for the flight crew to enter (and check) the details in the FMS. It can also be seen that if a descent restriction is not given until very close to the (normal) TOD point but on this occasion a tailwind exists which has not been accounted for in the controller’s calculation of the TOD point, then upon entering the details in the FMS the aircraft may have passed the ideal TOD point for those actual conditions. As explained, achieving the descent restrictions may therefore prove difficult or impossible. This situation is also exacerbated when the frequency is busy and therefore, after entering the details in the FMS in order to be given the TOD point prediction, there is an added period of time before the flight crew are able to make a transmission to ATC to request descent. There are also occasions when the process of entering descent restrictions into the FMS becomes a little lengthier. This often occurs when the restriction is given relative to an unknown waypoint or a waypoint that is different to that specified on the (flight crew’s) arrival chart and is therefore different to what the flight crew are expecting. Should this occur on fam flights, controllers will observe that this often results in the flight crew having a lengthy period of ‘heads down’ FMS button pushing and confusion. It may be useful for controllers to note that not all descent restrictions are given on the (flight crew’s) arrival charts and so in this case the flight crew are unable to pre-empt or appropriately prepare for any such earlier descent until such details are received from the controller’s transmission. Also, as stated, some arrival charts show restrictions relative to a waypoint that is different to the waypoint that the controller specifies in their instruction (although the restriction may be physically at the same point in space).

As an aside, but a human factors point worthy of note, controllers may observe during fam flights that when controllers issue an instruction that includes a descent restriction, then the same or a different controller issues a subsequent descent instruction that does not include the previously stated restriction, flight crews are often confused as to whether the previously issued restriction still applies or not. Although it is standard procedure that the present instruction cancels all previous instructions, when controllers apply this procedure pilots can often wonder whether the controller has simply forgotten to re-state that the restriction still applies. In such cases flight crews will therefore usually still aim to achieve the previously stated restriction in order to err on the safe side.

Finally, should the flight crew, after much effort of entering details into the FMS, forget to ‘execute’ the FMS entries, an appropriate TOD point will not be displayed on the ND. Consequently, should the flight crew not realise that the TOD symbol has not (suddenly) appeared on the ND or that the originally displayed TOD symbol has not moved by the required amount, then it is likely that the descent will not be commenced at the required point.

Allocation of Duties for Descent and Approach

As stated previously, in multi-crew (2 pilot) aircraft it is SOP for one pilot to be PF and the other pilot to be PM. It is a common SOP that whoever is designated as PF should remain as PF for the whole of the flight, that flight therefore being considered as ‘his/her’ sector. However, some operators have the SOP whereby at TOD, PF and PM swap roles. This continues until PM (for the descent) becomes visual with the airfield, at which point PM takes control of the aircraft – becoming PF once again and accomplishing the landing. The philosophy behind this type of descent and approach is that the original PM flies the descent and approach for the original PF, therefore allowing the original PF to monitor ‘his/her’ approach. Once the original PF becomes visual with the airfield, note that this may be as late as at the DA/DH or MDA/MDH, then the original PF takes control again in order to fly ‘his/her’ landing. This type of approach is often called the ‘Monitored approach’. During fam flights, depending on the operator’s SOPs, controllers may therefore witness this swapping of roles for the descent and then again for the landing.

Workload during Final Approach

During final approach the workload for the flight crew will increase at certain times due to the duties required for the final preparation of the aircraft for landing. Again, multiple items may be required at the same time and this may consequently mean that one or both pilots become ‘heads down’ in order to perform the required actions. The point at which the aircraft is re-configured to the landing configuration is usually the point of greatest workload for the flight crew during the approach. This point is typically at 2,000ft on the final approach and it is useful for controllers to note this point and the associated flight crew duties during fam flights.

At this re-configuration point the usual sequence of events is to:

  • Select the undercarriage and flaps to the landing configuration;
  • Select the required final approach speed;
  • Select other switchgear as required for landing;
  • Complete the before-landing checklist.

It will be observed during fam flights that each of the items in the list above requires multiple actions by the flight crew and this results in the process being lengthy and of a high workload. SOPs may also dictate that to accomplish the selections listed above, PF must call for the selection and PM must reply. This therefore requires much verbal communication between the flight crew. The selection of other switchgear may include items such as arming the speed-brake, selecting exterior lights and setting the missed approach altitude in the MCP. Also coinciding with the items in the list above may be items such as communications with ATC and changing the radio frequency, confirming the cabin is secure for landing, and the duties and calls that are required for the instrument approach e.g. “Outer marker, altitude checked”. If a non-precision approach is being flown this will also greatly increase the workload for the flight crew and such associated duties are worthy of note by controllers should a non-precision approach be flown during the fam flight. The workload is increased during a non-precision approach mainly due to two factors. One being that, in general, the automatic systems in aircraft are not capable or are not certified to ‘lock on’ to a non-precision final approach flight path. Therefore, although the autopilot may still be used, this requires PF to constantly calculate whether or not the aircraft is on the required lateral and vertical profile and actively command the aircraft to maintain or regain the required profile. If the aircraft is being flown with the autopilot engaged, typical modes used for this are ‘Heading’ and ‘Vertical Speed’. This is therefore different to the case of an ILS approach whereby the autopilot can ‘lock on’ to the localiser and glide-slope signals and the flight crew thereby have the reduced workload duties of monitoring the flight path. The other main factor is that PM must state at each mile the aircraft’s position relative to the required vertical profile and the altitude it should be at the next mile checkpoint e.g. “6 miles, 30 feet high. At 5 miles altitude should be 1,880 feet”. For PM this therefore gives increased workload not only from the increased verbal calls, but from the fact that PM must regularly move his/her attention to the approach charts in order to read the required checkpoint distances and altitudes (whilst still being responsible for monitoring the aircraft in general).

At a specified point on the final approach, SOPs will state a required verbal call by PM and response by PF to check that neither pilot has become incapacitated. The specified point on the final approach is typically at 500ft aal and the associated incapacitation call by PM is typically “Five hundred”. As additional information, operators whose flight crews are qualified to fly more than one aircraft type may also state in the SOPs that the required response by PF to the incapacitation call must also include the statement of the aircraft type e.g. “A330”. This is used as a reminder to the flight crew of the actual aircraft type being flown on that approach so that the pilot will use the correct flare and landing technique for that aircraft type.

Stabilised Approach Criteria

In order to reduce the likelihood of an unsuccessful landing due to the aircraft not being on the correct flight path or being outside of reasonable flight parameters, it is common for SOPs to state that at no later than a specified point on the final approach the aircraft must be within the criteria specified for a ‘stabilised approach’. If at the specified point or at any time after the specified point the aircraft is not within the stabilised approach criteria a go-around must be initiated.

The associated point used on the final approach is typically 500 feet aal or 1,000 feet aal depending on the operator in question. Some operators may specify both depending on the conditions e.g. 500 feet aal may be used during VMC but this must be increased to 1,000ft aal during IMC.

The stabilised approach criteria as specified in the SOPs will be such that they ensure that the aircraft is not only on the correct flight path (i.e. lateral and vertical profile) but also that the flight parameters are within limits (e.g. airspeed and bank angle), the engines are set at an appropriate power setting, the gear is down, and the flaps are set for landing. Examples of such criteria may be: Within one dot of the localiser and glide-slope scale, airspeed within –5kts to +15kts of the target final approach speed, and engines above idle power setting. Note that all of the specified criteria must be met in order for the approach to be considered as ‘stabilised’.

With reference to the engine power, this is due to the fact that (turbine) engines have a delay or ‘spool up’ time when commanding increased power from the idle setting. Should a go-around be necessary (for any reason) during the later stages of the final approach, if the engines were producing idle, or near to idle, power at the time the go-around was initiated, such a delay in the availability of the increased power would not be conducive to flight safety. Therefore SOPs will state an appropriate entry in the stabilised approach criteria with regards to the engine power.

SOPs may also state that at the specified point the PM is to make the required verbal call in order to confirm the aircraft’s status with respect to the stabilised approach criteria e.g. “Speed plus five, seven hundred feet per minute”.

Duties during Autoland

During the final approach and landing of an ‘autoland’ an observer in the flight deck will observe that the flight crew perform relatively few actions. Whilst it is true that during an autoland the flight crew duties are essentially those of a monitoring nature, the duties allocated to the flight crew during an autoland are more involved than may at first be thought. The following are therefore useful points to note should an autoland be witnessed during a fam flight.

During an autoland the captain will be the PF. Note that an autoland approach may still have a DH. Therefore, approaching Decision Height (DH) the PF must look out of the window in order to determine (at the DH) if the required visual reference is visible. If the required visual reference is not visible at DH a go-around must be initiated. Note also that the visual reference required at DH in order to continue the approach and autoland may be as little as one centre line light.

During an autoland approach and landing the PM’s duties are to monitor the primary flight and navigation instruments at all times. The PM must therefore remain ‘looking in’ at all times until the landing roll-out is complete. It is very important that the required instruments are monitored throughout the approach until the completion of the landing roll as a deviation or failure could mean that an autoland cannot or should not be continued. Note that as the PF will be ‘looking out’ approaching DH it must be the PM’s duty to remain ‘looking in’ until the landing roll-out is complete.

Note that at many of the ‘event points’ during an autoland, particularly those relating to failures or exceedance of limits, the flight crew must make an instant decision and do not have time to think about the situation. Therefore SOPs will often state that for certain events during an autoland a go-around must be initiated, this therefore removes the ‘thinking’ required by the flight crew and the correct action can be initiated immediately. It should also be noted from the explanations below that many of the failures associated with an autoland are of a ‘subtle’ nature, i.e. a warning is not generated to make the flight crew aware of such a condition. Examples of such ‘subtle failures’ are when a required FMA mode does not engage at the required time, or when an Instrument Landing System (ILS) signal deviation bar is removed from the PFD to indicate transmitter failure. Although in such cases a warning may not be generated for the flight crew, the consequences of not acting on any such condition can be very serious. It is therefore very important that the flight crew actively and accurately monitor the required instruments during an autoland.

With regards to the instruments that are required to be monitored during an autoland these are primarily:

  • The primary flying instruments (to ensure the correct airspeed etc is being flown);
  • The localiser and glide-slope indications (to ensure the ILS flight path is being maintained);
  • The FMA (to ensure the correct modes are engaged).

With regards to monitoring the localiser and glide-slope indications, the SOPs may state that below a specified height above the runway a go-around must be initiated for any deviation of either signal. The instruments must therefore be monitored by the flight crew for any such small indication. SOPs may also state that below a specified height a go-around must be initiated for a transmitter or receiver failure of either the localiser or glide-slope. Note that should such a failure occur it is not common for aircraft to have a system whereby a go-around is automatically initiated by the aircraft systems themselves. In this case the aircraft will therefore continue on its current flight path (as a default status) but will not be commanded by the ILS signal. Such failures must therefore be noticed by the flight crew (by monitoring the instruments) in order for the flight crew to initiate a go-around.

With regards to monitoring the FMA an in depth description of all the FMA modes associated with an autoland is not vital knowledge for controllers. If further explanation of an autoland is desired this can be sought from flight crews during fam flights. However, two examples of the importance of the change to the correct mode at the correct time during an autoland are that of the ‘Flare’ and ‘Idle’ modes, as described below.

During the autoland approach the ‘Glide-slope’ mode will be engaged so that the vertical path of the aircraft is commanded by the glide-slope signal. In order for an aircraft to accomplish a successful landing the aircraft must be ‘flared’ just prior to touchdown. The glide-slope signal cannot command a ‘flare’ as it is purely a straight line signal emanating from the glide-slope transmitter. A ‘Flare’ mode is therefore used whereby the glide-slope signal is not used, instead the aircraft’s flight computers use the radio altimeter data to calculate the rate of closure to the ground (i.e. runway) and this calculation is used in commanding an appropriate ‘flare’ manoeuvre. The ‘Flare’ mode will typically engage at 50 feet radio altitude (depending on aircraft type) and so at this height the PM must check that ‘Flare’ mode engages on the FMA otherwise the aircraft will impact with the ground at the current descent rate. Should ‘Flare’ mode not engage at the required height the SOP will almost certainly be for PM to immediately call “Go-around” and for PF to initiate the go-around immediately. It will be noted that should such a situation occur, at a height in the region of 50ft and with a descent rate in the region of 700fpm (12ft per second), the urgency to initiate a go-around is immediate. Consequently, flight crews will also be trained in the actions required should the wheels touch the ground during such a go-around.

During the ‘flare’ the engines must be commanded to idle power in order to accomplish a successful landing. The auto-thrust mode of the FMA must therefore be checked at the appropriate exact height, this typically being 30 feet radio altitude, to ensure that the ‘Idle’ mode engages.

Note that certain criteria during an autoland require the use of the radio altimeter rather than the barometric altimeter. It is therefore important that the radio altimeter is monitored and used correctly by the flight crew during an auto-land.

Upon touchdown, if the aircraft does not have a ‘roll-out’ mode the autopilot must be disconnected at touchdown so that the PF can control the aircraft and maintain the runway centre line during the landing roll. If the aircraft is fitted with a ‘roll-out’ mode the autopilot will use the localiser signal to continue tracking the runway centre line until such time as the autopilot is disconnected (at the completion of the landing roll). It is therefore important that the PM remains ‘looking in’ until the completion of the landing roll.

Difference in Final Approach Wind

In modern aircraft an accurate display of the wind currently being experienced by the aircraft is given on the ND. During the final approach it can be the case that the current wind displayed to the flight crew is significantly different to the surface wind as reported by the tower controller. This is often the cause of disbelief and concern by pilots, particularly if a tailwind exists on the final approach. Whilst it will be true that as the aircraft approaches the flare, the surface wind as reported by the tower controller will exist, any such difference in the wind on the approach may persist until relatively late in the approach i.e. until within the last few hundred feet. In particular when a tailwind exists, this may raise issues with flight crews with regards to the flight profile and configuration of the aircraft on the final approach and the choice of the direction of the runway-in-use for landing. Controllers may therefore find it useful to observe the wind displayed to the flight crew (on the ND) throughout the final approach during the fam flight.

Actions when Vacating the Runway

When vacating a runway, if the aircraft has simply ‘crossed’ the runway the only required action may be to switch off the strobe lights. When vacating a runway after landing however, this often involves many actions to be accomplished as part of the ‘after landing scan’. Add to this the need to change radio frequency, the receipt of taxi clearance and the need to check the taxi route on the aerodrome charts, then it can be seen that the PM in particular, and also possibly PF, becomes very much ‘heads down’ and is unable to appropriately monitor the taxiing of the aircraft and to maintain situational awareness.

Common actions when entering or vacating a runway are the selection of the aircraft’s exterior lights and selection of the transponder and the Traffic Alert and Collision Avoidance System (TCAS). These may be the only actions required depending on the situation, however, these (simple) actions still constitute the situation of one, or possibly both, crew members being ‘heads down’. When vacating a runway after landing the many (additional) items can include those associated with the flaps, speed-brake, auto-brake and weather radar.

Things to look out for
  • How the crew plan the descent;
  • How direct routings affect the descent profile;
  • How speed restrictions affect the descent profile;
  • How the crew manage being high or low on the descent;
  • How wind affects the descent;
  • How weather affects the descent;
  • Did the controller contribute to making the aircraft too high/low? If so, how?
  • How early/late the autopilot is disconnected and the PF takes over manually
  • The margins that the crew have for stabilising the approach considering the approach criteria (e.g. maintain 160 knots to 4 miles)


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