Cruise - A Guide for Controllers
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This article is an element of the NATS Flight Deck Procedures - A Guide for Controllers
During cruise the flight crew will generally provide a monitoring function, but will also manage ATC instructions and do any necessary paperwork. Constant weather updates will be obtained and occasionally the aircraft will be deviated around weather cells following negotiation with ATC.
The crew will also use this time to prepare for the return leg (if there is one) and will discuss fuel needs, weather, suitable diversion aerodromes, NOTAMS and any other relevant information.
Towards the latter stages of the cruise the PF will prepare for the approach which involves setting up the flight deck and briefing the PM on the approach and what the actions will be if they must perform a missed approach.
Selecting a Direct Route in the FMS
The autopilot typically operates in two system states:
- Strategic Operation: the aircraft follows the programmed route entered into the FMS;
- Tactical Operation: the aircraft responds to direct inputs from the flight crew, such as heading, level and speed (used when being vectored by ATC)
To achieve a direct route through the use of the FMS the required flight crew actions are usually, depending on the architecture of the specific FMS, to either: • Select the waypoint in the FMS. Input that waypoint to the top line of the FMS flight plan. Press the ‘Execute’ button on the FMS keypad, or; • Select the waypoint in the FMS. Press the ‘Direct to’ button on the FMS keypad. Press the ‘Execute’ button on the FMS keypad.
If the FD is already engaged in the FMS Strategic Mode, upon executing the direct route, the FD will give the appropriate commands to achieve the direct route. If the FD is engaged in a Tactical Mode, such as the ‘Heading’ mode, then in many aircraft this will require the FMS Strategic mode to be engaged by pressing the appropriate button on the MCP. During fam flights controllers may observe that it can be very often surprisingly easy for flight crews to forget to engage the FMS Strategic navigation mode (when in the ‘Heading’ mode) as the physical actions of selecting and executing the direct route in the FMS easily lead to the mindset that such a direct route has been accomplished and commanded, even though it has not until the FD is set to the FMS lateral navigation mode. In such a case the aircraft will therefore continue on its present heading whilst the FD is engaged in the Tactical ‘Heading’ mode rather than accomplishing the direct route. Note that if the heading of the direct route is similar to the present heading, the radar controller is unlikely to be aware that the flight crew has not correctly accomplished the direct route, until the next turning point occurs, upon which the aircraft will still continue on its present heading (in Strategic ‘Heading’ mode) rather than accomplishing the turn as per the FMS flight plan. In some aircraft the executing of a ‘Direct to’ on the FMS keypad automatically changes the FD mode to the FMS Strategic navigation mode and therefore the above situation of forgetfulness should not occur.
In the case where the instruction “Route direct [waypoint], when on track continue the heading” is given, again it may be observed that it can be surprisingly easy for flight crews to forget to engage the Tactical ‘Heading’ mode once the turn has been completed in the FMS lateral navigation mode. Again the radar controller is unlikely to be aware that the flight crew has not correctly complied with the ATC clearance until the next turning point occurs, upon which the aircraft will this time execute a turn as per the FMS flight plan, rather than continuing the present heading.
Minimum Speed Margin at High Altitude
For reasons of efficiency aircraft often cruise at levels that are at or close to the ‘maximum’ level with regards to the aircraft’s performance. When at or close to the ‘maximum’ level the aircraft is often therefore close to its ‘minimum’ speed. The ‘minimum’ speed may be due to one of a variety of aerodynamic reasons; however, the speed must be kept above the ‘minimum’ speed in order to ensure that the aircraft is not in danger of stalling. The ‘minimum’ speed is typically displayed on the Primary Flight Display (PFD) speed tape as an amber or yellow band. The ‘maximum’ level that the aircraft may operate at due to these performance reasons is specified by the regulatory authorities and will be such as to provide a specified margin to the ‘minimum’ speed. What is useful for controllers to note is that such a margin may be in the region of as little as 10kts. Consequently, should an engine failure or loss of thrust occur at a level where the margin to the ‘minimum’ speed is relatively small, the commencement of a descent will be required almost immediately due to the fact that the aircraft is unable to maintain a safe speed with the remaining thrust and therefore would be in danger of stalling if the descent was not commenced in order to maintain a safe speed.
During fam flights controllers may observe on the PFD the margin to the ‘minimum’ speed from the current speed as described above.
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