Appendix 1 – Primary Flight Information

ED Decision 2015/019/R

This appendix provides additional guidance for displaying primary flight information. Displaying primary flight information is required by CS 25.1303(b) and CS 25.1333(b). The specifications for arranging primary flight information are specified in CS 25.1321(b).

1.1     Attitude

Pitch attitude display scaling should be such that during normal manoeuvres (for example, approach or climb at high thrust-to-weight ratios) the horizon remains visible in the display with at least 5 degrees pitch margin available.

An accurate, easy, quick-glance interpretation of attitude should be possible for all unusual attitude situations and other “non-normal” manoeuvres sufficient to permit the pilot to recognise the unusual attitude and initiate an appropriate recovery within one second. Information to perform effective manual recovery from unusual attitudes using chevrons, pointers, and/or permanent ground-sky horizon on all attitude indications is recommended.

Both fixed aeroplane reference and fixed earth reference bank pointers (“ground and/or sky” pointers) are acceptable as a reference point for primary attitude information. A mix of these types in the same flight deck is not recommended.

There should be a means to determine the margin to stall and to display that information when necessary. For example, a pitch limit indication is acceptable.

There should be a means to identify an excessive bank angle condition prior to stall buffet.

Sideslip should be clearly indicated to the flight crew (for example, a split trapezoid on the attitude indicator) and an indication of excessive sideslip should be provided.

1.2     Continued Function of Primary Flight Information (Including Standby) in Conditions of Unusual Attitudes or in Rapid Manoeuvres

Primary flight information must continue to be displayed in conditions of unusual attitudes or in rapid manoeuvres (CS 25.1301). The pilot must also be able to rely on primary or standby instrument information for recovery in all attitudes and at the highest pitch, roll, and yaw rates that may be encountered (CS 25.1301).

In showing compliance with the specifications of CS 25.1301(a), CS 25.1309(a), CS 25.1309(b), and CS 25.1309(c), the analysis and test programme must consider the following conditions that might occur due to pilot action, system failures, or external events:

—          Abnormal attitude (including the aeroplane becoming inverted);

—          Excursion of any other flight parameter outside protected flight boundaries; or

—          Flight conditions that may result in higher than normal pitch, roll, or yaw rates.

For each of the conditions identified above, primary flight displays and standby indicators must continue to provide useable attitude, altitude, airspeed and heading information and any other information that the pilot may require to recognise and execute recovery from the unusual attitude and/or arrest the higher than normal pitch, roll, or yaw rates (CS 25.1301).

2.1     Airspeed and Altitude

Airspeed and altitude displays should be able to convey to the flight crew a quick-glance sense of the present speed or altitude. Conventional round-dial moving pointer displays inherently give some of this sense that may be difficult to duplicate on moving scales. Scale length is one attribute related to this quick-glance capability. The minimum visible airspeed scale length found acceptable for moving scales has been 80 knots; since this minimum is dependent on other scale attributes and aeroplane operational speed range, variations from this should be verified for acceptability. A displayed altitude that is geometrically derived should be easily discernible from the primary altitude information, which is barometrically derived altitude. To ensure the pilot can easily discern the two, the label '''GSL''' should be used to label geometric height above mean sea level. See Section 5.4.4 of Appendix 6 for HUD-specific airspeed considerations.

Airspeed reference marks (bugs) on conventional airspeed indicators perform a useful function by providing a visual reminder of important airspeed parameters. Including bugs on electronic airspeed displays is encouraged. Computed airspeed/angle-of-attack bugs such as Vstall warning, V1, VR, V2, flap limit speeds, etc., displayed on the airspeed scale should be evaluated for accuracy. The design of an airspeed indicator should include the capability to incorporate a reference mark that will reflect the current target airspeed of the flight guidance system. This has been required in the past for some systems that have complex speed selection algorithms, in order to give the flight crew adequate information for system monitoring as required by CS 25.1309(c).

Scale units marking for air data displays incorporated into primary flight displays are not required (“knots,” “airspeed” for airspeed, “feet,” “altitude” for altimeters) as long as the content of the readout remains clear. For altimeters with the capability to display both English and Metric units, the scale and primary present value readout should remain scaled in English units with no units marking required; the Metric display should consist of a separate present value readout that does include units marking.

Airspeed scale markings such as stall warning, maximum operation speed/maximum operating mach number, or flap limits, should be displayed to provide the flight crew a quick-glance sense of speed relative to key targets or limits. The markings should be predominant enough to confer the quick-glance sense information, but not so predominant as to be distracting when operating normally near those speeds (for example, stabilised approach operating between stall warning and flap limit speeds).

If airspeed trend or acceleration cues are associated with the speed scale, vertically oriented moving scale airspeed indications should have higher numbers at the top so that increasing energy or speed results in upward motion of the cue. Speed, altitude, or vertical rate trend indicators should have appropriate hysteresis and damping to be useful and non-distracting, however, damping may result in erroneous airspeed when accelerating. In this case, it may be necessary to use acceleration data in the algorithms to compensate for the error. The evaluation should include turbulence expected in service.

For acceptable means of compliance and guidance material on instrument graduations and markings, refer to the latest ETSOs and list of approved deviations on the Agency’s webse (www.easa.europa.eu).

Altimeters present special design problems in that: (1) the ratio of total usable range to required resolution is a factor of 10 greater than for airspeed or attitude, and (2) the consequences of losing sense of context of altitude can be detrimental. The combination of altimeter scale length and markings, therefore, should be adequate to allow sufficient resolution for precise manual altitude tracking in level flight, as well as enough scale length and markings to reinforce the flight crew's sense of altitude and to allow sufficient look-ahead room to adequately predict and accomplish level-off. When providing low altitude awareness, it may be helpful to include radio altimeter information on the scale so that it is visually related to the ground position.

2.2     Low and High Speed Awareness Cues

CS 25.1541(a)(2) states: '''The aeroplane must contain – Any additional information, instrument markings, and placards required for the safe operation if there are unusual design, operating, or handling characteristics.''' The CS-25 certification specifications related to instrument systems and their markings were not developed with modern day electronic displays in mind; consequently, these electronic displays are considered an “unusual design characteristic” per CS 25.1541(a)(2), and may require additional marking to warrant safe operation. In particular, it is considered necessary to incorporate additional markings on electronic airspeed displays in the form of low and high speed awareness cues to provide pilots the same type of “quick glance” airspeed awareness that was an intrinsic feature of round dial instruments.

Low speed awareness cues should provide adequate visual cues to the pilot that the airspeed is below the reference operating speed for the aeroplane configuration (that is, weight, flap setting, landing gear position, etc.); similarly, high speed awareness cues should provide adequate visual cues to the pilot that the airspeed is approaching an established upper limit that may result in a hazardous operating condition. Consider the following guidance when developing airspeed awareness cues:

—          Take into account all independent parameters that may affect the speed against which protection is being provided. This is most important in the low speed regime where all large aeroplanes have a wide range of stall speeds due to multiple flap/slat configurations and potentially large variations in gross weight.

—          The cues should be readily distinguishable from other markings such as V-speeds and speed targets (bugs). The cues should indicate not only the boundary value of the speed limit, but must clearly distinguish between the normal speed range and the unsafe speed range beyond those limiting values (CS 25.1545). Since the moving scale display does not provide any inherent visual cue of the relationship of present airspeed to low or high airspeed limits, many electronic displays utilize an amber and red bar adjacent to the airspeed tape to provide this quick-glance low/high speed awareness. The preferred colours to be used are amber or yellow to indicate that the airspeed has decreased below a reference speed that provides adequate manoeuvre margin, changing to red at the stall warning speed. The speeds at which the low speed awareness bands start should be chosen as appropriate to the aeroplane configuration and operational flight regime. For example, low speed awareness cues for approach and landing should be shown starting at VREF with a tolerance of +0 and –5 knots. Some Agency approved systems use a pilot selectable operating speed “bug” at VREF supplemented by system-computed low speed cues that vary in colour as airspeed decreases below certain multiples of the appropriate stall speed (for example, white below 1.3VS, amber below 1.2 VS, and red below 1.1 VS). Consider the specific operating needs of other flight regimes when developing the criteria for the associated visual cue.

—          Low speed awareness displays should be sensitive to load factor (g-sensitive) to enable the pilot to maintain adequate manoeuvre margins above stall warning in all phases of flight. The accuracy of this g-sensitivity function should be verified by flight tests. Flight tests should also be conducted in manoeuvring flight and expected levels of turbulence to evaluate proper functioning of any damping routines incorporated into the low speed awareness software; the level of damping should preclude nuisance/erratic movement of the low speed cues during operation in turbulence but not be so high that it inhibits adequate response to accurately reflect changes in margins to stall warning and stall during manoeuvring flight.

—          High speed awareness should be provided to prevent inadvertent excursions beyond limit speeds. Symbology should be provided to permit easy identification of flap and landing gear speed limits. A visual cue should be incorporated to provide adequate awareness of proximity to VMO; this awareness has been provided by amber bands, similar to the previously discussed low speed cues, and instantaneous airspeed displays that turn amber (or flash amber digits) as the closure rate to VMO increases beyond a value that sill provides adequate time for pilot corrective action to be taken without exceeding the limit speed.

—          The display requirements for airspeed awareness cues are in addition to other alerts associated with exceeding high and low speed limits, such as the stick shaker and aural overspeed warning.

3.      Vertical Speed

The display range of vertical speed (or rate of climb) indications should be consistent with the climb/descent performance capabilities of the aeroplane. If the resolution advisory (RA) is integrated with the primary vertical speed indication, the range of vertical speed indication should be sufficient to display the red and green bands for all TCAS RA information.

4.      Flight Path Vector or Symbol

The display of Flight Path Vector (FPV or velocity vector) or Flight Path Angle (FPA) cues on the primary flight display is not required, but may be included in many designs.

The FPV symbol can be especially useful on HUD applications. See Section 5.4.5 of Appendix 6 for HUD-specific FPV considerations.  .

The FPV or FPA indication may also be displayed on the HDD. In some HDD and most HUD applications, the FPV or FPA is the primary control and tracking cue for controlling the aeroplane during most phases of flight. Even though an FPV or FPA indication may be used as a primary flight control parameter, the attitude pitch and roll symbols (that is, waterline or boresight and pitch scale) which are still required primary indications by § 25.1303 must still be prominently displayed. In dynamic situations, such as during recovery from an unusual attitude, constant availability of attitude indications is required.

If the FPV/FPA is used as the primary means to control the aeroplane in pitch and roll, the FPV/FPA system design should allow pilots to control and manoeuvre the aeroplane with a level of safety that is at least equal to traditional designs based on attitude (CS 25.1333(b)).

There may be existing aeroplane designs where the HUD provides a FPV presentation and the HDD provides a FPA presentation. However, mixture of the two different presentations is not recommended due to possible misinterpretation by the flight crew. The designs that were accepted were found to have the following characteristics: correlation between the HUD FPV display and the primary flight display FPA display; consistent vertical axis presentation of FPV/FPA; and pilots’ ability to interpret and respond to the FPV and FPA similarly.

It should be easy and intuitive for the pilot to switch between FPV/FPA and attitude when necessary. The primary flight display of FPV/FPA symbology must not interfere with the display of attitude and there must always be attitude symbology at the top centre of the pilot's primary field of view, as required by CS 25.1321.

Aeroplane designs which display flight path symbology on the HUD and the HDD should use consistent symbol shapes (that is, the HUD FPV symbol looks like the HDD FPV).

In existing cases where an FPV is displayed head up and an FPA head down on an aeroplane, the symbols for each should not have the same shape. When different types of flight path indications may be displayed as head up and/or head down, the symbols should be easily distinguished to avoid any misinterpretation by the flight crew. A mixture of the two types of flight path indications is not recommended due to possible misinterpretation by the flight crew.

The normal FPV, the field-of-view limited FPV, and the caged FPV should each have a distinct appearance, so that the pilot is aware of the restricted motion or non-conformality.

Implementation of air mass-based FPV/FPA presentations should account for inherent limitations of air mass flight path computations.

Flight directors should provide some lateral movement to the lateral flight director guidance cue during bank commands.

To show compliance with CS 25.1301(a), CS 25.1303(b)(5), and CS 25.143(b), the FPV/FPA FD design must:

1.       Not have any characteristics that may lead to oscillatory control inputs;

2.       Provide sufficiently effective and salient cues to support all expected manoeuvres in longitudinal, lateral, and directional axes, including recovery from unusual attitudes; and

3.       Not have any inconsistencies between cues provided on the HUD and HDD displays that may lead to pilot confusion or have adverse affects on pilot performance.

Performance and system safety requirements for flight guidance systems are found in the following documents:

[Amdt No: 25/11]

[Amdt No: 25/12]

[Amdt No: 25/17]