Appendix 2 – Examples for Including Aural System Elements in an Alerting System

ED Decision 2011/004/R

1.       General

a.       Detailed guidance and examples are included in this appendix to help applicants with a means of compliance, requirements, and detailed design of an alerting system. They are based on the Agency’s experience with existing and proposed alerting systems that should comply with CS 25.1322. The extent to which this guidance and these examples are applied to a specific type investigation programme will vary, depending on the types of alerts that are presented, and the level of integration associated with an alerting system. The aural elements of an alerting system include:

—         Unique tones, including Master aural alerts

—         Unique Voice information (callouts)

b.       Each sound should differ from other sounds in more than one dimension (frequency, modulation, sequence, intensity) so that each one is easily distinguishable from the others.

2.       Master Aural Alert and Unique Tones

a.       Frequency

(1)     Use frequencies between 200 and 4500 Hertz for aural signals.

(2)     Aural signals composed of at least two different frequencies, or aural signals composed of only one frequency that contains different characteristics (spacing), are acceptable.

(3)     To minimise masking, use frequencies different from those that dominate the ambient background noise.

b.       Intensity

(1)     The aural alerting must be audible to the flight crew in the worst-case (ambient noise) flight conditions whether or not the flight crew are wearing headsets (taking into account their noise attenuation and noise cancelling characteristics) (CS 25.1322(a)(2)). The aural alerting should not be so loud and intrusive that it interferes with the flight crew taking the required action.

(2)     The minimum volume achievable by any adjustment (manual or automatic) should be adequate to ensure it can be heard by the flight crew if the level of flight deck noise subsequently increases.

(3)     We recommend automatic volume control to maintain an acceptable signal-to-noise ratio.

c.       Number of Sounds

(1)     Limit the number of different Master aural alerts and unique tones, based on the ability of the flight crew to readily obtain information from each alert and tone. While different studies have resulted in different answers, in general these studies conclude that the number of unique tones should be less than 10.

(2)     Provide one unique tone for master warning and one unique tone for master caution alerts.

(3)     We do not recommend a Master aural alert for advisories because immediate flight crew attention is not needed for an Advisory alert.

d.       Onset/Duration

(1)     The onset of the Master aural alert or unique tone should occur in a timeframe appropriate for the alerting condition and the desired response. Any delays between the onset of the Master aural alert or unique tone and its related visual alert should not cause flight crew distraction or confusion.

(2)     We recommend ramping the onset and offset of any aural alert or unique tone to avoid startling the flight crew.

(a)     A duration for onsets and offsets of 20-30 milliseconds is acceptable.

(b)     An onset level of 20-30 decibels above the ambient noise level is acceptable.

(3)     If more than one source of the Master aural alert or unique tone is provided, the Master aural alert or unique tone for the same condition should occur simultaneously at each pilot’s station. Any timing differences should not be distracting nor should they interfere with identifying the aural alert or unique tone.

(4)     Signal duration of the Master aural alert and unique tones should vary, depending on the alert urgency level and the type of response desired.

(5)     Unique tones associated with Time-critical warnings and Cautions should be repeated and non-cancelable until the alerting condition no longer exists (for example, stall warning), unless it interferes with the flight crew’s ability to respond to the alerting condition.

(6)     Unique tones associated with Warnings and Cautions should be repeated and non-cancelable if the flight crew needs continuous awareness that the condition still exists, to support them in taking corrective action. The aural warning requirements listed in CS 25.1303(c)(1) and CS 25.729(e) must be followed.

(7)     Unique tones associated with Warnings and Cautions should be repeated and cancelable by the flight crew if the flight crew does not need a continuous aural indication that the condition still exists (for example, Fire Bell or Abnormal Autopilot Disconnect) and if a positive acknowledgement of the alert condition is required.

(8)     Unique tones associated with Warnings and Cautions should not be repeated if the flight crew does not need continuous aural indication that the condition still exists.

(9)     Unique tones that are not associated with a Warning or a Caution (for example, certain advisories, altitude alert, or selective calling (SELCAL)) should be limited in duration.

(10)    Master aural alerts for Warnings and Cautions should be repeated and non-cancelable if the flight crew needs continuous awareness that the condition still exists, to support the flight crew in taking corrective action (CS 25.729(e)(2)). The requirements for aural Warnings in CS 25.729(e) must be followed.

(11)   Master aural alerts for Warnings and Cautions should be repeated until the flight crew acknowledges the warning condition or the warning condition no longer exists.

e.       Cancellation

(1)     For Caution alerts, if the flight crew does not need continuous aural indication that the condition still exists, the Master aural alert and unique tone should continue through one presentation and then be automatically cancelled.

(2)     If there is any tone associated with an Advisory alert, it should be presented once and then be automatically cancelled.

(3)     Provide a means to reactivate cancelled aural alerts (for example, the aural alert associated with a gear override).

(4)     When silenced, the aural alerts should be automatically re-armed. However, if there is a clear and unmistakable annunciation in the pilot’s forward field of view that the aural alerts have been silenced, manual re-arming is acceptable.

3.       Voice Information. For a Time-critical warning, use Voice information to indicate conditions that demand immediate flight crew awareness of a specific condition without further reference to other indications in the flight deck. A second attention-getting sensory cue, such as a visual cue, is still required (CS 25.1322(c)(2)). Additional reasons for using Voice information include:

a.       Limiting the number of unique tones.

b.       Transferring workload from the visual to the auditory channel.

c.       Enhancing the identification of an abnormal condition and effectively augmenting the visual indication without replacing its usefulness.

d.       Providing information to the flight crew where a voice message is preferable to other methods.

e.       Assuring awareness of an alert no matter where the pilot’s eyes are pointed.

f.       Voice Characteristics

(1)     General.

(a)     The voice should be distinctive and intelligible.

(b)     The voice should include attention-getting qualities appropriate for the category of the alert, such as voice inflection, described below.

(2)     Voice Inflection. Voice inflection may be used to indicate a sense of urgency. However, we do not recommend using an alarming tone indicating tension or panic. Such a tone may be inappropriately interpreted by flight crews of different cultures. Depending on the alerting condition, advising and commanding inflections may be used to facilitate corrective action, but the content of the message itself should be sufficient.

(3)     Voice Intensity.

(a)     Aural voice alerting must be audible to the flight crew in the worst-case (ambient noise) flight conditions whether or not the flight crew is wearing headsets (taking into account the headsets’ noise attenuation characteristics) (CS 25.1301(a)). Aural voice alerting should not be so loud and intrusive that it interferes with the flight crew taking the required action. The minimum volume achievable by any adjustment (manual or automatic) (if provided) of aural voice alerts should be adequate to ensure it can be heard by the flight crew if the level of flight deck noise subsequently increases.

(b)     We recommend automatic volume control to maintain an acceptable signal-to-noise ratio.

g.       Onset and Duration

(1)     The onset of Voice information should occur:

(a)     In a timeframe appropriate for the alerting condition and the desired response.

(b)     Simultaneously with the onset of its related Visual alert information. Any delays between the onset of the Voice information and its related visual alert should not cause flight crew distraction or confusion.

(c)      Simultaneously at each pilot’s station, if more than one source of the Voice information is provided for the same condition, so that intelligibility is not affected.

(2)     The duration of Voice information associated with Time-critical warnings should continue until the alerting condition no longer exists (for example, terrain warning). The Voice information should be repeated and non-cancelable during this time.

(3)     Voice information associated with Time-critical warnings and Cautions shouldnot be repeated if it interferes with the flight crew’s ability to respond to the alerting condition (for example, windshear warning, or ACAS II resolution advisory).

(4)     To support the flight crew in taking corrective action Voice information associated with Warnings should be repeated and non-cancelable if the flight crew needs continuous awareness that the condition still exists.

(5)     Voice information associated with Warnings should be repeated and cancelable if the flight crew does not need continuous aural indication that the condition still exists (for example, Cabin Altitude Warning or Autopilot Disconnect).

(6)     Reset the alerting mechanisms after cancelling them so they will annunciate any subsequent fault condition.

(7)     For voice alerts associated with a Caution alert, the corresponding Voice information should either:

(a)     Be limited in duration (for example, ACAS II Traffic Advisory or Windshear Caution), or

(b)     Be continuous until the flight crew manually cancels it or the Caution condition no longer exists.

h.       Voice Information Content

(1)     The content should take into account the flight crew’s ability to understand the English language.

(2)     When practical, Voice information should be identical to the alphanumeric text message presented on the visual information display. If that is not possible, the Voice information and alphanumeric messages should at least convey the same information, so it is readily understandable and initiates the proper pilot response.

(3)     For Time-critical warnings, the content and vocabulary of Voice information must elicit immediate (instinctive) directive corrective action (CS 25.1322(a)(2)). In order to do this, it should identify the condition triggering the alert. In some cases, it may also be necessary to provide guidance or instruction information.

(4)     For Warning and Caution alerts, the content of Voice information must provide an indication of the nature of the condition triggering the alert (CS 25.1322(a)(2)). The Voice information should be descriptive and concise.

(5)     The content should be consistent with any related visual information display (for example, Aural: “Pull up”; Visual: “Pull up” on the PFD.)

(6)     Structure Voice information that uses more than one word so if one or more words are missed the information will not be misinterpreted (for example, avoid the word “don’t” at the beginning of a voice message).

(7)     Design Voice information so the flight crew can easily distinguish one spoken word message from another to minimise confusion.

[Amdt 25/11]