AMC 25.773(b)(4) Pilot compartment non openable windows

ED Decision 2016/010/R

Total loss of external visibility is considered catastrophic. A sufficient field of view must exist to allow the pilot to safely operate the aeroplane during all operations, including taxi.

This field of view must remain clear in all operating conditions. Precipitation conditions such as outside ice, heavy rain, severe hail, as well as encounter with birds and insects must be considered.

This AMC material applies to conventional, multiple pane window systems, i.e. those which are composed of a main windshield and separate side panels assembled with structural posts. In the event a one piece ‘uni-body wraparound’ windshield is proposed, the applicant must meet the intent of the applicable rules, even though there are no separate side windows.

1.       Ice and heavy rain

—          Unless system failures leading to loss of a sufficient field of view for safe operation are shown to be extremely improbable, the following provides acceptable means to show compliance with CS 25.773(b)(4):

—          Each main windshield should be equipped with an independent protection system. The systems should be designed so that no malfunction or failure of one system will adversely affect the other.

—          For each forward side window it should be shown that any ice accumulations (Appendix C icing conditions and any applicable Appendix O icing conditions) will not degrade visibility, or the applicant should provide individual window ice protection system capability.

—          The icing accretion limits should be determined by analysis and verified by test. The extent of icing of side windows should be verified during natural or simulated icing flight tests with window ice protection systems unpowered. A limited number of test points, sufficient to validate the analysis, are required within Appendix C or Appendix O.

—          For the demonstration of compliance under Appendix O icing conditions, the applicant may use a comparative analysis. AMC 25.1420(f) provides guidance for comparative analysis.

2.       Hail, birds and insects

It should be shown by flight tests that exceptional pilot skill is not required to land the aeroplane using the normal aeroplane instruments and the view provided through the main or side windows having the degree of impairment to vision resulting from the encounter of severe hail, birds or insects. Appropriate test data should substantiate the estimated damage or contamination to the main or forward side windows during such an encounter.

It is unlikely that hail damage can be avoided. Rather than avoidance, the approach to ensure vision assuming hail strike has been to use damage assessment criteria contained in the ASTM International "Standard Test Method for Hail Impact Resistance of Aerospace Transparent Enclosures," ANSI/ASTM F 320-10 or equivalent. For the test set up to determine hail damage or windshield resistance to hail, reference can be made to ANSI/ASTM F 320-10, and "Global Climatic Data for Developing Military Products" MIL HDBK 310 (dated 23 June 1997).

For each impacted window, ANSI/ASTM 320-10 is used to characterize a damage pattern on a limited area of the window. For test purpose, the simulated damage patterns should be applied to the full impacted window surfaces in order to simulate in a conservative manner the visibility degradation through the windows.

The applicant should propose and substantiate the aircraft conditions when hail strike occurs. In the absence of such substantiation, the conservative assumptions will be to consider the maximum aircraft nominal speed combined with the hailstone falling speed.

When the damages are such that there is no remaining visibility through the windshield after hail encounter, or when the ice protection system is no longer operating after the hail encounter, a typical test configuration would be to block visibility out of the forward main windows for the pilot flying, and use simulated damage (if any) and ice accretions (if applicable) on the side window(s).

When conducting flight tests, adequate forward vision should be maintained for a safety pilot while providing appropriate forward view degradation for the test pilot.

Means of compliance to address birds and insects should be proposed by the applicant. The Agency is not aware of any in-service occurrence involving a total loss of visibility through the windshield after birds or insects encounter.

[Amdt 25/16]

[Amdt 25/18]