MOC VTOL.2600 Flight crew compartment

1.      External flight crew view

The following material is intended to serve as a guide, highlighting the elements to be considered when developing and assessing the external flight crew view of a VTOL capable aircraft. It offers a possible method to show compliance with VTOL.2600 for this design element.

The function of the external flight crew view in a piloted VTOL capable aircraft remains the same as assumed for any other aircraft in their respective Certification Specifications.

In the design phase of the pilot compartment, when considering the external flight crew view, applicants may therefore choose to start by using the guidance already available in AMC and AC material relevant to CS 27.773 “Pilot compartment view”, while keeping in mind the differences related with VTOL capable aircraft and Innovative Air Mobility (IAM) Operations. The AMC available for the different Certification Specifications include also Human Factors considerations.

(a)      Functions of the external flight crew view:

The external field of view should fulfil the following functions:

(1)     Provide sufficient external view so that the flight crew can perform their task of safely controlling the aircraft flight path.

(i)      The external field of view, or visual cues, will need to be assessed depending on the Flight Controls Laws, Kind of Operations and expected Meteorological Conditions (VMC or IMC)

(ii)     The external visual cues necessary to safely control the aircraft might differ depending on the phase of flight, as i.e. in the VTOL phase the flight crew may focus on ground details (“chin bubbles”) to fly a given trajectory or hold a position, while in forward flight they might only need to have a visible horizon.

(iii)     Depending on the design, the external view may be used for hazard awareness and/or mitigation, by showing that, by having parts of the aircraft visible by the crew, abnormal conditions can be identified to take proper actions and operate the aircraft safely.

(2)     Provide sufficient external view to see and avoid:

(i)      Traffic

(ii)     Ground obstacles

(b)     External field of view characteristics:

(1)     Optical distortions in the windshield or canopy, especially in the prime viewing areas should be avoided.

(2)     The design should allow for sufficient external field of view free of obstruction. Account can be taken of aircraft specific features (as “chin bubbles”) that provide the flight crew with sufficient visible external cues, in all day/night and weather conditions expected in operation.

(3)     The need for demisting devices/features should be considered during the development. Recent experience in electrically powered aircraft, where the amount of heated air that can be accessed and needs to be dissipated, has shown that the external view can get heavily impacted by fogging, and that the installation of an additional device/feature could be required for that purpose.

(4)     The area of the pilot compartment field of view that according with FAA AC 27.773[7] should be free from obstruction should be used as starting point for the design: years of experience show that this obstruction free area has ensured the functions listed in (a).

(i)      when using this material, applicants should consider the differences between the VTOL expected trajectories and flight attitudes envelope compared to conventional aircraft, and the CONOPS that will be carried out by the flight crew in terms of traffic/obstacle “see and avoid”.

(ii)     deviations from the current material can be justified by the reasons in (i) but also by  design characteristics of the VTOL capable aircraft (canards, lift/thrust systems forward of the flight crew compartment view).

(iii)     any obstructions should be assessed, and the suitability of the external field of view evaluated, in the entire flight test domain against its intended functions in the CONOPS.

(5)     If, for design reasons, the available external field of view does not allow the flight crew to perform their duties, the applicant may show compliance by using synthetic cues displayed to the flight crew. These synthetic cues should be designed to a high level of integrity and precision, in order to meet the intended function. They should be introduced as soon as possible in the design and be thoroughly assessed during the complete flight test campaign.

(c)      Loss of vision through windshield panel:

According to VTOL.2600 (c), for category Enhanced, the flight crew interface design must allow for continued safe flight and landing after loss of vision through any one of the windshield panels. The applicant should demonstrate by flight test, in case of a complete loss of vision through any panel, the remaining external field of view with the use of particular procedure (e.g., flight with sideslip) will allow for continued safe flight and landing.

(d)     Flight in precipitation[8] and operation in other environmental hazards:

(1)     The external field of view should be sufficient in day/night, and not impaired by precipitation conditions and other environmental hazards.

(2)     Precipitation conditions include, but are not limited to, rain, hail and snow.

(3)     While (e) provides specific guidance on evaluating the external vision obstruction resulting from a certain continuous exposure to snow conditions, no specific requirement applies for the obstruction when flying into inadvertent snow or rain.

(4)     Flight into hail should be considered taking into account the damage that can result from windshield structural integrity considerations as referred in (c), rather than concerning the expected obstruction due to its accumulation.

(5)     Other environmental hazards include, but are not limited to, operations into sand, dust and saline environment.

(6)     There is no specific requirement to determine any external vision impairment resulting from the exposure to environmental hazards.

(7)     The effect of operating into other environmental hazards should be taken into account during the aircraft systems qualification, including their effects on windshield wipers efficiency or the degradation of performance of any other alternative precipitation removing devices (i.e. hydrophobic coating or blowers), if installed.

(e)     Flight into known snow conditions:

CS-27 and AMC-27 contain no specific requirement or guidance for flight in precipitation conditions. In particular, no reference to falling and blowing snow is made in CS 27.773. There are no external vision requirements for flight into inadvertent snow.

This section intends to address the protection against potential accumulation of snow on windshield and windows when flying into known falling and blowing snow. 

So far, the pilot view obstruction in snow conditions has been addressed by the European Light Helicopter Manufacturers and the European Airworthiness Authorities during flight test demonstration for a turbine engine installation, as requested by the CS 27.1093(c). During these flight tests for helicopters powered by turbine engine, snow accretion was sometimes observed on the helicopter windshield, leading to a dangerous reduction in the pilot view. In these instances, only the use of wipers was able to restore acceptable visibility.

The Standardised European Rules of the Air establish in SERA.5010 the conditions under which an ATC unit can authorise a helicopter to operate within a control zone under Special VFR clearance, including certain weather minima. Therefore, it is assumed that a helicopter certified for day and night VFR can perform hover flights in re-circulating snow, take-off and land under snow falls, and fly with falling snow compatible with the Special VFR limit visibility.

Since the SERA Special VFR rules could still be applicable for VTOL capable aircraft, it is necessary to consider the pilot view of the flight path during a flight in snow fall that is compatible with these weather minima.

(1)     The external field of view should be sufficient in day/night, and not impaired by snow conditions.

(2)     If certification for flight in snow conditions is requested, it should be demonstrated that snow, both falling and blowing, does not accumulate on the VTOL windshield and windows so that flight crew external view of the flight path and surroundings is not unduly impaired during taxiing, hover flight, take-off, level flight and landing. Normal operations with no hazardous reduction in the pilot’s view of the flight path should be demonstrated under the following:

(i)      Conditions to ensure VTOL operation in falling and blowing snow without restriction:

(A)     Visibility: ½ mile as limited by snow, which represents a moderate/heavy snowstorm and is also consistent with the weather minima compatible with Special VFR. This value is a test parameter rather than an operational limitation to be imposed on the VTOL after the tests are completed.

(B)     Temperature:

(a)     Unless other temperatures are deemed more critical, -4°C to +1°C (25°F to 34°F) being  -2°C to +1°C desirable (28°F to 34°F) should be used, as conducive to wet snow conditions, which tends to accumulate on unheated surfaces subject to impingement.

(b)     Company development testing or experience with similar VTOL may be adequate to determine other critical ambient conditions for certification testing.

(C)     Operations:

(a)     Ground running, taxiing, and IGE hover operations are generally the most critical since the VTOL may be operating in recirculating snow. Twenty-five minutes, or the maximum allowed time in relation the aircraft limitations, under these extreme conditions is considered a reasonable maximum, both from the view of pilot stress and the maximum expected taxi time prior to take off in bad weather.

(b)     One hour of level flight operation, or maximum expected flight duration, under ½-mile visibility snow conditions is deemed to provide ample opportunity for accumulation to begin to build. Go-arounds and transitions to and back to wingborne flight, if applicable, should be included in these flight operations.

(c)      The durations reported in the table above are minimum test duration times based on experience with rotorcraft operations, to ensure that the snow accretion on the aircraft and windshield is representative of a worst-case scenario. Different durations can be agreed with the Agency depending on the actual aircraft limitations or the expected operations.

(D)     Provisions in the Aircraft Flight Manual:

(a)     Visibility restrictions or limitations, based on which falling and blowing snow operations can be allowed, are not considered appropriate, as visibility may fluctuate rapidly in snowstorms. It is affected by the presence of fog or ice crystals, is not measured or controlled by the flight crew, and is difficult to estimate.

(b)     Time limitations, other than possibly for ground and hover operations, are not considered appropriate:

1.      Since during cruise in snow conditions the aircraft is likely to be in and out of heavy snowfall, it is not practical for the flight crew to measure the time spent in snow in level flight conditions. Thus, it is not appropriate to include time limitations in the AFM for level flight snow operations.

2.      A practical ground and IGE hover time limitation of less than 25 minutes, or the maximum allowed time in relation the aircraft limitations, in recirculating snow may be considered. The expected action at the expiration of this specified time would be landing or transition to a safe flight condition where it has been shown that snow accumulations will not intensify or shed and so not cause unacceptable reduction in pilot visibility.

(ii)     Artificially produced snow should not be used as the sole means of showing compliance. While it is an excellent development tool, artificial snow production devices are usually restricted to use for hover and ground evaluations, and the snow pellets produced by these machines are not sufficiently similar to natural snowflakes to justify the use of artificial snow as the sole basis of certification.

(3)     Other test conditions:

(i)      The windshield and windows should remain free of excessive snow accumulation. Excessive accumulation is defined as accumulation that may cause hazardous reduction in flight crew’s view of the flight path.

(ii)     Actual flight demonstration should be performed in natural snow. The ground operations and IGE hover test conditions assume operation in recirculating snow. Blowing snow, resulting from rotor airflow recirculation, can be expected to be more severe than natural blowing snow if the VTOL capable aircraft continues to move slowly over freshly fallen snow. Thus, the blowing snow operational capability should be demonstrated by the taxi and hover operations in recirculating snow.

(iii)     Airspeeds:

(A)     For VFR VTOL capable aircraft, the airspeeds for the level flight test condition should include the maximum consistent with the visibility conditions.

(B)     For IFR operations, the airspeed should range from the minimum IFR speed or the minimum for snow operations up to the maximum cruise speed or the maximum speed specified for snow operations in the flight manual limitations, unless other airspeeds are deemed more critical. VTOL seeking VFR certification may later be IFR certified with a possible increase in airspeed in snow conditions. This factor should be considered if IFR certification is anticipated.

(iv)     Visibility measurements:

(A)     The specified visibility assumes that visual measurements are made in falling snow in the absence of fog or recirculating snow by an observer at the test site outside the tests VTOL capable aircraft’s area of influence.

(B)     An accepted equation for relating this measured visibility to snow concentration is V = 374.9/C^0.7734 where C is the snow concentration (grams/metre³) and V is the visibility (metres).

(a)     This equation can be reasonably applied to all snowflake type classifications and is credited to J.R. Stallabrass, National Research Council of Canada.

(b)     Other equations may be applied if they are shown to be accurate for the particular snowflake types for the test programme.

(v)     The likelihood that the desired concentration will exist for the duration of the testing is even more remote. Because of these testing realities, it is very likely that exact target test conditions will not be achieved. Those involved in certification should exercise good judgment in accepting alternate approaches. However, the applicant should strive to perform the test in conditions as close as practicable to ½ mile visibility.

(vi)     If it becomes apparent that snow accumulations in ground and IGE hover operations in recirculating snow are much more severe than in the level flight test, it is reasonable to accept prolonged IGE operations in recirculating snow and to accept durations of less than 1-hour level flight, or maximum expected flight duration. Best efforts should be made to ensure that at least some level flight time is accomplished at ½-mile visibility to assure that the spectrum is covered.

(vii)    For the level flight portion, if after a reasonable time it is noticed that there is no snow accumulation that would impair the pilot visibility, the duration of the level flight may be reduced accordingly.

(viii)   It should be determined that the visibility established at the test site is limited by snow and not by fog or poor lighting (twilight) conditions.

(ix)     Recirculation is necessarily a qualitative judgment by the test pilot. For test purposes, recirculation should be the highest snow concentration attainable in the manoeuvre, or that corresponding to the lowest visibility at which (in the pilot’s judgment) control of the VTOL is possible in the IGE condition. The visibility specification of ½ mile outside of the recirculation influence becomes inconsequential provided that fresh, loose snow is continually experienced during the ground operation and IGE hover testing phase. However, since it is intended that the test phases be accomplished sequentially to assure that transition to take off and other transients are considered, the conditions at take-off, level flight, and descent and landing should approximate the ½-mile visibility criteria.

2.      Controls and displays for use by the flight crew:

CS 27.1302 Amdt. 8, as per the guidelines defined in its AMC 27.1302, is accepted as a means of compliance with VTOL.2600 regarding the design and approval of installed equipment that is intended for use by the crew members from their normal seating positions in the cockpit with the following considerations:

(a)      CS 27.1302 and its AMC 27.1302 apply to the flight crew interfaces and system behaviour for all the installed systems and equipment used by the flight crew in the cockpit while operating the VTOL capable aircraft in normal, abnormal/malfunction and emergency conditions.

(b)     The functions that the flight crew members are able to perform from the cabin need to be considered if they can interfere with the ones under the responsibility of the cockpit flight crew, or if dedicated airworthiness requirements are included in the rules.

(c)      CS 27.1302 and its AMC do not apply to flight crew training, qualification, or licensing requirements.

(d)     The extent of the compliance demonstration necessary for each design may vary and not all the material contained in this MOC has to be systematically followed. The proportionate application of AMC 27.1302 will depend on criteria such as the VTOL category (Enhanced and Basic) and the maximum passenger seating configuration. 

(e)     The following proportional approach in the application of AMC 27.1302 supersedes AMC 27.1302 paragraph 3.2.9 “Proportional approach in the compliance demonstration”: