d. Policy/Compliance Methods. For guidance on compliance with this requirement, refer to the preamble of this rule and the following information. While flight tests to demonstrate compliance are not specifically required in this section, the functional tests identified are usually conducted during flight tests because the pressurization system, including the positive and negative pressure relief valves, can be exercised during other flight test activities, and many of the tests are more meaningful when conducted in flight. The functional tests involving environmental factors (temperature and humidity) are generally conducted on the ground under laboratory conditions.
(1) Pressure Tests For Compliance with § 25.843(a): The following policy is extracted from an FAA memorandum dated February 21, 1990, in response to concerns regarding cabin test pressures for demonstrating compliance with § 25.843(a).
(a) Some manufacturers conduct cabin pressure vessel tests to a pressure corresponding to the maximum relief valve setting, while others test to 1.33 times the maximum relief valve setting. Effective June 5, 1996, if the airplane certification basis includes Amendment 25-87, the factor 1.33 (ref. § 25.365(d)) applies for operation to 45,000 feet while a factor of 1.67 applies for operation above 45,000 feet. While there is not a regulatory requirement to test each and every production airplane to the 1.33 times maximum relief valve setting, many major manufacturers test to this pressure to enhance the fatigue characteristics of the structure. The FAA recommends that modifiers follow the lead of these major manufacturers. If the airframe was originally tested to the 1.33 times relief valve setting, then the modified airplane should also be tested to the same pressure.
e. Reference. The address for ordering the latest revision of the advisory circular listed below can be found in the Appendix to this AC.
AC 25-7A, Flight Test Guide for Certification of Transport Category Airplanes.
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Section 5. FIRE PROTECTION
35. SECTION 25.851 - FIRE EXTINGUISHERS.
a. Rule Text.
(a) Hand fire extinguishers.
(1) The following minimum number of hand fire extinguishers must be conveniently located and evenly distributed in passenger compartments:
---------- ---------- ------- ------ -- ------------- ------------- Passenger Capacity Number of extinguishers ---------- ---------- ------- ------ -- ------------- ------------- 7 through 30 1 31 through 60 2 61 through 200 3 201 through 300 4 301 through 400 5 401 through 500 6 501 through 600 7 601 through 700 8 -------------------------------------------------------------------
(2) At least one hand fire extinguisher must be conveniently located in the pilot compartment.
(3) A least one readily accessible hand fire extinguisher must be available for use in each Class A or Class B cargo compartment and in each Class E cargo or baggage compartment that is accessible to crewmembers in flight.
(4) At least one hand fire extinguisher must be located in, or readily accessible for use in, each galley located above or below the passenger compartment.
(5) Each hand fire extinguisher must be approved.
(6) At least one of the required fire extinguishers located in the passenger compartment of an airplane with a passenger capacity of at least 31 and not more than 60, and at least two of the fire extinguishers located in the passenger compartment of an airplane with a passenger capacity of 61 or more must contain Halon 1211 (bromochlorodifluoromethane CBrC1F2), or equivalent, as the extinguishing agent. The type of extinguishing agent used in any other extinguisher required by this section must be appropriate for the kinds of fires likely to occur where used.
(7) The quantity of extinguishing agent used in each extinguisher required by this section must be appropriate for the kinds of fires likely to occur where used.
(8) Each extinguisher intended for use in a personnel compartment must be designed to minimize the hazard of toxic gas concentration.
(b) Built-in fire extinguishers. If a built-in fire extinguisher is provided-
(1) Each built-in fire extinguisher system must be installed so that-- 91
(i) No extinguishing agent likely to enter personnel compartments will be hazardous to the occupants; and
(ii) No discharge of the extinguisher can cause structural damage.
(2) The capacity of each required built-in fire extinguishing system must be adequate for any fire likely to occur in the compartment where used, considering the volume of the compartment and the ventilation rate.
[Doc. No. 5066, 29 FR 18291, Dec. 24, 1964, as amended by Amdt. 25-54, 45 FR 60173, Sep. 11, 1980; Amdt. 25-72, 55 FR 29783, Jul. 20, 1990; Amdt. 25- 74, 56 FR 15456, Apr. 16, 1991]
b. Intent of Rule. This rule prescribes the standards for both hand held and built-in fire extinguishers. Section 25.851(a), which addresses hand fire extinguishers, is addressed in Advisory Circular (AC) 25-17. Section 25.851(b) is intended to ensure that the built-in fire extinguishing system does not introduce a hazard to occupants or the airplane structure, and that the system is adequate to control any fire likely to occur.
c. Background. Effective February 1, 1965, part 25 was added to Title 14, Code of Federal Regulations (14 CFR) to replace part 4b of the of the Civil Air Regulations (CAR). The standards in section 4b.380(b) of the CAR were carried over essentially unchanged to § 25.851 of 14 CFR. While § 25.851 has been amended several times, the standards for built-in fire extinguishers have not been not changed.
d. Policy/Compliance Methods. For guidance on compliance with this requirement, refer to the preamble of this rule and the following information. Flight tests are conducted, in which the fire extinguishing system is activated and the extinguishent concentration is measured. The common method is to utilize a gas spectrum analyzer to sample the extinguishent concentration in real time at several locations in the compartment. An advisory circular is being written that will provide guidance on certification of cargo compartment fire extinguishing or suppression systems. Guidance related to compliance with § 25.851(b)(2) is included under §§ 25.855 and
25.857 of this AC.
e. References. None.
36. SECTION 25.854 - LAVATORY FIRE PROTECTION.
a. Rule Text.
For airplanes with a passenger capacity of 20 or more:
(a) Each lavatory must be equipped with a smoke detector system or equivalent that provides a warning light in the cockpit, or provides a warning light or audible warning in the passenger cabin that would be readily detected by a flight attendant; and
(b) Each lavatory must be equipped with a built-in fire extinguisher for each disposal receptacle for towels, paper or waste, located within the lavatory. The 92
extinguisher must be designed to discharge automatically into each disposal receptacle upon occurrence of a fire in that receptacle.
[Amdt. 25-74, 56 FR 15456, Apr. 16, 1991]
b. Intent of Rule. This section provides standards for certification of lavatory smoke detection and waste chute fire extinguishing systems.
c. Background. Prior to the adoption of § 25.854, part 121 was amended in March 1985, by the addition of § 121.308, to require that each lavatory be equipped with smoke detectors, and that each lavatory trash receptacle be equipped with a fire extinguisher that discharged automatically when a fire occurs in the receptacle. Amendment 25-74, April 1991, added § 25.854 to require the same provisions for newly certificated airplanes.
d. Policy/Compliance Methods. For guidance concerning lavatory smoke detection testing plus an acceptable smoke generator, see AC 25-9A. For compliance with this requirement, refer to the preamble of this rule and the following information.
(1) Approval of Lavatory Smoke Detectors. The following policy is extracted from an FAA memorandum dated October 28, 1991, in response to an inquiry regarding certification of smoke detectors in transport category airplane lavatories.
(a) The following guidance provides policy for determining approval criteria for the lavatory smoke detectors that are required by Amendment 25-74 to 14 CFR. This amendment added § 25.854, which requires the installation of lavatory smoke detectors on all transport category airplanes with a passenger capacity of 20 or more. The preamble to the amendment contains a statement that "a commercially available smoke detector, such as the type commonly used in residential buildings, which is demonstrated to serve its intended function as installed, could be considered adequate under the proposal of Notice 89-1." When the economic evaluation was performed in support of the rule change, the cost impact was based in part on costs associated with inexpensive smoke detectors such as those noted above.
(b) When § 25.854 was added by Amendment 25-74, the justification used followed that contained in the preamble to the relevant part 121 amendment that was already in force. Section 121.308, effective March 29, 1985, requires lavatory smoke detectors for all airplanes operating under part 121. The original intent of the part 121 rule was to discourage passengers from smoking in the lavatories, and the smoke detectors were perceived as a costeffective way to provide that deterrent. This concern has become even greater now that virtually all US, and many international, commercial flights are "smoke free." The incentive to smoke in the lavatory is even greater because passengers cannot smoke at their seats.
(c) The primary FAA concern is to detect smoke from a lavatory fire in a timely manner, and the most likely location for a fire in the lavatory is in the waste bin containing paper towels. For that reason, the smoke detectors currently in use in lavatories are tested using burning paper towels, and are not necessarily optimized for the particulate size contained in tobacco smoke. The detectors continue to be effective as a deterrent to smokers, however, 93
because tobacco smoke is also detected, as evidenced by continuing reports of lavatory smoke alarms due to smoking in lavatories. To the extent that smoke detectors do not detect cigarette smoke, there is a perception that they are not properly designed and/or functioning. Therefore, it is the FAA position that industry be encouraged to do all they can to ensure that lavatory smoke detectors not only function well to detect smoke from lavatory fires, but also function to effectively deter lavatory smoking by efficiently detecting cigarette smoke as well.
(d) When the airframe manufacturers were approached by their customers to install smoke detectors as part of the type design, the FAA was requested to provide certification requirements covering the installation. The guidance provided was the following:
1 The detectors had to meet §§ 25.1301(a) and (d) and 25.1309(a). This meant that environmental, as well as performance, standards had to be considered.
2 Testing was to be performed in flight, and the combustible material used for testing was to be representative of what would be expected to burn in a lavatory waste bin,
e.g., paper towels.
3 As a design goal, the detector was to provide a warning within one minute after a fire started.
4 If unpressurized flight was to be allowed, testing under those conditions had to be performed.
(e) It has been suggested that Technical Standard Order (TSO) TSO-C1c might be appropriate for use in approving lavatory smoke detectors. TSO-C1c provides standards for approval of detectors to be used in cargo compartment smoke detection systems. The TSO requirements are not considered appropriate for lavatory smoke detection, as the environment and products of combustion to be detected are both different from that to be expected in a cargo compartment fire. This is not to imply, however, that a TSO-C1c detector would be unacceptable. With appropriate testing, and recognizing that the environment in a lavatory is different than a cargo compartment, a detector authorized under the TSO might be an acceptable choice.
(f) TSO-C1c authorization should not be a requirement for lavatory smoke detector certification. The certification basis for the lavatory smoke detector installation must include §§ 25.1301 and 25.1309, as discussed above, plus any other appropriate sections such as those addressing structural or electrical supply requirements. If application for certification occurs after the effective date of Amendment 25-74 (April 15, 1991), § 25.854 would also be applicable. For these installations, the intended function is to detect smoke resulting from a fire occurring in a lavatory.
(2) Approval of Lavatory Fire Extinguishers Containing Agents Other Than Halon.
The following policy is extracted from an FAA memorandum dated March 31, 1997, to distribute a minimum performance standard for use in certifying lavatory fire extinguishers.
(a) Sections 25.854(b) and 121.308(b) require that each lavatory on passengercarrying transport category airplanes with a passenger capacity of 20 or more, be equipped with a built-in fire extinguisher for each disposal receptacle for towels, paper, or waste located within the lavatory. The fire extinguisher must be designed to discharge automatically into each disposal receptacle upon occurrence of a fire in the receptacle. Currently, although not required by airworthiness regulations, the typical aircraft lavatory disposal receptacle fire extinguisher uses Halon as the fire extinguishing agent.
(b) Halon production was banned as of January 1, 1994, under the provisions of the Montreal Protocol for those subscribing nations, due to its identification as an ozone destroying compound. The Environmental Protection Agency has exempted the aviation industry from the ban on the use of Halon. However, the FAA established the International Halon Replacement Working Group to help identify acceptable replacements for halons. A key aspect of this work is to define minimum performance standards which can be used to assess the performance of candidate replacement agents to assure that they will provide protection equivalent to Halon. Standards are being developed for fire extinguishers used in: lavatory trash receptacles; cargo compartments; engines and auxiliary power units; and hand held extinguishers. The first of these minimum performance standards is published in report number DOT/FAA/AR-96/122, titled “Development of a Minimum Performance Standard for Lavatory Trash Receptacle Automatic Fire Extinguishers,” dated February 1997.
(c) The minimum performance standard in report DOT/FAA/AR-96/122 provides guidance on acceptable methods of compliance to § 25.854. The performance of an alternative agent needs to be measured against a standard test method. This document establishes fire load, trash disposal receptacle test article, test procedures, and pass/fail criteria for built-in extinguishers for lavatory disposal receptacles.
e. References. The addresses for ordering the latest revision of advisory circulars (AC), technical standard orders (TSO), and other referenced documents listed below can be found in the Appendix to this AC.
AC 25-9A, Smoke Detection, Penetration, and Evacuation Tests and Related Flight Manual Emergency Procedures.
Technical Standard Order TSO-C1c, Cargo and Baggage Compartment Smoke Detection Instruments.
FAA Report DOT/FAA/AR-96/122, Development of a Minimum Performance Standard for Lavatory Trash Receptacle Automatic Fire Extinguishers.
37. SECTION 25.855 - CARGO OR BAGGAGE COMPARTMENTS.
a. Rule Text.
For each cargo and baggage compartment not occupied by crew or passengers, the following apply:
(a) The compartment must meet one of the class requirements of 25.857.
(b) Class B through Class E cargo or baggage compartments, as defined in
25.857, must have a liner, and the liner must be separate from (but may be attached to) the airplane structure.
(c) Ceiling and sidewall liner panels of Class C compartments must meet the test requirements of Part III of appendix F of this Part or other approved equivalent methods.
(d) All other materials used in the construction of the cargo or baggage compartment must meet the applicable test criteria prescribed in Part I of appendix F of this Part or other approved equivalent methods.
(e) No compartment may contain any controls, wiring, lines, equipment, or accessories whose damage or failure would affect safe operation, unless those items are protected so that-
(1) They cannot be damaged by the movement of cargo in the compartment; and
(2) Their breakage or failure will not create a fire hazard.
(f) There must be means to prevent cargo or baggage from interfering with the functioning of the fire protective features of the compartment.
(g) Sources of heat within the compartment must be shielded and insulated to prevent igniting the cargo or baggage.
(h) Flight tests must be conducted to show compliance with the provisions of
25.857 concerning-
(1) Compartment accessibility,
(2) The entries of hazardous quantities of smoke or extinguishing agent into compartments occupied by the crew or passengers, and
(3) The dissipation of the extinguishing agent in Class C compartments.
(i) During the above tests, it must be shown that no inadvertent operation of smoke or fire detectors in any compartment would occur as a result of fire contained in any other compartment, either during or after extinguishment, unless the extinguishing system floods each such compartment simultaneously.
[Doc. No. 5066, 29 FR 18291. Dec. 24, 1964, as amended by Amdt. 25-15, 32FR 13266, Sep. 20, 1967; Amdt. 25-32, 37 FR 3972, Feb. 24, 1972; Amdt. 25- 60, 51 FR 18242, May 16, 1986; Amdt. 25-72, 55 FR 29784, Jul. 20, 1990; Amdt.
25-93, 63 FR 8048, Feb. 17, 1998]
b. Intent of Rule. This rule contains the material standards and design considerations for cargo compartment interiors; the statement that each cargo compartment must meet one of the class requirements of § 25.857; and the flight tests which must be conducted for certification.
c. Background. Effective February 1, 1965, part 25 was added to Title 14, Code of Federal Regulations (14 CFR) to replace part 4b of the Civil Air Regulations (CAR). The standards in section 4b.382 of the CAR, cargo and baggage compartments, and section 4b.384, proof of compliance, of the Civil Air Regulations (CAR), and were carried over essentially unchanged to 96
§ 25.855 of 14 CFR. This section contains the standards related to those requirements that must be demonstrated by flight test.
(1) Amendment 25-15 (September 20, 1967) added reference to the requirements of § 25.853 for materials, rather than stating the materials must be at least flame resistant.
(2) Amendment 25-32 (February 24, 1972) specifically addressed insulation blankets and cargo covers meeting the requirements of § 25.853(b-3).
(3) Amendment 25-60 (May 16, 1986) added more stringent flammability standards for cargo compartment materials.
(4) Amendment 25-72 (July 20, 1990) made editorial changes and relocated the material flammability requirements to § 25.853 and Appendix F of part 25.
(5) Amendment 25-93 (February 17, 1998) revised §§ 25.855(c) and 25.857(c)(2) to eliminate reference to Class D cargo compartments. . These amendments upgraded the fire safety standards for cargo or baggage compartments in certain transport category airplanes by eliminating Class D compartments as an option for future type certification. Compartments that can no longer be designated as Class D must meet the standards for Class C or Class E compartments, as applicable. The Class D compartments in certain transport category airplanes manufactured under existing type certificates and used in passenger service must meet the fire or smoke detection and fire suppression standards for Class C compartments by early 2001 for use in air carrier, or most other commercial service. The Class D compartments in certain transport category airplanes manufactured under existing type certificates and used only for the carriage of cargo must also meet such standards or the corresponding standards for Class E compartments by that date for such service. These improved standards are adopted to increase protection from possible in-flight fires.
(6) Harmonization. This regulation is the subject of a Federal Aviation Regulations/Joint Aviation Requirements (FAR/JAR) harmonization effort under the Aviation Rulemaking Advisory Committee (ARAC). The ARAC working group is recommending revisions to §§ 25.855 and 25.857, and a new advisory circular may evolve from that process.
As a result of this process, Class B compartments may be revised and a new Class F compartment may be proposed.
d. Policy/Compliance Methods. For guidance on compliance with this requirement, refer to the preambles of this rule and the following information. In accordance with § 25.855(h), flight tests must be conducted to show compliance with the standards concerning: compartment accessibility, smoke and extinguishing agent penetration into occupied areas, and the dissipation of extinguishing agent. Cargo compartment smoke detection testing should be conducted in flight, and is usually accomplished concurrently with the smoke penetration tests. Advisory Circular 25-9A contains guidance regarding smoke detection and penetration testing. Where suitable data is available from previous certification flight test programs of the same airplane 97
model, the cognizant ACO may review an applicant’s proposal on a case by case basis and, if satisfactory, may accept certification ground tests for smoke detection purposes.
(1) Halon Concentration. The following policy is extracted from an FAA memorandum dated March 9, 1998, regarding the adequacy of Halon concentration levels needed for certification of cargo compartments in loaded vs. empty configurations.
(a) Traditionally cargo fire extinguishing systems have been certified by demonstrating five percent initial Halon concentration levels and subsequent concentration levels of three percent, when the cargo compartment has been in the empty configuration. The majority of applicants in previous certification programs have not tested or analyzed a loaded cargo compartment configuration.
(b) Recent tests and analyses have raised concern regarding the adequacy of Halon concentrations in a loaded cargo compartment. Regardless of this concern, the Transport Airplane Directorate does not currently have any written guidance, stating that applicants must test or analyze the loaded configuration. Requiring testing or analysis to show Halon concentration levels remain at or above three percent in the loaded cargo compartment may be considered beyond the scope of what the FAA can require without public comment. Although some applicants have previously considered loaded cargo compartments, we do not require testing or analysis for the loaded cargo condition at this time.
NOTE: This may be considered a safety issue that the Transport Airplane Directorate may address in future policy.
(2) Class D to Class C Conversion. The following guidance is extracted from an FAA memorandum dated October 29, 1997, regarding criteria for certification of cargo compartments to be converted from Class D to Class C.
(a) On June 9, 1997, the FAA issued a Notice of Proposed Rulemaking, Docket No. 28937, Notice 97-10 (62 FR 32412, dated June 13, 1997), that proposes to upgrade the fire safety standards for cargo or baggage compartments in certain transport category airplanes by eliminating Class D compartments as an option for future certification. This notice also proposes that Class D compartments in certain transport category airplanes manufactured under existing type certificates and used in passenger service would have to meet the fire detection and suppression standards for Class C compartments by early 2001 for use in air carrier, commuter, on demand, or most other commercial service.
(b) A policy memorandum, dated August 18, 1997, was issued in response to questions concerning what guidance was available regarding the certification of smoke detection/penetration and fire suppression system evaluations in anticipation of the final rule associated with Notice 97-10. After considering industry objections to our memorandum of August 18, 1997, the information in our memorandum is revised to read: Fire Suppression Tests.
Several companies have objected to the guidance provided under “Fire Suppression Tests” in the memorandum dated August 18, 1997, even though some recent certification projects have been 98
approved using that guidance. Some manufacturers argued that they used a number of techniques to establish a minimum Halon concentration, including a volumetric averaging technique. The certification criteria contained under “Fire Suppression Tests” in our memorandum of August 18, 1997, is considered new policy for Class D to C conversions.
Therefore, the manufacturers may continue to use the certification techniques they have been using, including volumetric averaging, to establish the minimum concentration of Halon for fire suppression.
(aa) At the FAA/Industry workshop of April 22-24, 1997, the FAA’s Technical Center expressed concern that the current Halon measuring technique using volumetric averaging may allow a concentration of Halon insufficient to suppress a fire. The FAA Technical Center tests have shown that Halon, having a higher density than air, settled in the cargo compartment. Further, the tests showed that fires may reignite at the higher water lines in the cargo compartment due to insufficient Halon concentration even though the average volumetric concentration of Halon was considered adequate. There was no subsequent measured increase in Halon concentration near the fire due to convective stirring. This information was presented to and discussed with Industry at the April 22-24,1997, workshop and is the basis of the “Fire Suppression Tests” guidance provided in our memorandum of August 18, 1997.
(bb) The use of the technique of volumetric averaging to determine the minimum Halon concentration is questionable in light of the testing accomplished by the FAA’s Technical Center. Therefore, establishing minimum Halon concentrations near the ceiling should be considered. Applicants may elect to take advantage of this information in measuring the Halon concentration in their tests even though the FAA will not require this technique at this time. Furthermore, the Transport Standards Staff will develop an Advisory Circular (AC) that addresses measuring the minimum acceptable level of Halon in all cargo compartments.
(c) The rest of the memorandum, dated August 18, 1997, was revised as follows, and may be used for guidance:
(d) System Reliability. Use advisory material appropriate to the certification basis.
When applying AC 25.1309-1A, the following is suggested: Detection and suppression systems are considered complex in terms of paragraph 6d of the AC. A failed detection system and/or a failed suppression system in conjunction with a fire should be considered a catastrophic event.
Therefore, utilizing figure 2 of AC 25.1309-1A, knowing the system is complex and the failure event is a catastrophic event, the depth of analysis should include both a qualitative and quantitative assessment (reference paragraphs 8d, 9, and 10 of the AC).
(e) Dispatch. For dispatch relief, the systems should be tested in the proposed Master Minimum Equipment List (MMEL) configurations. Dispatch may be allowed with detection or suppression systems inoperative in a cargo compartment provided the AFM prohibits the carriage of cargo in the affected compartment.
(f) Smoke Detection/Penetration/Evacuation Tests. Use AC 25- 9A, “Smoke Detection, Penetration, and Evacuation Tests and Related Flight Manual Emergency 99
Procedures,” supplemented by ANM-100 memorandum, dated June 18, 1997, and the referenced video. The supplemental memorandum elaborates on a small smoldering fire for use in detection tests.
(g) If the applicant disagrees with any of the guidance above, then an issue paper can be created identifying the disagreement as an issue and coordinate it with the FAA.
(3) Protection of Critical Equipment in Class E Cargo Compartments. The following guidance is extracted from an FAA Generic Issue Paper regarding protection of critical equipment in Class E cargo compartments.
(a) Statement Of Issue.
1 A specific requirement to protect critical systems and equipment in Class E cargo compartments from fire damage does not exist. Both Class B and Class E cargo compartments are similar in that they do not have fire suppression systems. On Class B compartments, it is assumed that the fire can be controlled manually. On Class E compartments, the fire is controlled by shutting off ventilating airflow. In both types of cargo compartments fires can quickly reach dangerous proportions because no fast-acting suppression system is installed. In a Class E compartment, an uncontrolled fire could damage critical systems and equipment to compromise flight safety before ventilating airflow could be effectively shut off.
For this reason, protection of these critical systems and equipment must be ensured.
(b) Background.
1 The FAA has issued Airworthiness Directives (AD) 91-10-02, which requires certain design, equipment, and operational changes to maximize fire protection on "Combi" Class B cargo compartments. One of these requirements is to ensure appropriate protection of cockpit voice and flight data recorders, windows, primary flight control systems (unless it can be shown that a fire could not cause jamming or loss of control), and other equipment within the compartment that is required for safe flight and landing. If protective covers are used, they must be constructed of materials that meet the flame penetration resistance requirements of § 25, 14 CFR, appendix F, part III [Amendment 25-60].
2 The generic model airplane main deck Class E cargo compartment is similar to the main deck Class B cargo compartment on generic model "Combis." Therefore, similar protection must be afforded to the affected systems on the generic model aircraft.
(c) FAA Position.
1 Section 21.21(b)(2) of 14 CFR requires that no feature or characteristic of an airplane make it unsafe for the category in which certification is requested. Based on the similarities of the generic model aircraft Class E cargo compartment to the generic model aircraft "Combi" Class B cargo compartment, a concern exists that the unsafe conditions identified on "Combis" will also exist on the generic model aircraft.