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Final and approved; this changed AC section is included with AC 27-1B and will 

later be incorporated into the next published change or revision to AC 27-1B. 

CHAPTER 3 

AIRWORTHINESS STANDARDS 

NORMAL CATEGORY ROTORCRAFT 

MISCELLANEOUS GUIDANCE (MG) 

AC 27 MG 6. EMERGENCY MEDICAL SERVICE (EMS) SYSTEMS 

INSTALLATIONS INCLUDING: INTERIOR ARRANGEMENTS, 

EQUIPMENT, HELICOPTER TERRAIN AWARENESS AND 

WARNING SYSTEM (HTAWS), RADIO ALTIMETER, AND FLIGHT 

DATA MONITORING SYSTEM. 

 a. Explanation. This section pertains to EMS configurations and associated 

rotorcraft airworthiness standards. EMS configurations are usually unique interior 

arrangements that are subject to the appropriate airworthiness standards, part 27 or its 

predecessor CAR part 6, to which the rotorcraft was certificated. No relief from the 

standards is intended except by § 21.21(b)(1) or an exemption. EMS configurations are 

seldom, if ever, done by the original manufacturer. 

 (1) The FAA has specified in the operating rules the minimum equipment 

required to operate as a helicopter air ambulance service provider (identified by an “*” in 

this guidance). This equipment, as well as all other equipment presented for evaluation 

and approval, is subject to compliance with airworthiness standards. Any equipment 

not essential to the safe operation of the aircraft may be approved provided the use, 

operation, and possible failure modes of the equipment are not hazardous to the 

aircraft. Safe flight, safe landing, and prompt evacuation of the rotorcraft in the event of 

a minor crash landing, for any reason, are the objectives of the FAA evaluation of 

interiors and equipment unique to EMS. 

 (i) For example, a rotorcraft equipped only for transportation of a 

non-ambulatory person (e.g., a police rotorcraft with one litter) as well as a rotorcraft 

equipped with multiple litters and complete life support systems and two or more 

attendants or medical personnel may be submitted for approval. These configurations 

will be evaluated to the airworthiness standards appropriate to the rotorcraft certification 

basis. 

 (ii) Normal category rotorcraft should comply with flightcrew and passenger 

safety standards, which will result in the need to reevaluate certain features of the basic 

certified rotorcraft related to the EMS arrangement, such as doors and emergency exits, 

and occupant protection. Compliance with airworthiness standards results in placards 

or markings for doors and exits, exit size, exit quantity and location, exit access, safety 

belts, and possibly shoulder harnesses or other restraint or passenger protection means 

to be retained as part of the rotorcraft’s basic type design. These features, including 
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any placards and markings required as part of the rotorcraft’s basic type design, should 

be retained unless specific replacements or alternate designs are necessary for the 

EMS configuration to comply with airworthiness standards. 

 (2) Many EMS configurations of normal rotorcraft are equipped with the following: 

 (i) Attendant and medical personnel seats, which may swivel. 

 (ii) Multiple litters, some of which tilt. 

 (iii) Medical equipment stowage compartments. 

 (iv) Life support and other complex medical equipment. 

 (v) Human infant incubator (isolette). 

 (vi) Curtains or other interior light shielding for the flightcrew station. 

 (vii) External loud speakers and search lights. 

 (viii) Special internal (intercom) and external communication radio equipment. 

 *(ix) Flight data monitoring system (FDMS). 

 *(x) Radio altimeter. 

 *(xi) Helicopter terrain awareness and warning system (HTAWS). 

 *(3) All helicopter air ambulance service providers are required to operate at all 

times under a part 135 subpart L certificate. The equipment required to obtain 

operational approval includes: 

 *(i) FDMS. The installation guidance is in paragraph b.(13) of this MG. 

 *(ii) Radio Altimeter (RAD ALT). The installation guidance is in paragraph 

b.(14) of this MG. 

 *(iii) HTAWS. HTAWS is required for operation under part 135 subpart L, 

Helicopter Air Ambulance Equipment, Operation, and Training Requirements; 

§ 135.605, Helicopter Terrain Awareness and Warning System (HTAWS). The design 

standards are in Technical Standard Order (TSO)-C194 and the installation guidance is 

in AC 27-1 MG 18. 

 b. Procedures. 

 (1) General. 
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 (i) Original type design information and criteria may or may not be available 

from the manufacturer. Availability of this information is dependent on whether the 

information is considered “public” (i.e., non-proprietary) or proprietary. It may be 

appropriate to reference the helicopter manufacturer’s “standard” features, placards, 

and markings in the applicant’s modification design data. 

 (ii) The EMS modification presented for approval usually contains equipment 

of one manufacturer’s model or design. The type design of the modification will have 

features to power and restrain the equipment, maintain the rotorcraft systems integrity, 

and to otherwise protect the occupants. See paragraph b.(17), which refers to 

equipment substitution. 

 (iii) All equipment installations in the helicopter must be approved. EMS 

helicopters typically include operations in which large medical equipment is not installed 

in the helicopter but instead is carried onto the helicopter, as needed, such as isolettes, 

large medical equipment, and other medical items. This equipment is not included as 

part of the rotorcraft type design modification because it is not considered a permanent 

installation in the helicopter. However, carry-on medical equipment must be evaluated 

as to how it affects the safety of the helicopter and its occupants (including the occupant 

in the isolette) while being carried in the helicopter. This carry-on equipment (including 

the isolette) must be properly restrained so it is not a hazard during flight operations. 

Consequently, the means to stow or store the medical carry-on items must be evaluated 

for the appropriate load factors relative to the helicopter such that the means for 

stowage of the carry-on equipment does not fail. For instance, in the case of a carry-on 

isolette, the isolette is typically placed on top of an installed “mount” for the isolette. The 

“mount” is evaluated for carriage and restraining of the isolette. In some cases, an 

isolette may be completely self-contained and include other items, such as oxygen 

bottles required for the isolette occupant, which must also be evaluated for carriage of 

that equipment. 

 (2) Evacuation and Interior Arrangements. Access to the emergency exits or 

doors from any location in the cabin or compartment, access to and use of the exit or 

door opening means or release device, and the unobstructed area of an exit are 

potential problems that should be addressed in the early design stage. Multi-litter 

arrangements may be especially critical for normal category rotorcraft due to their 

limited space. 

 (i) The operation or use of devices for locking the position of swivel seats and 

for rapid installation and removal of litters (isolettes, etc.) should be labeled unless they 

are simple and obvious, and do not require exceptional effort. The design features of 

the device(s) and the seat or litter will influence the extent of information in any label 

necessary to ensure proper and safe installation for routine use and prompt evacuation 

when appropriate or necessary for the interior arrangement. The requirement for labels 

or markings (instructions, etc.) that applies to operation of seat or litter features, release 

devices, etc., is not relieved even if attendants are necessary for an evacuation as 
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discussed in paragraph b.(2)(iii). Placards or instruction cards that contain evacuation 

procedures do not necessarily contain detailed procedures for individual seats, litters, 

and so forth. Release devices that are simple and obvious and do not require 

exceptional effort are recommended. For example, a single central control for litter 

release would be preferred over multiple action release devices. However, seats and 

litters that require multiple actions to release or reposition may be acceptable if properly 

evaluated to determine that, in the event of an emergency landing, there is no 

obstruction or delay if rapid evacuation is necessary. 

 (ii) The passenger compartment should not be partitioned to impede access 

to exits. Exits and doors should be readily accessible. A door or exit is required on 

each side of the rotorcraft. A demonstration or a “walk-through” of appropriate 

evacuation procedures may be necessary to ensure the means and procedures are 

feasible and adequate. Rotorcraft with readily accessible exits, using simple and 

obvious means and procedures for an evacuation, may not require written procedures 

or a demonstration. Placards and durable markings, if necessary, may be sufficient to 

complement the exit markings and instructions required by the standards. 

 (iii) When an evacuation demonstration is determined to be appropriate for 

compliance, 90 seconds should be used as the time interval for evacuation of the 

rotorcraft. Attendants and the flightcrew, trained in the evacuation procedures, may be 

used to remove the litter patients. It is preferable for the patient to remain in the litter; 

however, the patient may be removed from the litter to facilitate rapid evacuation 

through the exit. The patients are not ambulatory during the demonstration. Evacuation 

procedures should be included if isolettes are part of the interior. The demonstration 

may be conducted in daylight with the dark of the night simulated and the rotorcraft in a 

normal attitude with the landing gear extended. For the purpose of the demonstration, 

exits on one side (critical side) should be used. Exits on the opposite side are blocked 

and not accessible for the demonstration. This is representative of a rollover or exits 

blocked due to a fire. 

 (3) Restraint of Occupants and Equipment. The emergency landing conditions 

specified in § 27.561(b) dictate the design load conditions. See AC 27-1, 

sections 27.561 and 27.785 for further information. 

 (i) Whether seated or recumbent, the occupants must be protected from 

serious head injury as prescribed in § 27.785. Swivel seats and tilt litters may be used 

provided they are substantiated for the appropriate loads for the positions selected for 

approval. Placards or markings may be used to ensure proper orientation for flight, 

takeoff and landing, and emergency landing conditions. The seats and litters should be 

listed in the type design data for the configuration. See paragraph b.(17) for 

substitutions. 

 (ii) For recumbent occupants, harnesses, straps, a padded headboard, a 

diaphragm, or safety belts may be used if in compliance with the load requirements of 

§ 27.561(b).and § 27.785(k). Harnesses or straps are recommended. When used, they 
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should prevent the occupant from significant forward motion in order to reduce occupant 

injuries. Infants in isolettes should be similarly protected by padding and containment 

within the isolette and the isolette restrained for the load cases noted in this paragraph. 

If the infant is strapped to a removable platform, there should be proper restraint of the 

platform and infant within the isolette for the load cases noted in this paragraph. Isolette 

materials are also subject to the flammability standards noted in paragraph b.(4). The 

litter(s) and isolette(s) should be listed in the type design data for the EMS 

configuration. 

 (iii) An isolette used for the transport of infants presents a special case, in that 

it may be included as part of an approved EMS configuration or it may be carried on the 

rotorcraft as needed for transport of infants. If the isolette is self-contained and is not 

part of an EMS type design, it may be considered a carry-on item and not part of the 

EMS type design. In these cases, there is typically a means to position the carry-on 

isolette and properly restrain the isolette for transport as part of the EMS design 

configuration. 

 (A) When the isolette is carry-on equipment, the operator must ensure that 

the isolette does not create a safety risk or interfere with aircraft operations. AC 135-14, 

Emergency Medical Services/Helicopter (EMS/H), provides information and guidance to 

air ambulance and EMS/H operators for large carry-on medical equipment such as 

isolettes. AC 135-14 includes the provision that isolettes are to be restrained in an 

appropriate manner and evaluated for specific emergency landing load factors as 

required by earlier amendments of the rotorcraft regulatory requirements. Since 

publication of AC 135-14, the emergency landing load requirements have changed for 

rotorcraft. Consequently, the load factors specified in that AC may not be appropriate 

for some rotorcraft, depending on the rotorcraft certification basis amendment level of 

the airworthiness standards. The minimum load factors should be no less than those 

specified in the certification basis of the rotorcraft transporting the medical equipment, 

such as isolettes. 

 

 (B) A placard indicating that the isolette should be evaluated per the 

guidance contained in AC 135-14 and restrained to the emergency landing load factors 

for rotorcraft occupants per 14 CFR 27.561(b)(3), or the appropriate reference based on 

the certification basis of the rotorcraft, should be placed in close proximity to the isolette 

mount location. 

 (iv) Galleys, medical supplies, and equipment compartments or modules 

should be restrained and the individual compartments should also contain the contents 

for the conditions noted in paragraph b.(3) of this guidance. Durable placards, decals, 

or markings should be used where appropriate to limit the maximum weight of any 

compartment and the whole module. Compartment latches having sufficient strength 

and displacement or engagement should be used to contain the contents for the 

conditions noted in paragraph b.(3) of this guidance. If necessary, a static load test or 

analysis should be employed to ensure the container or compartment remains intact 

and the latch does not disengage for the most critical conditions. Loose or unrestrained 
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contents in an individual compartment, in combination with similar compartments, 

should use a magnification factor with the design conditions. Prudent design and 

location of compartments having heavy, unrestrained (loose) equipment will mitigate the 

potential effects of landing impact loads. 

 (4) Flammability of Materials. 

 (i) Interior materials must meet the flammability standards in § 27.853, 

appropriate to the type design. The standard presently requires compartment materials 

to be at least flame resistant. The wall and ceiling linings, coverings of upholstery, 

floors, and furnishings must be at least flame resistant. 

 (A) Flash-resistant material may be characterized as that not exceeding a 

20-inch-per-minute (horizontal) burn rate. See AC 23-2, Flammability Tests, for further 

information. 

 (B) Flame-resistant material may be characterized as that not exceeding a 

4-inch-per-minute (horizontal) burn rate. 

 (C) Self-extinguishing materials that can meet the transport rotorcraft 

standards of § 29.853 of Amendment 29-17 are recommended for use in normal 

category rotorcraft. 

 (ii) When the isolette is included in the EMS configuration approval, the 

isolette materials are subject to the flammability standards of § 27.853. The current 

standards require that all materials, including transparencies, fabric (e.g., padding, 

covers), straps, etc., be flame-resistant for each compartment used by the crew or 

passengers. AC 23-2 also contains test information about flash and flame-resistant 

material. 

 (5) Exit Signs or Markings and External Markings. Doors and exits must have 

signs and markings (instructions) for prompt evacuation even in darkness. An 

emergency light system is not required by part 27. Refer to the RFM or maintenance 

manual for standard placards, decals, stencils, etc. Alternates may be approved as a 

part of the interior type design. 

 (6) Interior or “Medical” Lights. The view of the flightcrew must be free from glare 

and reflections that could cause interference. Use of a night vision imaging system 

(NVIS) should be a consideration in this evaluation. Curtains that meet the flammability 

standards (flame resistant) may be used. Complete partitioning or separation of the 

flightcrew and passenger area is not prudent. Means for visual and oral communication 

are usually necessary. Refer to AC 27-1 section 27.773, which concerns pilot visibility. 

 (7) Patient Interference. When passengers or patients are located in close 

proximity to the pilot and the primary flight controls of the rotorcraft, a guard or shield 

should be installed, or the patient should be restrained to prevent inadvertent or 
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potential patient interference with safe operation of the rotorcraft. The guard may be a 

part of the rotorcraft interior features. In addition, prompt evacuation should be ensured 

if a guard is used. 

 (8) External Devices. 

 (i) Search lights, loud speakers, baggage pods, etc., may be installed on the 

underside of or elsewhere on the rotorcraft. The strength of the attachments must be 

proven for the flight and landing conditions. The lights and the reflection from the lights 

should not adversely affect pilot view or visibility. Use of NVIS should be a 

consideration in this evaluation. 

 (ii) The device or pod located on the underside of the rotorcraft should not 

contact a level landing surface after “limit landing load” deflection of the landing gear. 

That is, the landing gear should deflect under limit load without causing damage to the 

device. For example, if the gear limit landing load deflection is 8 inches, the device 

would need to have at least an 8-inch ground clearance to avoid contact with the 

landing surface. 

 (iii) The physical characteristics of the rotorcraft landing gear design dictate 

the necessary clearance. The type design owner has this design information. A 

conservative deflection value may be chosen in place of obtaining design information. 

(The limit landing descent velocity specified in § 27.725(a) ranges from 6.5 to 8.3 feet 

per second.) 

 (iv) The device should also be designed and located to preclude penetration 

into a critical area of the fuselage such as fuel cell, fuel line, primary control tube, or 

occupant seat in the event of higher landing impact velocities. 

 (v) A flight evaluation is necessary to determine the effects of the device on 

the rotorcraft flight characteristics and on flight crew visibility. In addition, recent service 

history has shown that external equipment and external fixture modifications can affect 

main rotor mast bending loads. In lieu of a mast bending survey, a pre and post 

modification flight test may be conducted at the same gross weights, center-of-gravity 

(CG), power, and density altitude to compare a critical control position parameter 

(typically longitudinal cyclic stick position) at pre and post modification Vne airspeeds. 

 (A) If required, the post modification Vne should be reduced so that the 

post modification longitudinal cyclic stick position is slightly aft of (or less than) the pre 

modification stick position. This alternative procedure assumes that the static 

longitudinal stability of the helicopter has not been altered by the modification. For 

helicopters with neutral static stability, a more comprehensive investigation may be 

required. 

 (B) In some cases, a control position parameter other than longitudinal 

stick position may be critical. For example, a heavy external device mounted to the side 
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of the helicopter that gives a lateral CG close to the limit and an asymmetric yaw 

component would require pre and post modification lateral cyclic stick and pedal 

position measurements. Operating limitations other than Vne may need to be 

established, or reduced from pre modification limitations, to ensure pre-modification 

mast bending is not exceeded. 

 (9) Miscellaneous. Several paragraphs in this MG contain guidance for the 

standards cited in the Regulatory Sections reference list (paragraph c.(1)). These 

paragraphs should provide insight into designing an EMS configuration that would be 

acceptable under the standards. 

 (10) Oxygen. EMS oxygen installations are supplied by either liquid or gaseous 

oxygen. Both types of systems are discussed in this paragraph. 

 (i) Liquid Oxygen. 

 (A) System General Description. Most liquid oxygen systems in use are 

installed in military aircraft and, as a result, much of this material is based on experience 

with these systems. A rotorcraft liquid oxygen system should be comprised of a liquid 

oxygen converter, tubing, fittings, quantity gage, heat exchangers, and appropriate 

pressure and flow control components as shown in figures AC 27 MG 6-1 and 

AC 27 MG 6-2. The installation may provide for replenishing the liquid oxygen supply 

by use of a quick-removable converter or, in the case of a fixed installation converter, by 

providing external access for connection to a portable service trailer. More complicated 

systems such as those with multiple converter assemblies are not discussed here since 

installation of those systems are not envisioned in rotorcraft at this time. 

 (B) System Components. All components should be aircraft qualified and 

suitable for use in an EMS rotorcraft application. 

 (1) Liquid Oxygen Converter. A liquid oxygen converter assembly is a 

self-powered system for the storage of liquid oxygen and for its conversion to gaseous 

oxygen when required. A principal part of the converter assembly is a vacuum insulated 

container. Pressure relief valves should be provided to allow the escape of gas 

generated when oxygen is not being expended in the supply line. Oxygen losses from a 

converter assembly vary from 5 to 20 percent per 24 hours depending on the size of the 

container, its installation environment, and so forth. Aircraft qualified and approved 

converters suitable for EMS rotorcraft use are available in either 5 or 50-liter capacities. 

Size selection should be determined by flow rate and duration requirements. 

Performance characteristics of each converter size are available from the manufacturer. 

 (2) Shutoff Valve Assembly. This valve must be accessible to a 

flightcrew member and be mounted in the supply line on or as close as possible to the 

outlet of the converter. This valve provides for the confinement of the remaining supply 

of liquid oxygen to the converter in the event of an emergency. Since the system 

pressure is low, the use of an electrically actuated shutoff valve is satisfactory to 
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accomplish this function. In some installations, where the evaporating coil is 

immediately adjacent to the converter, a flow fuse has been used to accomplish this 

function. Use of a flow fuse must be supported by a system fault analysis and testing to 

show maximum normal flow will not result in nuisance trips, and reliable trips will be 

provided for malfunction conditions resulting in excess flow. 

 (3) Filler Valve. Some designs combine this function with the build-up 

and vent valve assembly as shown in figure AC 27 MG 6-2. 

 (4) Build-up and Vent Valve Assembly. This valve is positioned in the 

“vent” position when the system is being filled with oxygen and in the “build-up” position 

at other times. Some designs combine this function with the filler valve as shown in 

figure AC 27 MG 6-2. 

 (5) Pressure Build-up Coil Assembly and Pressure Closing Valve. 

With the build-up and vent valve in the “build-up” position, gas that is formed is allowed 

to apply pressure to the liquid to provide adequate flow through the check valve to the 

evaporating coil assembly. A connection to a pressure relief valve is also provided. 

 (6) Evaporating Coil Assembly. This is provided to convert the liquid 

oxygen into a gaseous form. The evaporating coil assembly should be of sufficient 

capacity to maintain the design flow quantity to the dispensing regulators at a 

temperature within +10 and -20°F of cabin ambient temperature. MIL-D-19326G 

contains a discussion of installation considerations for this unit. 

 (7) Vent Line. Gaseous oxygen escapes through this line. At the 

conclusion of the fill operation, liquid oxygen will flow overboard in a steady stream from 

this line to indicate the container is full of liquid oxygen. The vent line should be located 

to drain overboard at the bottom of the rotorcraft fuselage. Flow from the overboard 

vent should be directed so as not to create a hazard for personnel and not allow liquid 

oxygen to come in contact with the rotorcraft. The vent lines should be insulated to 

prevent frosting and sweating if they pass over equipment that will be harmed by water 

dripping from the lines, or drip pans should be installed under the lines. There should 

be no hydrocarbon fills or drains, forward or above, in proximity to the vent outlet. 

 (8) Regulator. A regulator should be installed in the supply line 

downstream from the heat exchanger. The regulator should reduce the liquid oxygen 

converter operating pressure to a supply pressure of 50 pounds per square inch gauge 

(PSIG) to be compatible with the normal operating pressure of medical oxygen 

equipment. 

 (9) Flow Control Valve. This valve provides a calibrated flow of 

gaseous oxygen from an operating supply of 50 ± 5 PSIG. A valve whose proof 

pressure is specified at 80 PSIG and has a burst pressure rating of 350 PSIG would be 

considered satisfactory. 
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 (10) Check Valve. This valve prevents gaseous oxygen in the supply 

system from backing up into the liquid oxygen in the container and increasing the 

vaporization rate of the liquid oxygen by exposure to the gas. This valve is normally an 

integral part of the liquid oxygen converter assembly. 

 (11) Quantity Indicators. A quantity indicator should be installed at the 

appropriate rotorcraft crew station to permit monitoring of the liquid oxygen supply. The 

indicator when installed in the rotorcraft should indicate the amount of liquid oxygen in 

the converter. Adequate clearance should be provided for the indicator connectors so 

that they can be readily disconnected by servicing personnel. Provisions should be 

made for the storage of the rotorcraft connectors to the liquid oxygen converter when 

they are disconnected. Liquid oxygen quantity indicating equipment is available in three 

types: capacitance gauging, electro-mechanical transducer indication, and differential 

pressure type indication. 

 (12) Pressure Relief Valves. Pressure relief valves are provided to 

vent overboard through the overboard vent system any excess pressures developing 

within the system. 

 (13) Lines. Lines should be either solid tubing or flexible hoses. 

Examples of acceptable solid tubing are aluminum alloy conforming to AMS 4071 or 

corrosion resistant annealed steel (304) confirming to MIL-T-8506. Flexible hoses 

should be used for rotorcraft system connections to removable converters and to other 

applications where relative movement may occur. Flexible hoses should be wire-braidcovered bellows or wire-braid-covered tetrafluoroethylene. Flexible hoses conforming 

to MS90457 or MS24548 are satisfactory. MS90457 hose is flexible to -297°F (-183°C), 

and MS24548 hose is flexible to -65°F (-54°C). Synthetic lines such as plastic, nylon, or 

rubber should not be used for lines subjected to continuous pressure, or for application 

where the line will not be visible. Lines that are not visible are those that are located 

behind liners or in the walls of the fuselage. 

 (14) Fittings. If in contact, dissimilar metals should be suitably 

protected against electrolytic corrosion. Line assemblies should be terminated with “B” 

nuts or a similar manufactured terminating connection. Universal adapters (AN 807) or 

friction nipples used in conjunction with hose clamps should be avoided in pressurized 

systems. 

 (15) Drain Valve. Systems that have permanently installed containers 

should include a drain valve located to allow for complete draining of the liquid oxygen 

container. An acceptable drain valve would be one in accordance with MK-V-25962 

that is suitably capped. A cap in accordance with AN 929-5 with a permanently 

attached chain is a suitable cap. 

 (16) Low Pressure, Low Level Warning System. It is recommended 

that provisions be included in the system to alert the appropriate aircraft crew member 

that the level of the oxygen supply has reached some low level. It is recommended that 
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low level be actuated when less than 10 percent of the full container capacity is 

available. If low system pressure is also monitored, the low pressure valve selected 

should be such that any drop in supply line pressure upon inhalation should not activate 

the low pressure warning function. 

 (C) Component Installation. The following are typical installation 

considerations that should be addressed when designing the oxygen system. 

 (1) Location. The oxygen equipment, lines, and fittings should be 

located as remotely as practicable from sources of flammable fluids, high heat and 

electrical items, fuel, oil, hydraulic fluid, batteries, exhaust stacks, manifolds, and so 

forth. Oxygen lines should not be grouped with lines carrying flammable fluids. If 

possible, converters should not be in line with the plane of rotation of a turbine. System 

components should not be installed in an environment that will exceed the temperature 

limit of the component, and no part of the system should be installed in an area that will 

exceed 350°F (176°C). To minimize loss due to heat, the liquid oxygen converter 

should not be located near equipment that dissipates a high quantity of heat. 

 (2) Converter Mounting. The oxygen container should be readily 

accessible to servicing personnel. If the container is not removable for servicing, the 

filler should be external to the aircraft with adequate contamination protection. 

Mounting provisions for the converter and plumbing to the evaporating coil assembly 

should include a drain pan with an overboard drain. 

 (3) Flexible Hoses. Hoses should be of sufficient length to provide 

unstressed connections and be protected against chafing on surfaces or objects that 

may damage the wire covering. The bend radius imposed on the hoses during 

installation and replacement should not be less than the minimum established by the 

hose specifications. 

 (4) Lubricants. No lubricants should be used on liquid oxygen pipe 

fittings. MIL-T-27730 Teflon tape may be used on male pipe fittings when required. 

Teflon tape should not be used on flared tube fittings, straight threads, coupling sleeves, 

or on the outer side of tube flares. None of the tape should be allowed to enter the 

inside of a fitting. Krytox fluorinated grease by E.I. Dupont De Nemours and Company, 

or an equivalent, may be used sparingly on seals. 

 (5) Tubing Routing and Mounting. There should be at least 2 inches of 

clearance between the oxygen system and flexible moving parts of the rotorcraft. There 

should be at least a 1⁄2-inch clearance between the oxygen system and rigid parts of the 

rotorcraft. The oxygen system tubing, fittings, and equipment should be separated at 

least 6 inches from all electrical wiring, heat conduits, and heat emitting equipment in 

the rotorcraft. Insulation should be provided on adjacent hot ducts, conduits, or 

equipment to prevent heating of the oxygen system. In routing the tubing, the general 

policy should be to keep total length to a minimum. Allow for expansion, contraction, 

vibration, and component replacement. All tubing should be mounted to prevent 
AC 27-1B Change 2/27/2014 

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vibration and chafing. This should be accomplished by the proper use of rubberized or 

cushion clips installed at 24-inch intervals (copper) or 36-inch intervals (aluminum) and 

as close to the bends as possible. The tubing, where passing through or supported by 

the rotorcraft structure, should have adequate protection against chafing by the use of 

flexible grommets or clips. The tubing should not strike against the rotorcraft structure 

during vibration and shock encountered during normal use of the rotorcraft. 

 (6) System Marking. The rotorcraft should be permanently and legibly 

marked, as applicable, in the locations specified below (a minimum letter height of 

1⁄4-inch is recommended): 

 (i) Adjacent to the overboard vent opening: 

CAUTION 

LIQUID OXYGEN VENT 

 (ii) On outside surface of filler box cover plate: 

LIQUID OXYGEN (BREATHING) FILL ACCESS 

 (iii) On underside surface of filler box cover plate: 

CAUTION - KEEP CLEAN, DRY, AND FREE FROM OILS 

 (iv) In prominent place when filler box is open, preferably near liquid 

oxygen drain valve: 

DO NOT OPEN DRAIN VALVE UNTIL DRAIN HOSE 

AND DRAIN TANK ARE CONNECTED 

 (v) Other placards, such as one at the converter cautioning about 

the presence of liquid oxygen, may also be appropriate. 

 (7) Other installation criteria are given in Chapter 6 of AC 43.13-2, 

Acceptable Methods, Techniques, and Practices-Aircraft Alterations, and should be 

given consideration. 

 (D) Precautions. The referenced Society of Automotive Engineers (SAE) 

report contains precautions peculiar to a liquid oxygen installation, and this material 

should be reviewed. It should also be emphasized that liquid oxygen equipment and 

the aircraft being serviced must be electrically grounded during servicing to prevent an 

accumulation of static electricity and discharge. The following considerations are 

included for special emphasis: 
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 (1) System Cleanliness. The completed installation should be free of 

oil, grease, fuels, water, dust, dirt, objectionable odors, or any other foreign matter, both 

internally and externally prior to introducing oxygen in the system. 

 (2) Closures. Lines that need to be disconnected during rotorcraft 

maintenance checks or overhaul, due to the location of the converter within the 

rotorcraft, should be capped to prevent materials that are incompatible with oxygen from 

entering the system when the system integrity is broken. Caps that introduce moisture 

and tapes that leave adhesive deposits should not be used for these purposes. All 

openings of lines and fittings should be kept securely capped until closed within the 

installation. 

 (3) Degreasing. All components of the oxygen system should be 

procured for oxygen service use in an “oxygen clean” condition. Parts of the oxygen 

system, such as tubing, not specifically covered by cleaning procedures should be 

degreased using a vapor phase trichloroethane degreaser. Ultrasonics may be used in 

conjunction with vapor phase degreasing for the cleaning of components. 

 (4) Purging. The system should be purged with hot, dry 99.5 percent 

pure oxygen gas in accordance with the manufacturers recommendations after: 

 (i) Initial assembly of the oxygen system; and 

 (ii) After system closure whenever the oxygen system pressures 

have been depleted to zero, or the system has been left open to atmospheric conditions 

for a period of time or is opened for repairs. 

 (5) Maintenance and Replacement. All parts of the oxygen system 

should be installed to permit ready removal and replacement without the use of special 

tools. All tubing connections and fittings should be readily accessible for leak testing 

with a leak test compound formulated for leak testing oxygen systems and for tightening 

of fittings without removing surrounding parts. 

 (ii) Gaseous Oxygen. 

 (A) General. This guidance is intended to supplement the existing 

guidance in AC 43.13-2, Chapter 6. If there are any differences within the two ACs, this 

guidance prevails since it pertains specifically to part 27 requirements. 

 (B) System Components. 

 (1) High Pressure Cylinders. Many installations utilize hospital type 

cylinders rather than aviation type cylinders. A concern with the hospital type cylinders 

is the yoke and the hard plastic washer that is commonly used with these cylinders. It is 

very difficult to properly attach these yokes since the rotorcraft provides a high vibration 

environment and no positive lock is provided. Leaks are a continuous problem with this 
AC 27-1B Change 2/27/2014 

Page MG 6 - 14 

configuration. Yokes are available for these bottles that provide for a positive lock. 

Improved washers that provide for a good elastometric seal and include a metal ring to 

limit crushing the washer are also available. If the hospital type bottles are to be used, 

only the modified yokes and improved seals should be considered for future 

installations. The preferred cylinder is the aviation type cylinder with the integral shutoff valve and regulator. All cylinders should be DOT approved. 

 (2) Lines. 

 (i) General. Any lines that pass through potential fire zones should 

be stainless steel. 

 (ii) High Pressure. Use of high pressure lines may be necessitated 

by the use of a pressure regulator that is remote from the cylinder. The intent is to 

locate the regulator as close as physically possible to the cylinder, and to minimize the 

use of fittings. Lines of 6-inch lengths are encouraged with 18-inch lengths being the 

maximum in unusual circumstances. Lines made of stainless steel are recommended. 

 (iii) Low Pressure. Although lines may only be subjected to low 

pressures, if they are located behind upholstery or for any reason are not 100 percent 

visible during normal operation, they should be solid metal lines or high pressure flexible 

lines that conform to SAE 100R14A specifications for stainless braided hoses. Other 

oxygen lines, so called “green lines,” should only be used in locations that are 

100 percent visible during normal operation. This would restrict their use to the run 

between the mask and the bulkhead disconnect in the aircraft cabin. Synthetic lines 

such as plastic, nylon, or rubber are not recommended for applications that will be 

exposed to continuous pressure (i.e., as opposed to pressurized when needed). These 

materials can cold flow. 

 (3) Fittings. 

 (i) High Pressure. Intercylinder connections are made with regular 

flared or flareless tube fittings with stainless steel. Usually fittings are of the same 

material as the lines. Mild steel or aluminum alloy fittings with stainless steel lines are 

discouraged. Titanium fittings should never be used because of a possible chemical 

reaction and resulting fire. 

 (ii) Low Pressure. Fittings for metallic low pressure lines are flared 

or flareless, similar to high pressure lines. Line assemblies should be terminated with 

“B” nuts in a similar manner to a manufactured terminating connection. Universal 

adapters (AN 807) or friction nipples used in conjunction with hose clamps are not 

accepted for use in pressurized oxygen systems. 

 (4) Shut-off Valve. Each system should contain a shutoff valve that is 

located as close as practical to the high pressure cylinder(s), and it should be 

assessable to a flightcrew member. High pressure cylinders should use slow opening 
2/27/2014 Change AC 27-1B 

Page MG 6 - 15 

and closing system shut-off valves. Where the regulator is part of the cylinder, and low 

pressure oxygen is controlled, the emphasis on slow acting valves is not as significant, 

and use of a flow fuse may be possible. Use of a flow fuse must be supported by a 

system fault analysis and testing to show maximum flow will not result in nuisance trips, 

and reliable trips will be provided for malfunction conditions resulting in excess flow. 

 (5) Regulators. The regulator should be mounted as close as possible 

to the cylinders (see paragraph b.(10)(i)(B)(8) of this guidance). If non-aviation qualified 

regulators are considered, their service history should be reviewed and careful 

consideration given to the manufacturer’s environmental qualification. Radio Technical 

Commission for Aeronautics Document D0-160 is a recognized and accepted standard 

for environmental considerations. As a minimum, consideration should be given to 

operation during altitude, temperature, and vibration extremes. 

 (6) Placards. Appropriate, durable placards should be provided with 

the installed system. Emphasis should be placed on any precautions that are 

appropriate during filling of the system and so forth. 

 (7) Filler Connections. When a filler connection is provided, it is 

recommended it be located outside the fuselage skin or isolated in a manner that would 

prevent leaking oxygen from entering the rotorcraft. Careful evaluation should also be 

made of any nearby sources of fuel, oil, or hydraulic fluid under normal or malfunction 

conditions. Each filler connection should be placarded. In addition, any valve (on 

aircraft or ground servicing equipment) associated with high pressure should be slow 

acting. 

 (C) “Provisions Only” Considerations. In some instances, systems are 

approved that only include provisions for a supply system consisting of the high 

pressure cylinders, regulators, and their associated lines and fittings. In these 

instances, a placard should be provided that refers to a supply system that is 

considered satisfactory for the remainder of the installation. An example of an 

acceptable placard for this situation is: 

Oxygen Supply System must be in accordance with the requirements 

given in STC SH _________. Deviations to the configuration specified 

must be evaluated and approved by the Manager (include reference to the 

appropriate FAA ACO). 

 (11) Medical Communication Equipment. This equipment is provided to allow for 

communication between the rotorcraft and ground medical personnel. It includes voice 

communication and may also include telemetry equipment for the transmission of 

graphic data. It should be demonstrated that this equipment functions properly and the 

range at which this determination was made recorded in the project file. The functional 

demonstration should include a 360° turn (clockwise and counterclockwise) to ensure 

no significant sections of signal blanking exist. The remainder of the emphasis on this 

equipment should be to ensure that operation of this equipment does not interfere with 
AC 27-1B Change 2/27/2014 

Page MG 6 - 16 

normal operation of any rotorcraft systems whose installation is required for safe 

operation of the rotorcraft. 

 (12) Cabin Lighting. EMS interiors normally include higher intensity cabin lighting 

than other interiors. This lighting capability should be carefully evaluated to ensure it 

does not interfere with operation of the rotorcraft. In some installations, a special 

curtain is required to separate the cockpit from any interference by the lighting. The 

FAA approved data should document the approach of how this evaluation was 

conducted. See paragraph b.(6) for other curtain considerations. 

 *(13) FDMS. If required under an operating regulation, an FDMS (not to be 

confused with a flight data recorder (FDR) certificated under § 27.1459) may be 

comprised of a system or combination of systems that record a helicopter’s flight 

performance and operational data. An FDR certificated under § 27.1459 and the 

appropriate operating rules would be acceptable to meet this requirement; however, an 

FDMS would not be adequate to meet the § 27.1459 requirement for an FDR. The 

FDMS should record digital or analog raw data, images, cockpit voice or ambient audio 

recordings, or any combinations thereof, according to a broadly defined set of 

parameters including information pertaining to the aircraft’s state, condition, and system 

performance. This data can be used to perform post flight analysis and provide critical 

information to investigators in the event of an incident or accident as well as to promote 

operational safety. When used in conjunction with an FAA-approved flight operations 

quality assurance (FOQA) program, part 135 certificate holders would be required to 

collect flight performance and operational data that characterizes the state of the 

helicopter and its subsystems that the certificate holder determines is pertinent to its 

safety program. FDMS data should be recorded and stored on digital media, and when 

selecting a location to install the hardware device used for storing the data, 

consideration should be given to the potential for survival in the event of a crash. The 

system should receive electrical power from the helicopter’s bus that provides the 

maximum reliability without jeopardizing service to essential or emergency loads, and 

capable of being operated continuously from the time power is applied to the aircraft 

until power is removed from the aircraft.

 (i) Safety. The FDMS equipment should not, under normal or fault conditions, 

adversely affect the airworthiness of the systems to which it is interfaced or of other 

aircraft systems. The equipment should be installed in accordance with all applicable 

safety regulations. The equipment should be tested under the standards of RTCA 

DO-160F, "Environmental Conditions and Test Procedures for Airborne Equipment," or 

subsequent issue. The European Organization for Civil Aviation Equipment 

(EUROCAE) specification ED-14 may be used in lieu of RTCA DO-160. Additional 

crashworthiness testing may be conducted according to EUROCAE specification 

ED-155, “Minimum Operational Performance Specification for Lightweight Flight 

Recording Systems.” Equipment testing should be conducted to the categories most 

applicable to the aircraft type, and the location of the equipment to be installed. The 

equipment manufacturer typically defines the test class within each environmental 

category. The objective of this level of testing is to ensure that the equipment does not 
2/27/2014 Change AC 27-1B 

Page MG 6 - 17 

present a hazard to the aircraft, and can survive and continue to operate under the 

environmental conditions to which it will be subjected throughout its life. Specific testing 

may be required to demonstrate that the equipment performs its intended function when 

operated over the full environmental conditions to which it has been declared to comply. 

Consideration should be given to the extremes at which it may be subjected during an 

incident or accident. These tests can be undertaken during the specified RTCA DO160F (or later revision) tests or separately. Analysis may be substituted for a test where 

its use can be shown to produce equivalent evidence of compliance. The system 

should be capable of recording up to 2 hours of image or acoustical data and 6 hours of 

aircraft parameter data. The applicant determines and maintains the data stream 

format and parameter documentation, including which parameters are recorded, how 

often the parameters are recorded, the bit resolution of each parameter, the operational 

range of each parameter, and the conversion algorithm from decimal units to 

engineering units. The Design Assurance Level (DAL) for an FDMS that is required by 

an operating regulation is DAL “D.” RTCA DO-178B (or later revision) provides 

acceptable software development standards, which in this case would be for DAL “D” 

software. RTCA DO-254 (or later revision) provides acceptable airborne electronic 

hardware (AEH) development standards, which in this case would be for DAL “D” AEH. 

Note: The duration between data downloads for the promotion of operational safety 

within a FOQA program is directly correlated to the recording capabilities of the 

system installed. 

 (ii) Recording. The FDMS should be capable of capturing and recording any 

combination of the following parameters in order to monitor the aircraft’s state, 

condition, and system performance: 

 Positioning system time. 

 Positioning system latitude. 

 Positioning system longitude. 

 Positioning system altitude. 

 Positioning system error. 

 Altitude. 

 Heading. 

 Pitch attitude. 

 Pitch rate. 

 Roll attitude. 

 Roll rate. 

 Yaw rate. 

 Air speed. 

 Ground speed. 

 Ambient acoustic data. 

 Engine parameters. 

 Main rotor revolutions per minute. 

 Transmission ambient audio. 
AC 27-1B Change 2/27/2014 

Page MG 6 - 18 

 Any other parameters deemed appropriate by the operator. 

Notes: Parameters may be recorded directly or deduced from recorded data from the 

FDMS. Additional guidance on parameters can be found in EUROCAE 

specification ED-155. 

 Recording individual pilots, using hot microphones, on separate pilot audio 

channels can provide useful information in the investigation of incidents and 

accidents. 

 (iii) Maintenance. The maintenance requirements to ensure the serviceability 

and continued airworthiness of the FDMS are typically established by the equipment 

manufacturer and installer. These maintenance instructions should be included in the 

applicable helicopter model instructions for continued airworthiness. 

 *(14) Radio Altimeter (RAD ALT). RAD ALTs installation is required. Its 

information display must be in the pilot’s primary field of view in all helicopters operating 

under a part 135 certificate. The minimum performance requirements for an FAA 

approved RAD ALT system can be found in TSO-C87. 

 (15) Other EMS Equipment. These items of equipment installed for the EMS 

mission are considered optional equipment and should be operated to ensure they 

function properly. This evaluation would normally be done by someone knowledgeable 

about the particular type of equipment, since correct operation of the equipment is 

essential to a valid determination that the required rotorcraft systems are not being 

interfered with. This includes all removable pieces of medical equipment that are used 

for patient care. The primary purpose of the evaluation of this equipment is to 

emphasize the possibility of any interference between operation of the EMS equipment 

and the systems whose installation is required for safe operation of the aircraft, the 

adequacy of the installation provisions, and assurance that failure modes will not result 

in a hazardous condition for the rotorcraft. 

 (16) Miscellaneous. The following areas are not peculiar to EMS installations; 

however, their significance is enhanced by the complexity of an EMS installation. 

 (i) Compatibility. Many EMS installations are a collection of several STCs and 

may also include some FAA field approvals. For this situation, it should be shown that 

the overall installation provides for safe operation of the aircraft. Operation of a search 

light, if included, should be addressed since in using this system it can be difficult to 

keep light from interfering with the pilot view. 

 (ii) Electrical Load Analysis. An electrical load analysis should be conducted, 

and additional guidance is available in AC 27-1 MG 1. If the analysis indicates the 

generator(s) can be overloaded, appropriate measures should be taken to account for 

the problem. In some instances (e.g., in a visual flight rules (VFR) approved rotorcraft), 

a placard that specifies certain operating limitations may be satisfactory, while in other 
2/27/2014 Change AC 27-1B 

Page MG 6 - 19 

instances (e.g., in an instrument flight rules (IFR) approved rotorcraft), an electrical 

interlock may be in order. In general, if the amount of overload is relatively small and 

the rotorcraft is not an IFR-approved rotorcraft, the placard solution will probably be 

satisfactory, whereas if the amount of possible overload is significant, it is more likely 

that an interlock scheme will be necessary. 

 (iii) Aircraft Grounding. It should be emphasized in an appropriate place in 

the STC data (e.g., RFM, maintenance information) that any time the EMS systems are 

being operated or serviced (e.g., oxygen) on the ground, the rotorcraft itself must be 

grounded. 

 (iv) Electrical Outlets. All electrical outlets provided in the cabin should be the 

three-prong grounded type. When not in use, these outlets should be suitably protected 

against the entry of fluids. 

 (v) Placards. All medical outlets (e.g., air, oxygen, vacuum) should be 

placarded. Electrical power outlets should be placarded for type of voltage and 

amperage capacity. A placard stating “No Smoking When Oxygen Is In Use” should be 

included. Other placards would include information appropriate to the oxygen system, 

operation of special controls, and so forth. 

 (vi) Equipment in Cargo and Baggage Compartments. When components are 

added to the baggage compartments, provisions should be made to protect the system 

components due to shifting cargo. In addition, when oxygen components are installed, 

the compartment should be placarded against the storage of oil or hydrocarbons. A 

smoke detector is recommended for a compartment if oxygen cylinders are installed in a 

closed, non-accessible compartment. Also, the cargo weight limitations placard should 

be changed. AC 27-1, section 27.787 pertains to cargo and baggage compartments. 

 (vii) Safety Assessment. When installing any new equipment or modifying 

existing equipment, a safety assessment must be made to assure the FAA that all 

possible failure conditions that could occur from these changes have been adequately 

addressed to show compliance to the regulations. 

 (17) Equipment Substitution. The EMS modification that is presented for 

approval will contain specific items of equipment, and the approval will make reference 

to this equipment. If other equipment (e.g., new model, manufacturer) is to be 

substituted, then an evaluation should be made to ensure the substitute equipment is 

also satisfactory. This evaluation would normally consist of comparing the attachment 

means, design features, failure modes, specifications, and operation of the two units. 

The purpose of the evaluation is to ensure there are no differences that have an 

adverse effect on the airworthiness of the installation. Other differences would not be 

considered significant. Specific seats and litters are generally approved as a part of the 

EMS configuration. Substitution may be approved in accordance with the standards. 

 c. Related Regulations and References. 
AC 27-1B Change 2/27/2014 

Page MG 6 - 20 

 (1) Regulatory Sections. 14 CFR 27.337, 27.471, 27.561, 27.773, 27.783, 

27.785, 27.807, 27.831, 27.853, 27.1301, 27.1309, 27.1353, 27.1357, 27.1365, 

27.1367, 27.1411, 27.1413, 27.1431, 27.1557(d), 27.1561, 27.1581(a)(2), 27.1583(d), 

27.1585, 27.1589, part 91, and part 135. 

 (2) Other References. Refer to the current version of each document. 

 (i) Helicopter Association International, Emergency Medical Services 

Recommended Guidelines. 

 (ii) National Highway Traffic Safety Administration, Air Ambulance Guidelines. 

 (iii) AC 23-2, Flammability Tests. 

 (iv) AC 43.13-2, Acceptable Methods, Techniques, and Practices-Aircraft 

Alterations. 

 (v) AC 135-14, Emergency Medical Services/Helicopter (EMS/H). 

 (vi) Oxygen Equipment for Aircraft, Society of Automotive Engineers 

Aerospace Information Report No. 825. 

 (vii) MIL-D-19326G, Design and Installation of Liquid Oxygen Systems in 

Aircraft, General Specification for Military Specification. 
2/27/2014 Change AC 27-1B 

Page MG 6 - 21 

FIGURE AC 27.MG 6-1 TYPICAL LIQUID OXYGEN SYSTEM 
AC 27-1B Change 2/27/2014 

Page MG 6 - 22 

FIGURE AC 27.MG 6-2 TYPICAL LIQUID OXYGEN SYSTEM - USING 

 COMBINATION VALVE