Special Conditions: Jetstream Aircraft Limited Model 4100 Series Airplanes, Main Cabin Aisle Arrangement

DEPARTMENT OF TRANSPORTATION

Federal Aviation Administration

14 CFR Part 25

[Docket No. NM-53; Special Conditions No. 25-ANM-45A]

Special Conditions: Jetstream Aircraft Limited Model 4100 Series

Airplanes, Main Cabin Aisle Arrangement

AGENCY: Federal Aviation Administration, DOT.

ACTION: Amended special conditions.

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SUMMARY: These amended special conditions are issued to Jetstream

Aircraft Limited for the Jetstream Model 4100 series airplanes. This

airplane has a novel or unusual design feature associated with the main

cabin aisle arrangement. Special Conditions No. 25-ANM-45 were issued

on July 9, 1991 (to British Aerospace Public Limited Company (BAe)),

addressing this unusual arrangement, and included a limitation on

passenger capacity. The applicant has requested that Special Conditions

No. 25-ANM-45 be amended to increase the passenger capacity limit from

29 to 30 passengers. Since Special Conditions No. 25-ANM-45 were

issued, the applicant has changed the company name and revised the

model number of the airplane. These amended special conditions reflect

those changes. Since the applicable airworthiness regulations do not

contain adequate or appropriate safety standards for this particular

design feature, these amended special conditions contain the additional

safety standards which the Administrator finds necessary to establish a

level of safety equivalent to that established by the airworthiness

standards for transport category airplanes.

EFFECTIVE DATE: November 30, 1994.

SUPPLEMENTARY INFORMATION: On May 24, 1989, BAe (currently Jetstream

Aircraft Ltd.) applied for a type certificate for the BAe Model 4100

(currently Jetstream Model 4101) airplane in the transport airplane

category. The Model 4100 was to be a derivative of the Model 3100,

which is a small airplane and is certificated under the provisions of

part 23. Like the Model 3100, the Model 4100 was a low wing, twin

engine turbo-prop design. The fuselage was stretched, however, to seat

29 passengers in a three-abreast arrangement. Since the Model 4100

airplane had the same fuselage cross section as the Model 3100, it did

not have a traditional main cabin aisle arrangement.

Section 25.815 specifies the minimum aisle width dimensions for

transport category airplanes based on the passenger seating capacity.

For airplanes with 20 or more passenger seats, a minimum 15-inch width

at heights 25 inches or less above the main aisle floor and a minimum

20-inch width at heights greater than 25 inches above the floor must be

maintained. Aisle width is measured at any point along the aisle,

normal to the centerline of the aisle. The main aisle envisioned by the

regulations would run in a straight line from one end of the passenger

cabin to the other and would satisfy these width criteria. Long-

standing FAA policy has permitted slight deviation from a straight line

where there is a transition from one cabin section to another, or where

there are interior features which dictate that the aisle move

laterally. For example, from tourist class to first class there may be

a change from five-abreast seating to four-abreast seating which moves

the aisle centerline laterally. This has been accepted provided the

required widths are maintained at all heights normal to the path that

an individual would take. This type of offset normally occurs at one or

two points in a main cabin aisle. In addition, there is no offset

permitted in the aisle vertically; that is, the required 15-inch

dimension must lie completely below the projected 20-inch dimension at

all points along the aisle.

The Jetstream main cabin aisle arrangement utilizes an offset at

each seat row. The left and right seat assemblies are offset from one

another longitudinally such that the seatbacks are not opposite each

other across the aisle. This arrangement permits a 20-inch measurement

between seatbacks (at an angle to the airplane centerline) and the

required 15-inch dimension is maintained within the projected 20-inch

dimension vertically. Thus the ``required aisle'' is not a straight

line from one end of the cabin to the other, but a series of

alternating angular segments from seatback to seatback. Nonetheless,

there is a straight path along the cabin length, and the aisle floor

does not deviate from side to side at all. The projected aisle width

along this straight path reaches 20 inches at a height of 43 inches

above the floor, which is the typical seatback height for transport

airplanes. Another feature of the design includes the use of overwing

exits which are offset longitudinally, corresponding to the seat

positions on the left and right of the airplane.

In order to provide design standards for the novel and unusual

aisle configuration in BAe Model 4100 airplanes the FAA issued Special

Conditions No. 25-ANM-45 on July 9, 1991. The criteria used to develop

Special Conditions No. 25-ANM-45 utilized existing regulations, and

made certain favorable design features in the Model 4100 mandatory.

Special Conditions No. 25-ANM-45 are summarized as follows:

a. There are no more than 29 passenger seats with no more than ten

seat rows on either side of the aisle;

b. The interior arrangement includes one pair of Type II and one

pair of Type III passenger emergency exits;

c. In addition to the requirements of Sec. 25.815, the projected

aisle width is at least 20 inches measured at a height of 43 inches

above the aisle floor;

d. The Type III exits and adjacent seat rows are offset

longitudinally such that persons approaching an exit from one end of

the cabin may use the exit without interfering with those approaching

the other exit from the other end of the cabin;

e. The location and part number of each passenger seat must be

defined by a drawing approved by the FAA or foreign civil airworthiness

authority. The seat arrangement may not be reconfigured without FAA

approval; and

f. An evacuation demonstration must be conducted to demonstrate the

efficacy of the aisle arrangement.

The content of Special Conditions No. 25-ANM-45 is based, in part,

on the maximum passenger capacity proposed by BAe. The specific

limitation of 29 passenger seats was the maximum capacity requested by

BAe, but the FAA considered that the fact that this capacity was well

below the maximum that could be allowed for the installed exit

combination was a significant factor in acceptance of the aisle

arrangement.

Subsequent to the issuance of Special Conditions No. 25-ANM-45 and

prior to issuance of the Type Certificate for that project, British

Aerospace Public Limited Company changed its name to Jetstream Aircraft

Limited. Also, the specific model of the 4100 series in question was

identified as the Jetstream Model 4101. Type Certificate A41NM was

issued to Jetstream Aircraft Limited on April 9, 1993, for the

Jetstream Model 4101 airplane. It is now anticipated that additional

models of the 4100 series may be added to this Type Certificate in the

future. In addition to the Model 4101, Special Conditions No. 25-ANM-45

would be applicable to any other Jetstream Model 4100 series airplanes

that incorporate the same novel or unusual aisle configuration.

Type Certification Basis

Under the provisions of Sec. 21.101, Jetstream must show that 30

passenger Model 4100 series airplanes comply with the regulations in

the type certification basis established for the 29 passenger Jetstream

Model 4101 airplane. The type certification basis for the Model 4101 is

as follows:

The certification basis for the Model 4101 is established in

accordance with Sec. 21.29, using the Joint Aviation Regulations (JAR)

as a reference point, with appropriate additional requirements

incorporated to provide an equivalent certification basis to the FAR

requirements for U.S. certification. British Aerospace and Civil

Aviation Authorities (CAA)-UK elected to have the type certification

basis be the following:

--JAR 25 as amended through Change 12 dated May 10, 1988;

--JAR 25 Orange Paper Amendment 88-1 effective October 18, 1988;

--JAR 1, definitions as amended through Change 4 dated June 1, 1987;

and

--any additional part 25 paragraphs necessary to provide a composite

type certification basis equal to the required part 25 type

certification basis.

Based on Secs. 21.29 and 21.17 and the type certification

application date, the applicable U.S. type certification basis was

established as follows:

--Part 25 dated February 1, 1965, as amended by Amendments 25-1 through

25-66;

--any applicable special conditions issued;

--any applicable exemptions granted;

--any equivalent safety findings made;

--the fuel venting requirements of Special Federal Aviation Regulation

No. 27, including Amendments 27-1 through the latest amendment in

effect on the date of Model 4100 is type certificated, and

--Part 36, including Amendments 36-1 through the latest amendment in

effect on the date of TC.

If the Administrator finds that the applicable airworthiness

regulations (i.e., part 25 as amended) do not contain adequate or

appropriate safety standards for Jetstream Model 4100 series airplanes

because of a novel or unusual design feature, special conditions are

prescribed under the provisions of Sec. 21.16 to establish a level of

safety equivalent to that established in the regulations.

Special conditions, as appropriate, are issued in accordance with

Sec. 11.49 after public notice, as required by Secs. 11.28 and

11.29(b), and become part of the type certification basis in accordance

with Sec. 21.17(a)(2).

Special conditions are initially applicable to the model for which

they are issued. Should the type certificate for that model be amended

later to include any other model that incorporates the same novel or

unusual design feature, or should any other model already included on

the same type certificate be modified to incorporate the same novel or

unusual design feature, the special conditions would also apply to the

other model under the provisions of Sec. 21.101(a)(1).

Novel or Unusual Design Features

The 30 passenger seat version of Jetstream Model 4100 series

airplanes incorporate the same novel or unusual aisle configuration as

described above for the 29 seat version.

As in the case of the 29 seat version, the required main cabin

aisle widths would be established using measurements taken between

seatbacks, which form a path in angular segments. The aisle width

measured normal to the fuselage centerline above 25 inches from the

floor, would extend from the seat back to the opposite cabin sidewall.

Thus, while the required 15-inch dimension at cabin heights below 25

inches from the floor would be completely within the projected 20-inch

width vertically, the 20-inch portion would not be centered over the

15-inch portion. (See Figure 1.)

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In establishing Special Conditions No. 25-ANM-45, the FAA carefully

evaluated the relevant design parameters of the BAe Model 4100 and

determined that the main aisle configuration is clearly not what was

envisioned by the regulations. The regulations do not specifically

prohibit the arrangement proposed; however, the policy in effect is

predicated on a largely straight aisle which has only one or two

lateral deviations; in other words, a traditional cabin arrangement.

Therefore, special conditions were developed to establish design

criteria which result in a level of safety equivalent to configurations

on which the regulations were based.

Jetstream has requested that the FAA amend Special Conditions No.

25-ANM-45 to allow a maximum passenger seating capacity of 30, to

account for a new customer demand, and be responsive to the market. In

their request Jetstream contends that there is no technical basis for

the limitation on passenger capacity, and that the Joint Aviation

Authorities (JAA) have, in fact, already granted such approval.

Jetstream has proposed that on the basis of harmonization with the JAA,

the FAA should consider their request favorably.

Jetstream further contends that the evacuation demonstration

conducted in accordance with the special conditions for the 29

passenger Model 4101 illustrated that there is no impediment due to the

aisle arrangement for 30 passengers. Jetstream argues that the concern

that prompted the requirement was therefore demonstrated not to exist.

In developing the special conditions, the FAA took account of the

several favorable cabin safety features that the 29 passenger Model

4101 incorporates. One of the major features was the good exit-to-

passenger ratio for the number of passengers requested. That is, one

pair of Type II exists and one pair of Type III exists for 29 passenger

seats, where the regulations allow up to 39. In this regard, 29

passenger seats was the largest arrangement proposed for a US customer.

Consequently, the FAA did not consider a greater passenger seating

capacity, and the issue of harmonization with JAA requirements did not

come up.

With respect to the evacuation demonstration conducted in

accordance with the special conditions, this demonstration was a

modified version of the demonstration specified in part 25, Appendix J.

The principal modification was the use of a single exit (in this case,

the forward Type I exit) instead of one half of the total number of

exits installed. The demonstration was conducted this way in order to

provide for maximum usage of the aisle, by the maximum number of

passengers. While this demonstration was completed in 54 seconds, the

resultant overall rates of egress were not particularly fast for this

type of exit. In addition, the evacuees appeared to steady themselves

on seatbacks to a higher degree than is typically seen in such

demonstrations. Thus, while there was no clear obstruction due to the

aisle, the overall egress rates did seem to be somewhat reduced. The

FAA therefore considers that the aisle arrangement should result in a

limit on maximum passenger capacity that is reduced over the

theoretical limit in the regulations.

Jetstream has requested that the FAA increase the current limit,

which they believe is arbitrary, and allow up to 30 passenger seats in

accordance with same special conditions. As noted by the applicant the

demonstration conducted for approval of the 29 passenger version Model

4101 did include 30 occupants in the passenger cabin (to account for

the JAA approved arrangement) and was completed well within the

allowable time. Therefore, from the standpoint of amending the special

conditions, an additional demonstration would not be required. While

the limit of 29 passenger seats is somewhat arbitrary, the FAA must

consider whether other conditions should apply if the limit is raised.

One of the existing conditions is that there be no more than ten

seat rows on either side of the aisle. The 30 passenger arrangement

proposed by Jetstream is consistent with this requirement. In fact, the

additional seat is located on the left side of the aisle at the extreme

rear of the airplane, and does not produce the same kind of aisle

offset that occurs throughout the cabin. This is because the seat is

opposite a windscreen at which point the aisle leads into the

passageway for the aft exit.

These special conditions are intended to provide requirements which

result in a cabin aisle that is as effective and safe as those

envisioned by the regulations for the maximum number of passengers on

the airplane. Where appropriate, requirements were drawn from existing

regulations. In other cases, new requirements were developed to

preserve the level of safety that is inherent in the design of more

conventional aisle arrangements. The FAA considers that an additional

passenger seat would not invalidate the basis for the original finding

of equivalency. It is noted however, that any further increase in

passenger capacity would violate one of the other conditions because it

would result in more than ten seat rows on at least one side of the

aisle. Therefore, any increase beyond 30 passenger seats would very

likely require the development of additional conditions. As noted

above, the FAA has determined that the staggered aisle should limit the

passenger seating capacity to something less than would otherwise be

allowed by the regulations. Under the terms of these special

conditions, 30 is considered to be the reasonable maximum.

Accordingly, in addition to the requirements of Sec. 25.815, these

amended special conditions are issued for Jetstream Model 4100 series

airplanes with a staggered main aisle. Other conditions may be

developed as needed based on further FAA review and discussions with

the manufacturer and the Civil Aviation Authority (CAA).

As discussed above, the amended special conditions would be

applicable initially to the 30-passenger version of the Jetstream

Aircraft Limited Jetstream Model 4101 airplane. Should Jetstream apply

at a later date for a change to the type certificate to include another

model of the 4100 series incorporating the same novel or unusual design

feature, these amended special conditions would apply to that model, as

well, under the provisions of Sec. 21.101(a)(1).

Jetstream, in their petition, requested that the amended special

conditions be promulgated without delay as a final rule and in time to

permit delivery of 30 passenger airplanes during August 1994. The FAA

determined that public comment was in the public interest, but

shortened the comment period to 20 days in order to expedite the

issuance of these amended special conditions.

Notice of Proposed Special Conditions No. SC-91-4-NM for the

Jetstream Aircraft Ltd. Model 4101 airplane was published in the

Federal Register on September 13, 1994 (59 FR 46939). Comments were

received from two labor organizations and two airlines.

Both operators comment on the improved productivity that could be

realized with the additional seat on the Jetstream 4101 and strongly

support the proposed amendment. One operator notes that the Joint

Aviation Authorities have already approved the airplane for 30

passengers and only the FAA special conditions limit its capacity.

The third commenter is not in favor of the proposed amendment to

the special conditions. This commenter argues that the exit arrangement

and flight attendant seat location on the airplane are confusing and

awkward. The commenter specifically criticizes the location of the

flight attendant seat and the potential for the flight attendant to

disrupt evacuation through the right rear exit. The FAA does not agree

that the exit arrangement on this airplane in conjunction with the

flight attendant seat location is in any way inadequate. The exit

arrangement complies fully with all applicable regulations and provides

good uniformity for the number of passengers on board. The FAA also

participated in an evaluation of the flight attendant's ability to

assist passengers through the right rear exit and found that this could

be accomplished quite effectively.

This commenter also does not feel that the emergency evacuation

conducted as part of the original special conditions was adequate, and

should be repeated before an increase in passengers could be

considered. The commenter notes that there has been a change to the

regulations regarding evacuation demonstrations since the issuance of

the special conditions and believes that the new provisions were not

complied with in the original demonstration. This commenter also

questions whether the demonstration met the FAA's stated objective of

maximizing the number of passengers who must use the aisle, and feels

that the aft right-hand door should have been used for the

demonstration. Additionally, the commenter interprets FAA comments to

mean that the passengers were ``zig-zagging'' around the seats backs.

The commenter also suggests that Jetstream should relocate the flight

attendant seat to the forward bulkhead. The commenter believes that

this would enhance both passenger and flight attendant safety. This

last comment is not germane to these amended special conditions.

With respect to the demonstration conditions, these were chosen by

the FAA for the purpose of complying with the special conditions, and

not to show compliance with the specific regulations regarding

evacuation demonstrations. Therefore, changes to these regulations

since the special conditions were issued are not relevant. In any case,

the new provisions in the regulation were encompassed in the

demonstration done for the special conditions.

In regard to the second concern, that the demonstration did not

maximize usage of the aisle, the FAA does not agree. The evacuation

demonstration was conducted with a full passenger and crew complement

using only the forward entry door. This scheme was chosen for several

reasons. First, the end of cabin location meant that passengers at the

extreme end of the cabin would have to traverse the entire cabin in

order to reach an exit, thus making maximum use of the aisle. Second,

the forward exit, a Type I exit, was specifically selected because it

was expected to provide for an increased evacuation rate capability

over the aft exit, which is a Type II. Thus, the door rate would not

mask any effect due to the aisle. Had the aft exit been used, it is not

clear that the rate capability of the door is sufficiently higher than

that of the aisle to identify effects due to the aisle only. In

addition, the aft location of the flight attendant seat meant that

passengers had to negotiate the aisle without crew assistance, which is

the more critical case.

Finally, the FAA's observations of the evacuation demonstration, as

expressed above, were not intended as an implication that passengers

were required to ``zig-zag'' around the seat backs. The passengers

proceeded in a straight line along the aisle, but did seem to use the

seat backs as hand-holds to a higher degree than is usually seen.

In summary, the FAA will not require a new evacuation demonstration

since the results of the previous demonstration are still valid.

The fourth commenter opposes the amendment to the special

conditions, and reiterated opposition to the original special

conditions. The commenter's principal objection is the projected width

of the aisle at heights more than 25 inches above the floor. The

commenter notes that the special conditions require a 20 inches

projected width at 43 inches above the floor, but expresses frustration

at not knowing the projected width below that height. The commenter

assumes that 43 inches represents the tops of the seat backs. This

commenter also interprets the FAA's observations of the evacuation

demonstration as indicating that there is a safety problem with the

airplane.

Regarding the projected aisle width, this varies essentially

linearly from approximately 16 inches (at a height of 25 inches above

the floor) to 20 inches (at a height of 43 inches above the floor).

However, since the seat rows are longitudinally offset, there is never

any point where the transverse measurement between seats is less than

20 inches. As noted in the original final special conditions, the

actual seat back height is almost 50 inches. Forty-three inches

represents a typical seat back height for airplanes where the seats are

mounted at the same level as the main aisle.

As noted previously, an indication of some additional problems

should not be inferred from the FAA's observations of the evacuation

demonstration. They are simply the factual observations of the

demonstrations.

Conclusion: This action affects only certain novel or unusual

design features on one model of airplanes. It is not a rule of general

applicability, and it affects only the manufacturer who applied to the

FAA for approval of these features on the airplane.

List of Subjects in 14 CFR Part 25

Air transportation, Aircraft, Aviation safety, Safety.

The Amended Special Conditions

Accordingly, pursuant to the authority delegated to me by the

Administrator, the following amended special conditions are issued as

part of the type certification basis for the Jetstream Model 4100

series airplanes with an offset main cabin aisle.

The authority citation for these special conditions is as follows:

Authority: 49 U.S.C. 1344, 1348(c), 1352, 1354(a), 1355, 1421

through 1431, 1502, 1651(b)(2); 42 U.S.C. 1857f-10, 4321 et seq.,

E.O. 11514; 49 U.S.C. 106(g) (Revised Pub. L. 97-449, January 12,

1983).

1. Jetstream Model 4100 series airplanes may be approved with an

offset main aisle provided:

a. There are no more than 30 passenger seats with no more than ten

seat rows on either side of the aisle;

b. The interior arrangement includes one pair of Type II and one

pair of Type III passenger emergency exits;

c. In addition to the requirements of Sec. 25.815, the aisle

projected aisle width is at least 20 inches measured at a height of 43

inches above the aisle floor; and

d. The Type III exits and adjacent seat rows are offset

longitudinally such that persons approaching an exit from one end of

the cabin may use the exit without interfering with those approaching

the other exit from the other end of the cabin.

2. The location and part number of each passenger seat must be

defined by a drawing approved by the FAA or CAA-UK. The seat

arrangement may not be reconfigured without FAA approval.

3. An evacuation demonstration must be conducted to demonstrate the

efficacy of the aisle arrangement.

Issued in Renton, Washington, on November 8, 1994.

Stewart R. Miller,

Acting Manager, Transport Airplane Directorate, Aircraft Certification

Service.

[FR Doc. 94-28286 Filed 11-29-94; 8:45 am]

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