# Bjorn’s Corner: The challenges of airliner development. Part 6. Adding seats.

By Bjorn Fehrm, Henry Tam, and Andrew Telesca.

June 04, 2021, ©. Leeham News: Last week, we examined operating and product certification rules related to 9-seater air taxis and commuters. We took the example of the new Tecnam P2012 Traveller to study the certification rules for a 9-seater. Now we upsize the aircraft to understand the pros and cons of adding extra seats.

The Viking Twin Otter, the only in-production 19-seater. Source: Viking Air

##### Adjusting a Nine-Seat Aircraft to a 19-Seat Aircraft

If we very roughly scale the P2012’s maximum takeoff weight, we can use it as a “rubber” aircraft to illustrate the pros and cons of a 19-seater.

For a 19-seater, the aircraft needs to accommodate 10 more passengers than the P2012.  Assuming each passenger weighs 100 kg, the aircraft would need an extra tonne of payload.  An aircraft’s empty weight (including structure and systems) is, historically, somewhere around 50 to 60% of the maximum takeoff weight.  The remaining capacity is split between fuel and payload.

As a result, one extra tonne of payload requires an additional tonne of structure and systems to support the payload.  This brings the maximum takeoff weight from 3,680kg/8,113lb to 5,680kg/12,522lb.  Because the aircraft is heavier, it is likely to consume more fuel.  Let’s add, for argument’s sake, 150 kg of fuel and structure to compensate for the higher fuel burn.  The resulting maximum takeoff weight would then become 5,830kg/12,853lb.

The Beech 1900, a common 19-seater that has been out of production since 2002. Source: Beech Aircraft.

So, what does an aircraft with a 5,830kg/12,853lb maximum takeoff weight and 19 passenger seats mean to commuter operators?

The good news: it still doesn’t require a flight attendant in many jurisdictions.  For example, EASA Part-CAT states that:

“ORO.CC.100 Number and composition of cabin crew

(a) For the operation of aircraft with a MOPSC of more than 19, at least one cabin crew member shall be assigned when carrying one or more passenger(s).”

From an operating economics standpoint, the per-seat cost goes down compared to that of a nine-seater.  The operator can now divide crew costs by 19 instead of nine.  The maintenance cost could be higher (e.g. bigger engines) but again it would be shared by 19 passengers instead of nine.

And then there’s the bad news:

First, the aircraft needs more equipment.  When an aircraft operating under EASA Part-CAT exceeds 5,700 kg/12,566lb, it is required to have two independent static pressure systems, a terrain awareness warning system, a cockpit voice recorder, etc.  These are just a few examples and not an exhaustive list.

Second, there are additional training (or type rating) requirements for pilots operating larger or more complex aircraft.  For instance, under FAA 14 CFR Part 61 — Certification: Pilots, Flight Instructors, and Ground Instructors, it states:

§61.31   Type rating requirements, additional training, and authorization requirements.

(a) Type ratings required. A person who acts as a pilot in command of any of the following aircraft must hold a type rating for that aircraft:

(1) Large aircraft (except lighter-than-air). [Large aircraft means aircraft of more than 12,500lb/5,670kg, maximum certificated takeoff weight.]

(2) Turbojet-powered airplanes.

(3) Other aircraft specified by the Administrator through aircraft type certificate procedures.”

Third, under FAA Part 135 (the operating rules for our Traveller), a commuter aircraft can only have up to nine passenger seats.  An operator needs to be under Part 121 to take full advantage of all 19 seats.  Yet, §121.157 Aircraft Certification and Equipment Requirements clearly states “(h) Newly type certificated airplanes. No person may operate under this part an airplane for which the application for a type certificate is submitted after March 29, 1995, unless the airplane is type certificated under part 25 of this chapter.”

So for the US market, Part 23 certification for our 19-seater is no longer enough, our aircraft has to be designed to the tougher Part 25 rules.

Why have we not seen a clean-sheet 19-seat commuter in the past decade, or two?  There are many plausible reasons.  Changes in FAA rules over time could have eroded the demand for 19-seat commuters in the US.  It is true that Cessna is currently developing the SkyCourier, a 19-seat turboprop.

Cessna SkyCourier, an unpressurized utility aircraft that is available as a 19-seater. Source: Cessna.

Yet, the company is likely targeting FedEx’s cargo missions, as it’s an unpressurized aircraft and would have to fly below 10,000ft when carrying passengers.  As a result, the SkyCourier is a cargo aircraft first and a 19-seat commuter second, or perhaps even lower priority.

Price could be another factor that puts 19-seaters at a disadvantage.  A Twin Otter 400 costs \$6.5M as per Business & Commercial Aviation 2020 Purchase Planning Handbook’s estimate.  For those who do not need 19 seats, the cabin volume, or the STOL/floats/skis capabilities, one could get a faster aircraft with better fuel economy, such as PC-12 or HondaJet, for \$5.3M.

A nine-seat Cessna Caravan that cruises at a similar speed with a lower fuel burn costs \$2.0M.  Keep in mind, these are list prices, not actual net prices.  Electric aircraft, or other new technologies, may tilt the balance in the future.

##### Many new projects

There are many 19-seat projects with alternative propulsion systems proposed at the moment. Will these aircraft have a market when the present market seems to sit still and it has not attracted any new OEMs for 20 years? We will need to wait a few years to see whether these new products can deliver on their promises and whether there is a market for them.

### 13 Comments on “Bjorn’s Corner: The challenges of airliner development. Part 6. Adding seats.”

Thanks for the article

2. The Twin Otter is also unpressurized so that is a market the SkyCourier could attack. At least the non-STOL part of it.

• I think Cessna have been very clever with positioning the Skycourier 408 or has been very clever to have FedEx as its customer. The Skycourier 408 can carry three LD3 Unit Load Devices. This means those regional airports now being serviced by passenger turboprops and small jets (Embraer’s, DASH-8,, Saab Commuters or even B737) that can’t carry LD3 can now directly receive an LD3 via a Skycourier. FedEx already operate an extensive network of Caravan 208 and understand how to run courier flight but can now use a standard LD3. There is really not much point compromising performance in order to negligibly increase the market by allowing the aircraft to get into a few more short grass strips in an airfield lacking logistics. I suspect it will develop a global market, perhaps in Africa, and once support is established this will also drive a passenger market.

• I think we can safely say that FedEx issued the specs for the 408.

With FedEx as a base customer the rest is vastly easier. Much like the Caravan success its all due to FedEx.

The aircraft are very expensive to develop and costly build and it takes a special set of circumstances to get one into production.

Base reality of why its relatively rare. For passenger ops the whole thing is so costly its no longer viable (or you get Cape Air on conventional piston)

3. Bjorn

You say a 19 seater must be certified under Part 25. But in the case of the SkyCourier at least one report states:

“The non-pressurized, fixed-gear airplane will be certified to Part 23 standards.”

Is that just wrong, or could it be the plan is to certify first under Part 23 limiting the aircraft to 9 passengers and cargo, then later upgrade the certification if there is interest in a passenger version.

• There seems to be recent ‘new part 23 rules’

Specifically, the new part 23 revolutionizes standards for airplanes weighing 19,000 pounds or less and with 19 or fewer passenger seats by replacing prescriptive requirements with performance-based standards coupled with consensus-based compliance methods for specific designs and technologies.

Along with some video of the situation -mostly blather 8min
https://www.faa.gov/tv/?mediaId=1258

• Thanks for the links. Part 23 is also being used to certify some of the eVTOL aircraft now flying as prototypes. The FAA seems very proud of its new FAA Part 23 rules and the new performance based approach that allows greater innovation. I suspect the application of the rule will grow upwards and either displace parts of Part 23 or encourage a similar rewrite of Part 25.

• Its a bit of irony that the Part 23 video talks about and shows lots of small and innovative planes with only a few seats and yet the first new build to go through the process ( 3 flying test aircraft) is the mundane/ no surprises ’19 seater’ Skycourier from Cessna – continuing their highly conservative heritage. Its almost as though some one snuck the larger 19,000lbs- 19 seats numbers in while no one was looking ?

• Gents,

you all raised interesting points. It took some time to get a good answer as I let Henry and Andrew, my experts, formulate it. Here goes:

@jbeeko, you actually caught one of the key points that we were trying to illustrate! Glad that you brought this up.

When designing an aircraft, we need to look at both product certification (e.g. Part 23/25/etc, the area where FAA has introduced performance-based rules for Part 23) and operation rules (e.g. Part 91/121/135/etc) as we mentioned in the last article.

One can certify an aircraft with up to 19 seats and 19,000 lb MTOW under Part 23. But operation rules make this a difficult use case in the US. Under Part 135 (Commuter and On-Demand), a “commuter” needs to have nine passenger seats or less AND 7,500 lb of payload or less.

If an operator wants to use an aircraft with 19 passenger seats for scheduled services, then the operation cannot be under Part 135. It needs to be under Part 121 (Airline), and this, as we mentioned in the above article, requires the use of Part 25 aircraft (with some grandfather rules).

Cargo-only operation is different. By FAA’s definition, an all-cargo operation is considered a non-scheduled operation. So, the operator needs to figure out whether it falls under Part 121 or Part 135. The threshold is 7,500 lb payload. Since the SkyCourier Freighter has a 6,000 lb payload, it could be operated under Part 135 On-Demand.

Now, it doesn’t mean that people in the US won’t buy the 19-seat version. Private owners (such as Companies) can still use it. It could be used for Part 135 On-Demand (nonscheduled) passenger services. Plus, the 19-seat version can be used in many countries, also for scheduled services.

Marketing and Product Strategy teams at an aircraft OEM need to understand opportunities in different regions and market segments to determine what configurations or optional features to invest in to get the best return on investment.

To be clear, the later part of the discussion focuses on the changes in Part 23. But this is not the catch, it’s the operating rules, Part 135 being replaced by Part 121, that stops the SkyCourier from 19-seat scheduled services in the US.

• One of the more interesting 19 seat commuter studies is a German Government sponsored one for a 19 seat all electric commuter. that would presumably be certified to the EASA equal of Part 23 if it saw the light of day. That would be the opposite of ‘conservative’

https://www.intelligent-aerospace.com/commercial/article/14170220/19-seat-commercial-commuter-plane

“According to our research, 56 percent of 19-seaters worldwide fly distances of less that 200 kilometres (108NM) and 83 percent fly less than 350 kilometres. (189NM)”

It’s an interesting statistic and shows that the Skycourier passenger variant would spend less than an hour in transit and is unlikely to need pressurisation.

• Dear Bjorn, Is there a pathway for aircraft such as the Skycourier from Part 23 to Part 25 certification so that it can operate under Part 121 as an airliner. Do they need to start again with the certification process or can they just add features.

4. Good question William. Here’s what Andrew Telesca answered:

I believe they would need to start again (can’t just certify the new features as major changes), but there are opportunities to streamline the compliance demonstration.

For all those areas with common regulations between the parts you would propose to utilize the previously approved data and do a new showing of compliance with that data, accounting for continued applicability if the data is potentially impacted by any of the design changes being introduced to meet the new Part. You would of course also have to show compliance to the additional Part 25 requirements not common with Part 23.

An interesting item would be whether you could claim service credit to avoid or reduce the 150-300 hours of function and reliability flight testing [21.35(b)(2)] during the Part 25 certification process.