I have spent a lot of time talking about aircraft cost - comparing- tracking and benchmarking them. If you take a look at the new aircraft ads- you don't see much mention of costs- but you do see a lot about performance.

David Wyndham  |  01st July 2011
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David Wyndham
David Wyndham

David Wyndham is the founder of David Wyndham + Associates. He was previously a Co-owner and President...

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Volume- Speed- Range:
Tips to evaluate aircraft Performance & Specification.

I have spent a lot of time talking about aircraft cost - comparing- tracking and benchmarking them. If you take a look at the new aircraft ads- you don't see much mention of costs- but you do see a lot about performance. There are essentially three main questions a prospective buyer will have about an aircraft:

• How big is the cabin?
• How fast does it fly?
• How far does it fly?

Cabin size is very important to most buyers. In its data- Conklin & de Decker lists cabin height- width- length and volume. Those are maximum dimensions. The volume shown in turbine airplanes is calculated from the cockpit divider to the aft pressure bulkhead. So when you are comparing these- you need to also consider:

Cabin Shape: A round cabin versus a ‘bread-loaf’ shaped cabin. In a round cabin you tend to have the maximum width at- or just above the hip. With the bread-loaf shape- you get a more straight-sided cabin and thus tend to have greater shoulder width. A cross section diagram will show this quite clearly.

Cabin Length (passenger seating area): Two aircraft may have a maximum cabin length of 28 feet. However- one may have a bigger galley or lavatory. While that may be desirable- it would detract from the area for passenger seating. A cabin floor plan will show lavatory- galley and passenger seating areas. Be sure to evaluate them and consider the trade-offs between passenger seating space and galley/lavatory requirements.

Baggage Volume: Bags can be stored in the cabin- and on many aircraft- in external baggage compartments. So while total baggage volume may be important to you- also consider whether you need access to bags in-flight. Large items may require specific lengths and width for easy storage. Also- for larger jets- how high off the ground is the external access to the baggage? Loading bags externally can help prevent added wear on the interior and allow passengers to board during baggage loading.

Take a moment to consider your speed requirements relative to the trips that you will fly. If all your trips are well within the range capabilities of the aircraft- then you can plan on flying these trips at high-speed cruise if time is of the essence. If you are flying trips near the range capability of the aircraft- those trips may have to be flown at the slower- fuel-efficient long-range cruise speed.

This can be important. In one analysis we did- Aircraft A cruised 55 knots faster than Aircraft B. Speed was important to our customer. However- for their long-range trip- Aircraft A needed to fly at long range cruise speed while Aircraft B- with greater range capability- could fly that same trip at its high-speed cruise (which- as it happened- was the same as Aircraft A's long-range cruise speed).

While Aircraft A arrived sooner than Aircraft B on all the shorter trips- on the critical long trip- both aircraft flight-planned for the same elapsed time. So with speed- it is not only a question of how fast- but how far.

In evaluating the required range of the aircraft- generally you should have an aircraft that can do at least 85% of your trips non-stop. The remaining 15% may be done either with one fuel stop or non-stop by chartering a more capable aircraft.

In comparing aircraft ranges- you need to consider what passenger/payload you need and what sort of alternate airports may be required. With business airplanes- the NBAA has its IFR Fuel Reserve formula for alternate airports that are either 100nm or 200nm distance from the destination.

When comparing a turboprop to a jet- or even a light jet to a mid-size jet- make sure both range figures are using the same criteria. With turbine aircraft- we always compare with a 200nm alternate. However- for long-range and especially over-water trips- 200nm may be insufficient. For one client who flies regularly to Russia from Western Europe- their travel dictated an aircraft with about a 450nm alternate. Flying trans-Pacific may dictate a 600nm alternate depending on the weather.

Further- if you regularly fly with four passengers- then your range requirements should be based on four passengers- not eight. Remember- with increased capability comes increased complexity and cost. So while more may be better- it may not add the value in relation to the cost. Some aircraft may offer long range- and they may have a large payload- but they may lack range with a large payload.

When calculating range- you should also consider the distances with expected winds. Headwinds slow an aircraft down- just as a current in the river slows a boat down. An aircraft cruising at 450 knots into a 50 knot headwind will travel only 400 knots over the ground. So a trip that starts at 1-500nm- with expected headwinds and via airways routing may require an aircraft with a range of 1-700nm to be able to fly the trip non-stop.

There is more to the aircraft performance analysis- but the above should help answer the three biggest questions many owners have. Remember- all aircraft are compromises. Just make sure that you don't compromise on the important parameters.

David Wyndham is an owner of Conklin & de Decker. The mission of Conklin & de Decker is to furnish the general aviation industry with objective and impartial information in the form of professionally developed and supported products and services- enabling its clients to make more informed decisions when dealing with the purchase and operation of aircraft. With over 1-800 clients in 90 countries around the world- Conklin & de Decker combines aviation experience with proven business practices.

More information from; Tel: +1 508 255 5975.


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