The following report aims to assess the performance characteristics of the Jetstream HP137 Mk 1. The aircraft is operated by the College of Aeronautics.
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2. Methodology
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The aircraft performance data was provided in the form of graphs which were produced and collated between the seven groups, from the data that was recorded in the Jetstream. These interim performance charts are found in Appendix A. In addition to this, fuel flow data was recorded during taxi, approach and take-off.
2.1 Performance Summary Tables
The objective is to calculate the range and time taken to complete a mission with a given amount fuel and payload. Each mission is split into 2 main parts – the ‘en-route’ and the ‘diversion’. Each phase (ie climb, cruise, descent, etc) is operated in a certain way and relevant parameters such as climb rates and cruise altitudes are usually stipulated by company policy. Using the data available in the fuel flow tables and interim performance charts, fuel burn for each phase of the flight was calculated.
The fuel, time and distance required for each stage are calculated as described above and entered into the table in a systematic manner – starting with the ramp weight of the aircraft and proceeding through the mission profile. A column is dedicated to monitoring the decrease in aircraft weight. This is balanced by the increase in fuel consumed.
The analysis was performed at the following combination of payload and fuel on board to define the range payload diagram:
Point 2: Take-off at MTOW, comprising maximum payload and remaining fuel allowable
Point 3: Take-off at MTOW, comprising maximum fuel and remaining payload allowable
Point 4: Take-off weight the sum of OEW and MFW
When all flight phases are calculated, the sum of the fuel burn, time taken and distance covered is calculated. The distance and time contributions are taken only for those phases that fall under the en-route mission, while the fuel required to perform the en-route and diversion are calculated separately and summed.
2.2 Strategy Adopted
Overall guidelines were followed in the form of rounding the results for fuel as to give a conservative figure.
For all the diversion stages the priority was to minimise fuel burn. This was achieved by comparing the fuel burn for different climb speeds and descent rates. It was minimised by climbing at 110 kts and descending at a rate of 3000 feet per minute.
Some data was strongly dependent on weight, for example, holding performance. Therefore it was necessary to estimate the weight of the aircraft at that particular point of the flight. This was done by looking at the weight column of the table followed by the extrapolation of data from the series of graphs.
The appropriate weights for the aircraft (heavy or light) were selected by looking at the aircraft weight at that particular phase of flight. For points 3 and 4 it was necessary to calculate the long-distance range at the expense of payload until the ferry range, or the extremity of a zero payload was reached. In these cases, the light data was used.
2.3 The full passenger configuration
It was found that time and fuel data calculated were far apart and there was no data between the 175nm to 1100nm set. An additional analysis was performed for the ’10 passenger and baggage’ loading situation (Point 5).
Point 5: Take-off at MTOW, comprising 950kg payload and remaining fuel allowable
This gave us an extra point to add certainty to the plots generated.
2.4 Producing the Graphs
For each analysis performed, we obtained the following data:
* Distance covered excluding diversion (range)
* Time taken excluding diversion (block time)
* Fuel required to perform en-route part of mission (en-route fuel)
* Fuel required to perform diversion (reserve fuel)
With the range and payload known for each point, the range-payload diagram is produced. The total fuel required for the mission is called the block fuel. With the above data, we can also plot graphs of:
* Block time vs range
* Block fuel vs range
* Reserve fuel vs range
2.5 One Engine Inoperative (OEI) Ceiling
The OEI ceiling chart is produced by looking at the climb gradient chart. The ceiling corresponding to a climb gradient of 1.1% for each of the light and heavy flights is obtained and re-plot on altitude vs weight axes.
3. Summary of Results
3.1 Assumptions and errors
It must first of all be stated that the calculations were based on the central assumption that the readings obtained were for International Standard Atmosphere (ISA) conditions at sea-level. This assumption is fairly valid as the ground temperature during the test flights was about 12 to 15oC. The second major assumption made is that there was no wind – which was obviously not achievable but compensating for it would be beyond the scope of this exercise.
The interim performance charts jointly produced were of varying accuracy. Charts like the Specific Air Range (SAR) data points did not show smooth trends and it is likely that errors are introduced in trying to draw a smooth curve through these points.
3.2 Comments on Results
Range-Payload Diagram
The range payload diagram is shown in Figure 1. The maximum payload (limited by the structural maximum zero-fuel weight) is calculated to be 1365kg (Point 1). When fuel is added to the Maximum Take-off Weight (MTOW) limit, the range with maximum payload is 175nm (Point 2).
With the aircraft seating capacity filled with 10 passengers and their associated baggage (950kg payload) the aircraft is capable of flying 587 nm. Since the aircraft weight is limited by the MTOW, if greater range is required, a trade off would be made to reduce payload in return for more fuel. Point 3 on the payload range diagram is where the fuel tanks are filled and the maximum distance that may be travelled is 1100 nm with a reduced payload of 460 kg.
In the extreme, the range can be extended further by not carrying any payload (Point 4) such as during delivery flights. This is termed the ferry range and is found to be 1112 nm. This extension arises solely from the decreased fuel consumption due to decreased weight.
Block Time
The Block time graph is found in Figure 2. Block time takes into account the aircraft taxiing out, climb, cruise, descent, approach, landing and taxi back. This aids scheduling and gives the block time required for a certain stage length. For example, the block time for the aircraft to travel 587 nm with 10 passengers and their baggage is 205 minutes.
The ferry distance of 1112 nm will would take 372.5 minutes.
Block Fuel
Block Fuel is the amount of fuel required for a mission including the amount of reserve fuel needed in case of diversion. The variation of block fuel with stage length is given in Figure 3. With 10 passengers onboard the aircraft will use 925.5 kg of fuel for a stage distance of 587 nm.
Reserve Fuel
The reserve fuel required takes into account a diversion to an alternate airfield 100nm away. In the case of the 10 passenger flight, the Jetstream needs to carry 292.65kg which is included in the block fuel. Figure 4 shows the increase in reserve fuel required with distance travelled. This increase arises due to the fact that a provision (Route Reserve) of 5% of Block Fuel is included.
OEI Ceiling
In the event of one of the engines failing, the thrust and hence speed decreases and the lift generated may be insufficient to balance the aircraft weight if at altitude. The aircraft needs to descend to a lower altitude where the air is denser to generate sufficient lift. For safety reasons, the OEI ceiling is specified as that where the aircraft can maintain a 1.1% climb gradient.
This ceiling is shown in Figure 5. The altitude decreases with an increase in aircraft weight due to increasing lift required. There will be some error associated with this as the aircraft weight at the exact time of the drift-down manouevre was not recorded. The graphs were produced using the Take-off weight of that particular flight.
4. The Aircraft’s suitability for Commercial operations
The Jetstream 31 aircraft is a very versatile aircraft that could and is used for a variety of roles currently in the industry. It has a reasonably high maximum payload and a good range with a reduced payload. It also has the capability to operate to airports with short runways.
Such roles include freighter conversions for feeding integrators and mail requirements, the use as an air ambulance for medical transfer of patients and organs, a training platform for pilots switching to multi-engine aircraft and various military and research purposes.
The aircraft has low operating costs, which allows the aircraft to be considered for use in economically deprived or developing countries as well as those developed. The only problem is its speed limitation when compared to jet aircraft. This speed penalty is not significantly felt over short distances.
This combination of low cost and low seating capacity would be suited for the operation of commercial routes where demand is low over fairly short distances or where surface transport links are poor. An example of this are the air taxi services in remote regions or between islands.
Based in the south east of the UK, the aircraft fitted in 10 seat configuration would have sufficient range to fly to most locations in the UK and even to most parts in Belgium, France and Germany. There might be a market in corporate travel for companies wishing to have employees transferred at low cost between different operating locations where direct air services and not available or to small airports.
