Learjet 35A D-CGFD – a measurement aircraft for deployment in the upper troposphere and lower stratosphere

The Aircraft

The Learjet 35A is a twin-engined jet aircraft that was originally conceived as a business jet. The basic version has seating for two pilots, a flight attendant and seven passengers. Its flight characteristics and construction also make it ideally suited for use in atmospheric research.

The Learjet 35A has the following characteristics:

• excellent flight performance, i.e. long range and good rate of climb
• maximum height of 45,000 feet, also attainable with externally mounted equipment
• economical operation
• adequate body size for the installation of measurement equipment in 19-inch racks
• large cargo door allowing easy access
• robust construction enabling easy modification

Technical Data for the Learjet 35A

General Information
Certification Status		FAR 25
Approved Operations		Day, Night, VFR, IFR and flights under Known Icing Conditions
Minimum Flight Crew		Pilot and Copilot
Seating Capacity		Up to Eight Passengers
Maximum Operating Altitude		45.000 ft	13.714 m
Dimensions
Length		48 ft, 8 in	14,83 m
Height		12 ft, 3 in	3,73 m
Wing Span		39 ft, 6 in	12,04 m
Wing Area		252,3 sqft	23,53 m2
General Performance
Average Rate of Climb (max. T.O., SL) **
up to FL 410		1.708 ft/min	521 m/min
up to FL 370		2.300 ft/min	701 m/min
up to FL 100		4.000 ft/min	1.220 m/min
Average Rate of Climb (T.O. mass 14.000 lbs, SL) **
up to FL 450		2.050 ft/min	625 m/min
up to FL 370		3.700 ft/min	1.128 m/min
up to FL 100		5.500 ft/min	1.694 m/min
Range		2.200 NM	4.070 km
Weights & Capacities
Max. Takeoff Weight **		19.600 lbs	8.909 kg
Empty Weight **		10.400 lbs	4.727 kg
Fuel Capacity **		7.000 lbs	3.182 kg
** following refit in Autumn 1997

The Learjet as a „Special Mission Aircraft“

The Learjet is ideally suited for special missions on the basis of its characteristics and robust construction.

The wings are derived from those of a Swiss fighter plane and allow external loads of up to 450 kg to be mounted under each wing. The generously dimensioned jet engines allow high rates of climb. The maximum height of 45,000 feet can be reached even with a full load of measurement equipment (including inlet systems and external measurement pods under the wings).

The use of the Learjet 35A/36A for operations involving target drones

Our partner company, GFD (Gesellschaft für Flugzieldarstellung mbH, Hohn) operates a total of eleven Learjets of type 35A/36A.

The main task of the Learjets is the deployment of target drones on long steel cables and is carried out under contract to the Bundesamt für Wehrtechnik und Beschaffung (BWB; Federal Office for Defence Technology and Acquisitions). The drones serve as realistic targets for ground troops. They are also used for the purpose of training personnel in radar tracking and for making optical and dynamic measurements of antiaircraft systems.

The Learjet was chosen for these tasks because it enables reliable routine operation. Furthermore, due to low repair, maintenance standby and operating costs it is also economical to operate, while at the same time it is well suited for refitting with specialized equipment.

The aircraft have been modified as follows:

• increased take-off weight through V-tails;
• increased range through enlarged wing tip fuel tanks;
• extra power supplies;
• equipment mounts below the wings;
• special radio and navigation systems.

GFD currently employs 18 pilots, 3 of whom are trainers/LBA specialists, and 29 technicians, 5 of whom are also examiners for aeronautical equipment (Class 1 and Class 2). The company is under the supervision of the Luftfahrt-Bundesamt (LBA; Federal Aviation Bureau) in Braunschweig and is registered with the Bundesminister für Verkehr (Federal Ministry of Aviation) as an aviation enterprise with the registration number. EG 039. GFD also carries out maintenance and repair work and is officially recognized as meeting the European standard JAR 145.

The aircraft currently fly 2500 hours annually during both daytime and nighttime operations and it is planned to increase the flight time to 3500 hours in the near future.

Measurement equipment

In line with our concept for the dual use of the aircraft, both the aeroplanes and the measurement equipment are available for use as part of a co-operation between the company GFD and enviscope. The scientific equipment has been specially developed to meet the specialised requirements of atmospheric environmental research.

A basic set of scientific equipment has been put together which supports the installation of a wide range of instruments. A major criterion was the development of a modular construction that can be easily adapted to the specific requirements of each project.

The following components are currently available for use:

• 19-inch instrument racks
• power supplies for the instruments
• vacuum supply system
• two exchangeable side windows for use as instrument platforms
• exchangeable roof section for use as an instrument platform
• isokinetic aerosol inlet system
• universal air inlets and sampling systems
• large external pods mounted under the wings
• instrument racks for the pods
• data collection system; well tested under flight conditions
• registration of avionics data from the on-board navigation system
• data collection system for meteorological parameters

The cabins can be outfitted with up to eight 19-inch instrument racks. Seating for a maximum of three people allows the manual operation of the instruments. The racks in the front section of the aircraft are available for the client’s equipment. Depending on the project requirements it is possible to fit racks of various different sizes. The racks are partially outfitted with special mounting plates, so that the equipment can be installed or removed within a few minutes. This allows maintenance and laboratory calibration to be carried out immediately before and after the flight.

A total of approximately 110 HU (1 HU= 44.45 mm) of space are available in the 19-inch racks for the installation of research equipment. The lower sections of the racks can be utilized for the installation of further equipment, e.g. vacuum pumps.

The power supply is mounted in the racks in the rear of the aircraft and can supply a total of 9 kW. This is available as 28V DC supply and can be partially converted into 115 V / 400 Hz and 220 V / 50 Hz AC current.

Various inlet systems allow the instruments to be supplied with outside air to enable measurements to be made. Several external apertures exist for this purpose.

One window has on each side of the plane been equipped with an exchangeable mounting platform. This allows a fast refitting of the aircraft and helps realize our goal of a modular design. These ‚window plugs‘ can be outfitted with user specified inlets or sensors. Various frequently required inlet systems, e.g. an isokinetic inlet for aerosol measurements and an adjustable „universal inlet and sensor housing“, are also available.

A further aperture has been made in the aircraft roof. This is similar to the side windows as it is equipped with an exchangeable platform which can be outfitted with a range of sensors, etc. Sensors which are permanently mounted there provide basic meteorological data which is recorded by a computer system. This PC-based data collection system has been well tested under flight conditions and is equipped with peripheral devices which allow the client to simultaneously record measurement data. The software has been specially adapted for this purpose and allows ‚on-line‘ and ‚quick-look‘ functions, as well as the possibility of exchanging data with other computer systems as required by the client.

The supports below the wings of the Learjet also offer unique opportunities as the external pods can carry loads of up to 450 kg on each side.

A new external pod was also approved for use during the refitting of the Learjet for environmental research purposes. This is approximately 2.9 m long and has a diameter of 40 cm. An instrument rack can be mounted in the pod within a few minutes. A total weight of up to 150 kg is permitted for the pod, which translates to an allowable user load of 120 kg. The external pods are attached using so-called NATO locks, which allow the pods to be attached and detached easily. This allows the entire pod, complete with measurement instruments to be changed within a few minutes.

Deployment Possibilities

The long term availability of these aircraft for projects in the field of environmental research is ensured through contracts between the BWB and GFD on the one hand and between GFD and enviscope on the other.

The development of a modular infrastructure allows the Learjet to be refitted with measurement equipment within a short time. Depending on the composition of the measurement equipment and the degree of preparation, it is possible to install and then remove the equipment again in only a few hours.

In extreme circumstances it is possible to employ the external pods as autonomous measurement units and to use a ‚plug and measure‘ concept. In such a situation it is possible to exchange the equipment within approx. 5 minutes by attaching the pods to the supports and plugging in the equipment to the power supply.

This special capability makes the Learjet 35A ideally suited for use as a temporary research aircraft. The spectrum of conceivable applications, which is already very broad, can thus be extended to include an even wider range of activities. Alongside the planning of conventional measurement campaigns, such as the  POLSTAR 98 measurement campaign, it is possible to carry out routine measurements with a small set of suitably selected equipment. These could be carried out alongside other users activities, e.g. at the same time as routine operations involving target drones.