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  What's Really Involved in the Certification of that Jet?

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Robert Baron

President and chief consultant of, a worldwide aviation safety consulting firm that specializes in Crew Resource Management and Human Factors Training for aviation departments. Mr. Baron also owns and operates Learjet Crews International, Inc. He holds an Airline Transport Pilot Rating and has over 15 years of aviation experience as a Line Captain, Instructor and Check Airman in Learjet aircraft. He's also type-rated in the Cessna Citation and holds a Flight Engineer Rating for Turbojet aircraft. His academic achievements include a Bachelor's Degree in Professional Aeronautics/Aviation Safety and a Masters Degree in Aeronautical Science with specializations in Aviation Safety/Human Factors from Embry-Riddle Aeronautical University. If you have any questions or would like additional information on CRM/Human Factors training, please don't hesitate to contact Robert through e-mail or via phone at 800-294-0872, 954-803-5807.

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     What's Really Involved in the Certification of that Jet? (Pg. 2)
     Robert Baron


Learjet 35a Safety Certification Highlights


            Weight and C.G. certification require extensive testing under a variety of weather and weight conditions. Certain criteria must be met for atmospheric and weight/c.g. variables such as:

  • Relative humidity must be 80%, at and below standard temperatures; and

  • Relative humidity must be 34%, at and above standard temperatures plus 50F.

    Between these two temperatures, the relative humidity must vary linearly.

  • The center of gravity limit must not lie beyond:

  • The extremes selected by the applicant.

  • The extremes within which the structure is proven; or

  • The extremes within which compliance with each applicable flight requirement is shown.

  • Maximum weights corresponding to the airplane operating conditions (such as ramp, ground, takeoff, en-route, and landing), environmental conditions (such as altitude and temperature), and loading conditions (such as zero fuel weight, center of gravity position and weight distribution) must be established so that they are not more than:

  • The highest weight selected by the applicant for the particular conditions.

  • The highest weight at which compliance with each applicable structural loading and flight requirement is shown.

  • The highest weight at which compliance is shown with the certification requirements of Part 36 of the FAR's.


    Structural Members: The Learjet structural members, throughout the aircraft, are in close proximity of each other. This design and construction offers a FAIL-SAFE structure. If a structural member should fail, this will not create a catastrophic situation since the nearby structural members are capable of absorbing this additional load. The fuselage is built around a main keel beam in the cabin area and utilizes round shape frames, which are ideal for a pressurized vessel.

    Learjet 24 Structural Members (Similar to the Learjet 35a)

    Cabin Door: The Learjet cabin door is designed with two halves (an upper half and a lower half), which are locked together with ten heavy steel pins. When the door is closed, it is an integral part of the cabin structure.


    Amongst other things, FAR 25.1351 specifies that:

  • The generating system power sources function properly when independent and when connected in combination.

  • No failure or malfunction of any power source can create a hazard or impair the ability of remaining sources to supply essential loads.

  • There are means accessible, in flight, to appropriate crewmembers for the individual and collective disconnection of the electrical power sources from the system.

  • There are means to indicate to appropriate crewmembers the generating system quantities essential for the safe operation of the system, such as the voltage and current supplied by each generator.

  • It must be shown by analysis, tests, or both, that the airplane can be operated safely in VFR conditions, for a period of not less than five minutes, with the normal electrical power (electrical power sources excluding the battery) inoperative, with critical type fuel (fuel from the standpoint of flameout and restart capability), and with the airplane initially at the maximum certified altitude.

    Compliance with the above requirements is provided by a dual generator system. Each engine has a respective DC generator rated at 30V and 400AMPS. Both generators provide DC power to all the required DC (and, through inverters, AC busses). However, with the failure of one generator, the remaining generator can provide the same buss powering. Therefore, the failure of one generator will not adversely affect the total electrical systems in flight.

    If both DC generators should fail in flight, the aircraft's two 24VDC batteries will provide an additional backup (redundancy) to a double-generator failure for approximately 30 minutes in night IFR conditions.

    If both DC generators, and both ship's batteries become depleted, a standby emergency battery will provide DC power to only a few select systems. These are the standby gyro and its light, the landing gear circuit, the flaps circuit, and landing gear green locked-down lights. This battery will last 3:45 minutes in the STANDBY position (gyro and light only), or: 30 minutes in the ON position (powering all the above components). Should the standby battery become depleted, there would be no additional sources of electrical power in the aircraft.

    Means of providing control and monitoring of the electrical systems is provided by switches and gauges accessible to the flightcrew. Each respective generator has a switch and a reset switch for generator control. Each battery also has a respective switch that can be placed in the ON or OFF position.


                Should an engine fail during takeoff in the Learjet 35a, certification under Part 25 guarantees a positive climb gradient on the remaining engine, provided certain criteria are met. The criteria must be the most restrictive of the following:

  • The aircraft is at or below its maximum certified takeoff weight.

  • Maximum takeoff weight for airport altitude and temperature.

  • Maximum takeoff weight for the runway and ambient conditions.

  • Maximum takeoff weight for obstacle clearance considerations.

    If the aircraft is above the maximum weight as required from above, there will be no guarantee of the climb performance advertised in the performance charts.


                The fuel system in the Learjet 35a is self-sustaining once the engines have been started. The fuel is supplied through Jet Pumps (Motive Flow System). Jet pumps have no moving parts and require no electrical power. They operate by the Venturi Principle (pressure differential causes the fuel to enter the pump). Therefore, a total loss of DC electrical power in flight will not adversely affect the engine fuel system. Further, the aircraft can be descended below 25,000 feet, if necessary, where the engines can suction feed fuel through their respective fuel pumps.

    Fuel Jet Pump

                Additional requirements of the fuel system include venting, filtering, and draining capabilities. Since the Learjet 35a does not have fuel heaters, a fuel anti-icing additive must be added to the fuel to prevent fuel icing at higher altitudes.

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