Aircraft Sales and Parts

An old hobbs meter (soviet union 1970s)

The Hobbs meter is a device that records elapsed time. It is frequently used in aviation applications to record the time that the electrical power (battery master switch) is 'on'. Hobbs is a genericized trademark for products generically called Engine Hour Meters. Hobbs meters are made by Honeywell, but there are other manufacturers such as ENM and Datcon. The meters all run electrically and indicate hours and tenths of an hour (1 tenth = 6 minutes,) but there are several ways that a meter can record the 'Hobbs time':

It can simply measure the length of time that the master switch has been on. This tends to overstate Hobbs time.

It can be activated by oil pressure running into a pressure switch and therefore only runs while the engine is running. Many rental aircraft use this method to preclude the possibility of flying with the master switch off to improperly reduce Hobbs time.

It can be activated by another switch, either an airspeed sensing vane under a wing (as in the Cessna Caravan) or a pressure switch attached to the landing gear (as in many twins.) In these cases, the Hobbs only measures the length of time the aircraft is actually flying. This is for those who wish to record Time In Service. Things like Turbine Actual Runtime are kept like this for purposes of overhaul cycles, and are usually kept by commercial operators under FAR Parts 135, 121, or 125.

It can be activated when the engine alternators are online (as in the Cirrus SR series).

General Aviation Use

For purposes of GA (general aviation,) Hobbs time is usually recorded in the pilot's log book, and many Fixed-Base Operations (FBOs) that rent airplanes charge their hourly rate based on Hobbs time. Tach Time is recorded in the engine's log books and is used for such things as determining when the oil should be changed and the TBO (time between overhauls.) Tach (tachometer) time differs from Hobbs time in that it is linked to the revolutions per minute (RPM) of the engine. Tach Time records the length of time at some specific RPM 2400 RPM for example. That means that it is most accurate at cruise RPM, and least accurate while taxiing or stationary with the engine running. At these times, the clock runs slower. Depending on the type of flight, tach time can be 1020% less than Hobbs time. Many organizations such as flying clubs charge by Tach Time so as to differentiate themselves from FBOs by the fact that 10-20% less time recorded makes it 10-20% cheaper to fly (if the hourly rate is the same.)

The technologically advanced equipment that provides the capability for modern warfare demands that people responsible for its maintenance are much more technology literate than any previous generation.

New high performance, fast jet, aircraft systems, such as EFA Typhoon and Joint Strike Fighter (JSF), are defined as half jet, half computer. The maintenance crews of these aircraft will be working with sophisticated computer systems unheard of with today’s legacy aircraft. This, by its very nature, redefines the required maintenance skills and offers new opportunities in the way knowledge is acquired.

Additionally, as the military strives to operate within ever tightening defence budgets, there is less likely to be money available to fund additional pieces of equipment for strictly training purposes. All equipment procured must be available for operations, and it is becoming increasingly common for maintenance technicians to only interact with and gain system knowledge when the new equipment is already in service.

To address these issues, the construct of the maintenance classroom is changing. Where students were primarily taught using text books, wiring diagrams and old or out of service physical equipment, today’s computer literate students utilise Commercial Off The Shelf (COTS) computer-based training devices that provide a desktop ’virtual system’ that looks, feels and reacts exactly like the real system.

Properly managed and modelled virtual maintenance training systems can recreate any complex system, to any level of detail. This is then dependent on a system creating a truly virtual free-play environment that allows the student to view and interact with the system in any way they want, and be confident that the consequences of their actions replicate precisely any interactions with the real equipment.

The real value of such a virtual free-play environment comes when an instructor has the ability to inject faults, the effects of which propagate through the equipment and result in symptoms which can be observed and then diagnosed by the student. This enables students to learn maintenance tasks such as fault isolation/detection, remove/replace procedures, operational/functional check, and maintenance task rehearsals.

This learning experience can be further enhanced by students’ ability to interface real or modelled equipment, such as test sets and prognostic systems, directly with the virtual system. This furthers the learning experience by allowing the maintenance technicians to learn how to operate the tools that they will go on to use in the operational role.

The main benefits of this approach over using real equipment can be summarised as:

1. Increased student throughput - The system is always available to the student. There is no requirement for the real system to be available, enabling maintenance procedures to be replicated many times on many single ‘virtual’ systems, such as high performance, fast jet aircraft.

2. Lower costs - providing real equipment requires a higher initial cost and incurs a high budget to support the in-service life span in terms of spares and repairs to frequently used equipment.

3. Safe training environment - students can not damage the equipment and can learn a job in a potentially harmful working environment without risk to themselves.

4. Ability to inject more realistic faults - Instructors can inject faults with ease and then immediately reset the system for the next task. The faults include diagnostic procedures that would be hard to replicate on real equipment without causing it serious damage.

5. Ability to aid instructor functionality - Instructors can monitor students as they undertake tasks; demonstrate particularly complex procedures for the students on their PC; record student performance and playback for debrief as well as evaluate and store student progress through an integrated learning management system.

6. Team Training Tasks - Many maintenance training tasks require maintenance technicians to work in teams. The virtual maintenance system allows students on individual computers to interact with each other and simultaneously undertake a team training task.

7. Multi-Configuration Scenarios - The majority of new military equipment now requires simultaneous training on a range of variants. An example of this is the JSF which comprises conventional takeoff and landing (CTOL), short takeoff vertical landing (STOVL) and carrier suitable (CV) variants. Systems such as the JSF are also likely to be in service for at least the next 30 years and there will be a requirement to upgrade component systems of the aircraft as technology continues to advance. Using a virtual maintenance training system, the instructor is able to quickly reconfigure the training simulation to any number of concurrent operational builds.

The economic and operational benefits that virtual maintenance training systems can deliver are well proven. However, some – such as VEGA group - believes it is the extent to which these maintenance training systems are now deployed that will determine the level of improved performance in front line equipment.

By: Martin Mcallister

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