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Theory of Operation (Resistive FQM Systems)

Theory of Operation (Resistive FQM Systems)

 

For the purposes of discussing the operation of the Resistive Fuel Quantity Measurement System, in the paragraph below, the following assumptions are to be made:

 

1.   The aircraft utilizes a 28 Volt DC Power Supply

2.   The meter movement within the indicator is the most common type used,      which is a 100mv, d’arsonval type. This means that it requires 100mv of  electrical current to drive the pointer on the meter movement from zero  deflection to full deflection.

When power (V+) is applied to one side the resistive element and ground is applied to the other end and the desired maximum electrical current is 100mv, then Ohm’s Law can be applied in order to determine the required value for the resistive element as follows:

R = E/I

Where R= Resistance, E= Voltage and I = Current.

Since the values of E and I are known, 28 Volts DC and 100mv respectively, then R is resolved as follows:

28/.1 = 280

Therefore the value of the resistive element to be used is equal to 280 Ohms at its minimum value, which would equate to a maximum current flow of 100mv.

A reasonable design, would dictate that the system would change 1mv for each 1% of fuel used. Using that model then 1mv (1% of the fuel remains) would be our zero point on our indicator. Once again Ohm’s Law dictates the value of our resistive element as follows:

28/.01 = 2800 Ohms

Therefore our resistive element must be variable from 280 ohms up to 2,800 Ohms.

The operation of the system is now very straight forward, when the fuel tank is empty the float is at its very lowest point and the wiper resistive element follows the float. Therefore the wiper is at the point of the resistor which is closest to ground and furthest from the source of power or 2800 Ohms. This would provide only 1mv of power to the meter movement and only 1% deflection of the pointer or our Zero (EMPTY) setting.

When the fuel tank is full the float and wiper both will be at their highest point, closest to the source of power or 280 Ohms. This would provide 100mv of power to our meter movement and 100% deflection of the pointer or our FULL setting.

As the voltage is constant and both electrical current and resistance are perfectly linear then our pointer travel will also be perfectly linear.

 

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Get to Know the Fuel Quantity Indicator

Get to Know the Fuel Quantity Indicator

 

The indicator for a resistive fuel quantity system is typically a very simple instrument consisting mainly of a meter movement housed within a standard 3-1/8” case. On occasion the indicator may have some signal conditioning element within it, however that is quite rare and when it does occur even that will be a very simple voltage divider or a single stage amplifier.

The most common meter movement used in these indicators is of the d’arsonval type and which is typically 100 milli-volts full scale.

 

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Fuel Measuring Systems- Tank Unit

Fuel Measuring Systems- Tank Unit

 

The tank unit is the component which gives the system its name “Resistive Fuel Quantity Measuring System”. The tank unit is comprised of a float which is mechanically linked to a variable resistive element. The float rides on top of the fuel and will move up or down depending on the level of fuel within the tank. The floats movement is coupled via a linkage arm and gear assembly to a variable resistive element which then mimics the movement of the float.

The variable resistive element is comprised of two parts; a resistive strip, typically manufactured with Nichrome wire, and a wiper assembly. Nichrome wire has a predictable and stable resistance to electricity per inch of wire. Therefore it is possible to cut and form a piece of nichrome wire to a very exact electrical resistance value simply by adjusting the length of wire.

The wiper assembly is a moveable electrical contact which slides across the length of the nichrome wire while making direct physical contact with the wire. In our fuel measuring system one end of the nichrome wire will be connected to the aircraft power source and the other end will be connected to ground. The wiper will tap a varying amount of electricity off of the nichrome due to its physical contact. The amount of electricity that is tapped is determined by the overall resistive value of the wire and precisely where on the wire that the wiper is making physical contact. If the wiper has moved 24% down the length of the wire then 75% of the electricity is tapped off; at 50% movement then 50% is tapped off; at 75% movement then 25% is tapped off. This relationship is very predictable and very consistent and operates precisely under the principles of Ohm’s Law (E=I/R).

The electricity that is tapped off by the wiper is then transmitted by wire directly to the Fuel Quantity Indicator and is used to drive the pointer indicating fuel qty.

 

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Fuel Measuring Systems- Regulated Power Source

Fuel Measuring Systems-Regulated Power Source

 

In most resistive type fuel qty. systems the aircrafts own power source (14 VDC or 28VDC) is used as the power for the quantity system. While the aircraft power is reasonably well regulated it is not a precise regulation, hence the first negative for resistive systems. Aircraft power can vary from approximately 12.6 volts up to 17 volts on a nominally 14 volt aircraft. On a 28 volt aircraft the power supply can vary from 24 volts up to 31 volts. The aforementioned variances are under normal operating conditions, if the power supply is experiencing technical problems then the variance can be significantly more.

 

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Overview of Resistive Type Fuel Qty. Measuring Systems

Overview of Resistive Type Fuel Qty. Measuring System

 

The Resistive Type Fuel Quantity measuring system utilizes a variable resistive element in order to vary a precise electrical DC voltage based on the quantity of fuel in the fuel tank. The varying electrical signal, in turn, is used to drive the pointer on a fuel quantity indicator in a manner which is proportional to the amount of fuel in the tank, thereby visibly indicating remaining fuel quantity to the pilot.

The typical components in this type of system include the following:

a) Regulated DC Power Source
b) Tank Unit (Fuel Qty Sender, Fuel Probe)
c)  Indicator

Stay tuned to the TGH Aviation for additional information pertaining to these components

 

 

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General Overview Fuel Measuring Systems

General Overview Fuel Flow Systems

 

The purpose of the Fuel Quantity Measuring System is to provide the pilot with a visual representation of the amount of useable fuel which is currently being held within the aircrafts fuel tanks.

There are two basic methods of providing this measurement:

Resistive Type Measuring Systems

Capacitive Type Measuring Systems

Both of these systems provide fuel quantity information by varying a precisely controlled electrical signal. However the method of varying that signal is considerably different between the two. In general, while resistive type systems are reasonably accurate and reliable they do not provide the high level precision and extended long term reliability that is provided by the capacitive type system. As a result, in modern aircraft, resistive systems are typically relegated to lower cost, lower performance aircraft while higher cost, and higher performance aircraft will almost always utilize the capacitive systems.

This training document will provide a general knowledge about both of these systems without going too deeply into the engineering and technical aspects of either system. This document is not intended to provide technical training at a level that would permit the student to perform repairs or maintenance on this system but rather is intended to provide a general knowledge of those systems at a level required by a Sales or Marketing person. Therefore only following subjects will be addressed:

a) A General Theory of System Operation
b)Components of the system
c) System pros and cons
d) Typical failure modes
e) Typical end users
f) Common manufacturers

 

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Airspeeds- Selecting a Replacement Indicator

Airspeeds – Selecting a Replacement Indicator

 

Typically, you can replace any airspeed indicator with another manufacturer’s indicator of the same type and range. However, there are some considerations, which should be taken into account.

Aerosonic and Kollsman indicators often have a longer case than other manufacturers, this could cause an installation problem in some aircraft.

On typical indicators the Pitot, input will be in the center of the back of the indicator, with the Static input to the right, when viewed from the rear. Occasionally you will find some older indicators where this was reversed. This may cause installation difficulties on some aircraft.

If the customer has requested a specific range and we do not have that range, you may offer a different range. However, you do not want to vary by a large amount. For example, an alternate range for 0-200 might be 0-180 or 0-240. Unacceptable alternates for 0-200 would be 0-150 or 0-300.

Airspeeds- Frequently Asked Questions

Airspeeds – Frequently Asked Questions

 

What would cause my airspeed indicator to be stuck at zero all the time?

A common cause of this is a problem with the Pitot source. If the line from the Pitot tube has disconnected, then pressure will not be applied to the instrument.

If the Pitot tube is clogged, then pressure will not be applied to the instrument.
It is common for Leaf Bugs to build nests in Pitot tubes and effectively clog them.

 

What would cause my indicator to be stuck at a high speed all the time?

A common cause of this is over pressure being applied to the Pitot tube. Aircraft are often washed using a high-pressure water hose. A careless mechanic may spray the hose across the Pitot tube applying high pressure to the air speed indicator. This will usually destroy the diaphragm. Units damaged in this way normally cannot be repaired and must be replaced.

 

Can you re-screen and/or range mark my dial?

Yes, we can. The repair department will advise you of the fee for this process. Screening dials is not done in house, this function is done at an outside vendor. The repair shop will advise the lead time. In order to ensure that the proper range marks are applied we require a copy of the airspeed specifications that are in the Pilot’s Operating Handbook.

 

The temperature bulb on my true airspeed indicator is broken can you fix it

No, we cannot. The bulb on a true air speed indicator is filled with gas. All of the gas escaped when the bulb broke. The bulb must be replaced. However, these bulbs are extremely expensive and extremely difficult to find.
Is the indicator you are selling me, Tso’d ?

United Instruments, Edo-Aire, Kollsman, Garwin, Aerosonic – Yes

UMA – No

If an instrument is certified to a TSO, it must state such on the ID plate. If it does not state the TSO on the name plate then you must assume that it is not approved to the TSO.

 

What is a TSO ?

TSO stands for Technical Standard Order. This is an FAA document, which, defines how a specific type of instrument should work in order to be considered airworthy.

 

What is the tolerance (accuracy) on an airspeed indicator?

Typically +/-2 (Knots or Miles per hour) up to 200 after which it is typically +/-3.

 

Why does it cost so much to overhaul my airspeed indicator?

Overhauling an airspeed indicator is not as simple as you would think. A typical overhaul includes all of the following steps:

Total dis-assembly and inspection.
Determine and correct the cause of failure.
Ultrasonic cleaning of all jewels, pivots, and gears.
Reassemble internal components.
Silk-screen dial and/or range mark
Calibrate the unit to manufacturers and/or FAA specifications
Assemble unit into case.
Seal Case.
Verify calibration of final assembly.
Accomplish cosmetic touch-up.
Complete all FAA mandated paperwork and certifications.

The typical time required to overhaul an airspeed indicator can range from 1 hour for a simple unit to over 3 hours for complex units.

Calibration of an airspeed indicator is a complex process. Airspeed is a non-linear function, which must be displayed on a linear dial. There are at minimum seven calibration points that are all interactive with each other.

Airspeeds – Get to Know the Operation and Manufactures

Airspeeds – Get to Know the Operation and Manufactures

 

How it works

Pitot pressure is forced into the diaphragm causing it to expand like a balloon. Static pressure is contained within the indicator case and surrounding the diaphragm. As the static pressure changes it, will either cause the diaphragm to compress, as the aircraft loses altitude or allow it to expand as the aircraft gains altitude. This expansion and contraction of the diaphragm is mechanically linked to the pointer causing it to move around the dial thereby displaying the speed of the aircraft as a function of the difference between the Pitot and static pressures.

 

Range Marks

Range marks are a reminder to the pilot of the aircrafts basic operating envelope as it pertains to airspeed. Typical range marks found on an air speed dial are:

White Arc – VFE This is the maximum speed at which the aircraft can operate safely with the flaps extended.

Green Arc –  This is the normal operating range

Yellow (Orange) Arc – Caution

Red Radial – VNE Never exceed speed

Blue Radial – This is the minimum operating speed using one engine on a two engine aircraft.

Note: Maneuvering speed is not marked on the dial, it is normally on a placard, which is located on the instrument panel.

 

Manufactures

The following companies all have manufactured air speed indicators and are the most common that you will see:
United Instruments

Kollsman
Aerosonic
Aeromarine
McCleod
UMA
Garwin
Edo-Aire

 

Airspeeds- Basic Principle of Operation

Airspeeds- Basic Principle of Operation

 

Main Components

The major components of an air speed indicator are:

  1. Case
  2. Diaphragm
  3. Dial
  4. Pointer
  5. Mechanical Linkage
  6. Hair Spring
  7. Jewels and Pivots


Case
A standard air speed indicator for general aviation comes in a 31/8  inch diameter case. This is a standard size for most general aviation indicators. It is important that an air speed case be air tight as the case contains the static pressure input.

A leaky air speed case will cause the indicator to give erroneous readings. There are two input ports on the back of the case. These ports are the Pitot port and the Static port. These ports are connected to aircraft Pitot tube and Static port respectively.

 

Diaphragm
A diaphragm is essentially a balloon made of very thin metal. Typical metals used for this purpose are copper or brass. The diaphragm is sealed at all points except for one very thin pressure input tube. This tube is located directly in the center of the back face of the diaphragm. The pressure tube is connected directly to the Pitot port. The thickness of the metal used in manufacturing the diaphragm will determine the range of the air speed indicator (thin = low speed, thick = high speed). It is important to note that this diaphragm is extremely delicate. Picking up a unit a blowing air into the input port will damage the diaphragm.

 

Dial
The dial will contain the speed information and any pertinent range marks.

 

Pointer
The pointer points to the current speed of the aircraft as indicated on the dial.

 

Mechanical Linkage
The mechanical linkage connects the pointer to the diaphragm.

 

Hair Spring
The hair spring returns the pointer to zero when pressure is removed from the indicator.

 

Jewels and Pivots
The pivot is the spindle or axle for the pointer. Jewels are glass bearings on which the pivot rotates.