Vacuum System
DEFINITION
The vacuum system includes the equipment and instruments (AI, HI) that use the directed flow of air to drive gyroscopes to indicate attitude and heading.
SAFETY FACTORS
Thorough understanding of normal operating characteristics of the vacuum system, adherence to checklists including the vacuum system instruments and ability to detect vacuum system failure is essential to safe flight.
· Proper, regular setting of HI to agree with magnetic compass necessary for safe, accurate navigation
· Additional electric vacuum pump recommended to back-up engine driven vacuum pump
· Over-dependence on AI may lead to difficulty with partial panel flight control after vacuum system failure
TOLERANCES
Instrument Rating PTS (FAA-S-8081-4D)
II. B. 1. Exhibit adequate knowledge of the elements related to applicable aircraft flight instrument systems and their operating characteristics to include
e. attitude indicator
i. heading indicator
k. vacuum systems
II. C. Instrument Cockpit Check
1. Exhibit adequate knowledge of the elements related to preflighting instruments, avionics and navigation equipment cockpit check by explaining the reasons for the check and how to detect possible defects
2. Perform the preflight on instruments, avionics and navigation equipment cockpit check by following the checklist appropriate to the aircraft flown
3. Determine that the aircraft is in condition for safe instrument flight including
d. heading indicator
e. attitude indicator
k. power source for gyro instruments
4. Note any discrepancies and determine whether the aircraft is safe for instrument flight or requires maintenance
OBJECTIVES
To develop the student's knowledge of the vacuum system to meet the FAA Practical Test Standards. To develop the student's understanding of vacuum system instruments (AI, HI) operating characteristics, the habit of proper preflight instrument checks and ability to detect abnormal or unsafe operation.
PROCEDURES
Lecture/Discussion:
a. Vacuum system
AI and HI gyros driven by engine-operated vacuum pump
Monitor suction gauge (4.5 - 5.4 inHg)
Additional back-up electric vacuum pump recommended
Cessna TR182 vacuum system
b.attitude indicator (AI)(attitude instrument, artificial horizon, gyro horizon)
1)Gyroscopic rigidityin space
2) Operating mechanism: small brass wheel (gyroscope) with a vertical spin axis
a) Stream of air (via vacuum system) (C-TR182), or
b) Electric motor
3)Horizon diskfixed to gimbals remains in same plane as gyro
a) Pitch marks
b) Bank index
4)Symbolic aicraftmounted on instrument case over horizon disk
5)Erection mechanism(2 to 5 minutes)
6) Older gyroscaging mechanism
7) Errors
a) Slight nose up or down during rapid acceleration or deceleration respectively
b) Possible small bank and pitch errors after 180 degree turn
c) Errors are small and correct themselves within a minute or so in SLF
c.Heading indicator (HI, directional gyro, DG)
a) HI gyroscope
i) Double gimbal mounted
ii)Horizontalspin axis - senses rotation about aircraft vertical axis
iii) Rigidity in space causes HI to maintain heading indication without the oscillation and other errors inherent in the magnetic compass
b) Must be periodically (every15 minutes) set to the heading indicated by the magnetic compass
c) Usually air-driven by vacuum system
COMMON ERRORS
· Failure to study POH for details of vacuum system(s) in a specific aircraft
· Failure to complete preflight vacuum system instuments check
· Failure to set HI to magnetic compass before takeoff, before approach and every 15 minutes
· Failure to detect abnormal operation or failure of vacuum system
· Over-dependence on AI leading to difficulty with partial panel flight control after vacuum system failure
References
14 CFR parts61and91
Instrument Flight Maneuvers 4th Edition, Gleim, 2004
Instrument Flying Handbook, FAA-H-8083-15, 1999
Instrument Rating PTS, FAA-S-8081-4D, April 2004
POH for aircraft flown (e.g., C-TR182)