In my previous article (see https://midwestflyer.com/aircraft-systems/), the ignition and charging systems were covered in detail. As mentioned in that article, most pilot applicants, be they private, commercial, instrument or even flight instructor, do not understand aircraft systems. Pilots do not need to have a mechanic’s level of understanding of systems to be safe. However, the more a pilot knows about the aircraft they are flying, the more comfortable he/she tends to be. In this article, I will cover the “Pitot Static System,” the “Vacuum System” and the “Brake System.” The level of knowledge presented here is at or beyond what an examiner would expect in a private pilot applicant.
Pitot Static System Definitions:
“Static Source,” the port or ports from which the surrounding air pressure is measured.
“Alternate Static Source,” a static port located inside the fuselage of an aircraft, for use if the static source is blocked.
“Pitot Tube,” the port from which ram air pressure is measured.
“Aneroid,” a bellows type arrangement used in sensing pressure.
“Altimeter,” a gauge which senses surrounding air pressure and reads altitude.
“Vertical Speed Indicator,” a gauge which measures the pressure rate of change and converts that information to a climb or descent rate.
“Airspeed Indicator,” a gauge which compares static air pressure to ram air pressure and converts that information to airspeed.
The pitot static system gauges, the airspeed and vertical speed indicators, and altimeter, seem fairly straight forward in their operation. It gets complicated once something goes wrong with the pressure sources. The vertical speed indicator (VSI), the altimeter and the airspeed indicator all use the static source or alternate static source as a source of static or surrounding air pressure. The airspeed indicator uses the pitot tube as a source of ram air pressure. All three gauges are comparing the static air pressure with another pressure and converting that difference to a reading.
The altimeter compares static pressure to the pressure in a sealed aneroid. As the aircraft climbs, the static pressure drops, and the pressure trapped in the aneroid becomes relatively greater than the static pressure. The aneroid expands as the static pressure decreases. The expanding aneroid causes the hands on the altimeter to rotate. Setting the altimeter to local pressure adjusts the reading to reflect current conditions.
The vertical speed indicator also compares static pressure to that in a modified aneroid. The aneroid in a vertical speed indicator has a small, calibrated port that is open to the static pressure. When the aircraft climbs, the static pressure lessens. The slightly higher pressure in the aneroid is mostly trapped by the small port, causing the aneroid to expand. It takes about 7 seconds for the static pressure and aneroid pressure to equalize. The 7-second lag is important to remember. A vertical speed indicator will instantly show a trend in pressure change… that is, a climb or descent is shown instantly in the VSI needle, and lag for the rate.
For instance, when a climb is initiated, the vertical speed indicator immediately indicates a climb by the needle deflecting upward. If the rate of climb is held constant, it takes the vertical speed indicator about 7 seconds to stabilize and show +500. This is the same for descents and even leveling off. Expect a 7-second lag for the needle of the vertical speed indicator to catch up to the rate of altitude change.
The airspeed indicator compares the static pressure from the static source with the ram air pressure from the pitot tube.
Assume the aneroid is connected to the pitot tube, and the airspeed gauge housing is connected to the static source. The ram air pressure increases as the airspeed increases. This causes the aneroid to expand. The expanded aneroid drives a geared mechanism and moves the airspeed indicator needle.
Problems arise when the static source or the pitot tube is blocked. If the static source is blocked, all three gauges will be affected. Since the trapped static pressure will not change as the aircraft climbs, both the altimeter and vertical speed indicator will not change readings. The altimeter will remain at airport elevation and the vertical speed indicator will remain at zero. The airspeed indicator will read accurately while accelerating down the runway, and will read low as the aircraft climbs, reaching zero indicated airspeed at about 300 feet above ground level (AGL) for small general aviation aircraft. This is because air pressure drops as altitude increases. The trapped surface level static air pressure quickly equals, then exceeds the ram air pressure as the aircraft climbs into less dense air.
How a blocked pitot tube affects the airspeed indicator depends on the level of blockage. If there is some leakage, the airspeed indicator will read zero. This is what happens during icing if the pitot heat is not activated, or when an insect blocks the pitot tube.
A good friend once had his Cessna 182’s pitot tube blocked by striking a large grasshopper. This resulted in zero indicated airspeed. If the pitot tube has a complete blockage, then the air pressure in the airspeed indicator’s aneroid will remain at the level at which the complete blockage occurred. As an aircraft climbs, the trapped pressure becomes much higher than the static pressure. This results in a higher indicated airspeed than actual. In this case the higher you climb, the higher the airspeed indicates. Some have described this condition as “the airspeed acting as an altimeter.” Complete blockages can be caused by tight fitting pitot tube covers, such as those used during painting or de-icing, being left on. Complete blockages have been contributing factors in fatal accidents with the airlines.
Vacuum System Components Definitions:
“Vacuum Pump,” a device driven by the engine that pulls air through the two instruments.
“Vacuum Gauge,” a meter showing pressure drop in the vacuum system.
“Vacuum Filter,” the filter which the air being sucked into the vacuum system passes.
“Vacuum Regulator,” a device which regulates the maximum vacuum in the vacuum system.
“Directional Gyro” (DG), a vacuum-operated gyroscopic instrument showing compass direction when set.
“Artificial Horizon,” a vacuum-operated gyroscopic instrument showing aircraft attitude and bank angle.
The Operation of the Vacuum System
The vacuum systems in older aircraft have one vacuum pump. All the Cessna single-engine piston aircraft built in 1996 and later have two vacuum pumps. Vacuum pumps are turned by the engine. The pump or pumps suck air through the vacuum air filter, then through the directional gyro and artificial horizon. The air coming through the gyroscopic instruments passes through a nozzle which directs the airflow over the gyroscopes located in the instruments. This air stream spins the gyroscope. A spinning body tends to remain rigid in space.
Think of how a top acts once spun up. The pressure difference or vacuum below the surrounding air pressure is shown on the vacuum gauge. Normal operating range for a vacuum system is from 4.5 – 5.5 pounds per inch. A vacuum pressure regulator manages the system and keeps the pressure within this range. Low pressure will cause the gyros to drift or precess. Higher pressure will cause accelerated wear on the gyroscopic instruments and the vacuum pump or pumps.
When a single vacuum pump fails, the first indication is the vacuum gauge goes to zero. Unless the pilot is very diligent on his/her scan or happens to catch the needle movement of the gauge when it goes to zero, they will not notice the gauge. Without airflow, the gyros will spin down becoming less and less rigid in space. This will result in precession. Precession will cause the artificial horizon to read inaccurately, showing a bank and/or climb in straight and level flight. Precession of the directional gyro will cause the indicated heading to drift. Single-engine Cessna aircraft with two vacuum pumps will show a failure light on the annunciator panel when one pump fails.
A vacuum pump failure in visual meteorological conditions (VMC) is a nuisance, but not a cause for concern. A vacuum pump failure in instrument conditions is an emergency. A pilot experiencing the loss of the artificial horizon and directional gyro in instrument conditions should immediately declare an emergency and proceed with caution.
A good backup instrument is the Garmin G5 Electronic Flight Instrument, which will provide up to 4 hours of power with its internal battery pack. This space-saving, STC’d electronic flight instrument can serve as a standalone primary source for aircraft attitude information or as a directional gyro/horizontal situation indicator. As a primary flight instrument, the G5 combines attitude information with such secondary information as altitude, airspeed and vertical speed in a single digital display that makes flight information easier to scan.
Brake Components & Definitions:
“Hydraulic Fluid,” a light oil used in brake systems. This oil will absorb moisture from the atmosphere.
“Master Cylinder,” a hydraulic fluid pump activated by pressing on the top of a rudder pedal.
“Brake Line,” generally an aluminum tube connecting the master cylinder to the brake housing.
“Brake Housing,” a part attached to the landing gear housing, a brake piston or pistons.
“Brake Calipers,” one floating and one fixed. Brake pads are attached to the calipers.
“Brake Pads,” composite pieces riveted to the calipers. Pads grip the brake disc.
“Brake Disc,” the metal disc, generally stainless steel, attached to the wheel. The brake disc is gripped by the brake pads.
In most fixed gear general aviation aircraft, the brake system is the only hydraulic system on the aircraft. Examiners like asking if there is a hydraulic system on the aircraft, so take note!
When the pilot presses the top of a rudder pedal, the mechanical linkage attached depresses a plunger in the master cylinder, causing the hydraulic fluid to flow through the brake line to the brake housing. This fluid causes the piston or pistons in the housing to extend, pinching the brake disc between the floating or movable brake caliper and the fixed caliper. The brake pads grip the brake disc, adding drag to the wheel.
With piston-powered aircraft, brakes should be used sparingly. To slow a piston-powered aircraft, the first step should be to reduce power, then use brakes if needed. One of my pet peeves is students that taxi with power and brakes. For some reason some pilots feel that 1000 RPMs is the lowest power setting acceptable when taxiing. Remember to throttle back first, then brake!
Recently I reviewed the manual for a Piper M600. This single-engine, pressurized turboprop has on its prelanding checklist, the following: place your toes on the BOTTOM of the rudder pedals prior to touch down. The danger of landing with brakes on is real. Landing with the brakes on will at best result in a flat spot on your tires, and at worst, end up with a blown tire or tires and possible wheel damage.
I would strongly suggest that every pilot incorporate placing their toes on the bottom of the rudder pedals into their pre-landing checklist. I also feel strongly that placing the pilot’s toes on the bottom of the rudder pedals should be done prior to every takeoff. It is very easy to drag the brakes on takeoff. Every Cessna flight instructor has witnessed the left main tire stop spinning immediately after rotation. This is a clear indicator of their student dragging the brakes on takeoff.
Prior to starting the aircraft, the brakes must be tested. The pedal when pressed should be firm with a small amount of travel. Spongy brakes indicate air in the brake line or low brake fluid. The brakes need to be serviced prior to flight if this condition exists.
Those of us who fly in cold weather have a few additional considerations with preflight and brakes. In below-freezing weather, the brake pads can freeze to the brake disc. Test for frozen brakes by pushing the aircraft and observing if the wheels turn. If the wheels slide rather than turn, then the brakes are frozen. On preflight with high-wing aircraft, simply kicking the tire from the front, while holding on to the strut, will break the ice free. If the day is particularly cold, it is possible that the moisture absorbed by the brake fluid could freeze. The pilot can easily tell if the brake fluid is frozen as the brake pedals will not move when pressed. If this is the case, then the brakes will simply NOT WORK! If when you test the brakes, and they are solid, with no movement, do not start the engine, as you will not have any brakes.
Understanding how the pitot static system, the vacuum system and the brake systems operate is important to every pilot. Knowing not only what happens, but why things happen, makes pilots safer. Knowing what to expect when these systems malfunction is especially important for safe flying!
DISCLAIMER: The information contained in this column is the expressed opinion of the author only. Readers are advised to seek the advice of their personal flight instructor, aircraft technician, and others, and refer to the Federal Aviation Regulations, FAA Aeronautical Information Manual, and instructional materials concerning any procedures discussed herein.
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