[Ref. PPL Instruction Kit Vol1 (306) – 9th Edition, V9.2 P.44-53, Pilot’s Handbook of Aeronautical Knowledge, FAA-H-8083-25B Page 8-10, 8-11, 8-12] Blockages [e.g pitot cover, wasp nests, mud (dirt) and ice formation] OR leaks in the pitot-static sysem will cause inaccuracies in pressure instruments, i.e., the ASI, VSI and altimeter.
Alternate Static Source is located in the cockpit where is usually not the same as the static pressure outside – pressure can be either lower, higher or the same as the actual static pressure. Consequence based on theory !!!
Notes: Before that, let’s review the concept of indicated airspeed (IAS): IAS = Pitot Pressure – Static Pressure IAS = Total Pressure – Static Pressure IAS = (Dynamic Pressure + Static Pressure) – Static Pressure So, now let Dynamic Pressure = 2 ; Static Pressure = 3 @ 1000′, then Total pressure = 2+3 =5 Static vent(s) or line(s) blocked (fixed static pressure = 3):
Pitot tube or line blocked (fixed total pressure = 2+3 =5) – but altimeter (ALT) and VSI will be still functioning correctly:
Monique, a student pilot from Kansas, writes: I think the guys at the hangar are trying to pull one over on me, so I’m hoping you can give me the straight dope: Is there any truth to the story that if your static port gets blocked you can break the glass on the vertical speed indicator to restore the system? “Break glass in case of emergency” was common advice back in the day, and not just for fire alarms. And in some circumstances in an airplane, even today, it might save your life. To understand how and why this is so, we need to do a quick review of static ports and their ecosystems. In a traditional “six-pack” airplane, three of the flight instruments — the airspeed indicator, altimeter, and vertical speed indicator — function by being connected to a system of air hoses called the pitot-static system.  This system of hoses has two openings: The pitot mast and the static port. Keeping it simple for today, the pitot tube is a little arm out on the wing that samples moving air — or more correctly stated — the pressure generated when the plane moves through the air. The static port, on the other hand, is in a nice cozy little spot out of the slipstream, and it measures changes in air pressure as the plane ascends and descends. Actually, that’s badly stated. The port is just a vent that allows the static line to quickly reflect changes in ambient air pressure. It’s the individual instruments that do the measuring. The altimeter uses changes in static pressure to calculate your altitude, while the vertical speed indicator, also called the VSI, uses static air to show how quickly that altitude is changing. Meanwhile, the airspeed indicator compares the static air to the impact air coming through the pitot tube to calculate airspeed. It’s actually the only instrument in the airplane that uses the pitot, so it’s really a bit silly calling the whole affair a pitot-static system. We should call it the static-pitot system, don’t you think? Anyway, moving on… If the static port gets blocked, the air in the static lines doesn’t change with altitude as designed. The altimeter “freezes” at the altitude at which the blockage occurred, the VSI ignores changes in vertical speed, while the airspeed indictor gives wonky readings. All problematic, especially if you’re flying in IMC. This is why many modern general aviation planes have two static ports, or an alternate static source, which is often inside the plane. But there are still plenty of aircraft out there with only one static port and no backup, so what to do if the port gets clogged or blocked? Simple: Just create a new opening in the static system. After all, a static “port” is just a frickin’ hole in the static hose. Hence the advice to break the glass. This is simply a quick way of creating a new static port. Breaking the glass frees the system to respond to changes in air pressure once again. Up is down and down is upBut what the guys didn’t tell you — and something that you really need to know — is if you break the glass on the VSI without completely destroying the instrument, it can read backwards. Why takes a little time to explain…If you buy a VSI on eBay, you’ll find it has two hose connections on the back. One leads to the inner casing of the instrument, the other to a diaphragm that lives inside that casing. The connector that leads into the diaphragm is a wide open hole, while the port that leads into the case itself has a tiny pin hole called a calibrated leak. This architecture allows the diaphragm to respond to changes in air pressure instantly, while causing the air inside the instrument case to lag behind. Similar to how the airspeed indicator compares pitot air to static air, the VSI compares the updated pressure from outside the plane to the delayed pressure inside the case, and uses the difference to measure vertical speed through the atmosphere. It’s really quite clever. So let’s think about how this works with an intact system: Flying straight and level the two pressures are the same, so our VSI shows no climb or descent. But if I throttle up and pitch the nose up into a climb, the air pressure inside the diaphragm instantly drops with the outside air, while the air inside the casing, restricted by the calibrated leak, needs to play catch up. When going up, it holds a higher pressure for a time. Descending, it’s the opposite — the diaphragm is now over-pressurized, compared to the air in the casing. With the glass fractured, however, the calibrated leak is now in the wrong place in the system. With the static air supply coming in from the front of the instrument, instead of from the back, the case over-pressurizes when it would normally be depressurized and vice versa. It reads backwards, showing a climb when you are descending and a descent when you are climbing. Of course, if the glass is completely pulverized, it’s likely that the casing and the diaphragm will be at the same pressure and the instrument will read zero. Meanwhile, thanks to the venturi effect of air flowing around the fuselage in flight, the air pressure inside the cockpit is slightly lower than the air pressure outside, (which is why the static port is on the outside of the plane in the first place), so if you smash the VSI to “restore” your static system, the altimeter will error slightly high using cockpit static, and the airspeed indictor will error slightly fast. How much of a pressure difference we’re talking about is speed-dependent but, given the speeds most of us fly, this is a small issue and it sure beats the heck out of no altitude and speed info in the clouds. Or, if you like VSIs…Actually, any of the pitot-static instruments can be smashed to accomplish emergency static venting, as they are all linked together via a single static line. So why is the standard recommendation to sacrifice the VSI? Some folks say it’s because the VSI is the least needed of the flight instruments. I’m not sure I agree with that, but the VSI isn’t required for instrument flight by the regs, and the other two are, so I guess it makes sense that if we’re going to vote someone off the island, we should make it someone we don’t (legally) need. Or maybe it’s simple practicality. While VSIs aren’t cheap — for certificated airplanes they run between $400 to $800 — altimeters and airspeed indictors cost significantly more. Hammer timeAll of this said, while it’s easy to break a wine glass putting it in the dishwasher, which happened to me last night, intentionally breaking the glass face of a cockpit instrument isn’t as easy as it sounds. Smashing it with your fist is more likely to break your hand than the instrument. So what to do? Look to your elders and plan ahead — many ol’ timers carry those paramedic glass-breaking hammers with them when they fly. Of course, it goes without saying that you should only consider breaking the glass if you have one static port, no alternate, and your static fails in the soup. If it’s a nice VFR day, you can fly just fine on a three pack. And, naturally, this doesn’t work in pressurized aircraft. Oh, right. And it doesn’t work with glass cockpits, either. If you break a $50,000 Garmin G1000 with a hammer, the owner will likely break your face. |
What happens when VSI static port blocked?
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