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Learning Centre: Aircraft General Knowledge - The Pitot/Static System
Posted by Gavin on Sunday, June 25 @ 07:19:55 EST
Training and Education Good morning Airmen

The target for tonight is the Pitot and Static system and I’m going to try and explain it in English for all you new people. Have you ever looked out the left hand side of your favorite C172 and noticed a big chunk of metal coming out from under the wing? I bet you didn’t even notice it and I bet that when your wife pointed it out and asked the question, you told her exactly what it was didn’t you? Yep, well it’s not the fuel drain and it’s not to hold your hat and coat. Although I have seen it used for that purpose a few times down at the aero club. If you said that the Pitot tube was for measuring your speed and altitude you would still be wrong. The Pitot tubes only job is to measure pressure and nothing else. It measures total pressure which is static pressure, that’s measured by the static vent and I will explain the static vent further in a minute. The Pitot tube measures Static Pressure + Dynamic Pressure to = total pressure. Which brings me to the reasons why the Pitot is on the left hand wing as apposed to anywhere else on the airplane? Dynamic pressure, Dynamic is a fancy word for motion and that’s why when you talk about Aerodynamics your talking about motion through the air. Dynamic pressure is the measure of the airflow going into the Pitot tube. That is why the Pitot tube is on the wing and not on the dash board in the cockpit. So why does it have to be on the left? Well it doesn’t but it is a good idea because that little tube is very prone to icing and in American made aircraft the Captain sits in the left seat. So the Pitot tube is on the closest side to where the pilot sits.

Pitot Tube
Pitot Tube

The Pitot tube is also used to determine airspeed by sensing dynamic pressure coming down the tube. Now this is simple enough to understand X amount of dynamic pressure = Y amount of Indicated Airspeed. The reason we call it indicated airspeed and not just airspeed is because it’s an indication of how much air is going through the Pitot tube and not how fast you’re flying or moving relative to the ground. If you have a 60kt headwind when lined up on runway 24 your airspeed indicator is going to indicate 60kt but really you haven’t even let off the breaks. So if we are flying at 160 knots with a headwind of 60 knots we are going roughly 100kt and visa versa for a tailwind component. However there is one more thing to consider for us GA pilots and that’s the effect of altitude on our airspeed. As we climb the air gets thinner which means we have to go faster so that the same set amount of dynamic air molecules entering our Pitot tube that it takes to indicate 100kt stays the same. If we are flying at 1000ft AGL at 100kt our ground speed will be close to 100kt. If we then climb to 5000ft and maintain that 100kt Indicated airspeed we have to fly faster and our ground speed will now be 110kt. We put on our oxygen masks and climb our poor C172 with its WW2 Rolls Royce Merlin V12 turbocharged engine to 30,000 feet and maintain 100kt IAS our ground speed will be closer to 160kt. Now we have one more thing that we have to do and I bet you can’t wait to find out what it is. We have to do a little math and that’s to find our Calibrated True Airspeed. The CTAS is simply indicated airspeed adjusted for altitude and the formula is very simple for rough mental calculation. At 5000ft you would take 8% of your indicated airspeed and add it on top. And then at 10,000ft you take roughly 16% of your indicated airspeed and add it on top. And you continue that as long as you’re climbing. So if you’re flying at 5000tf at 100kt 8% of 100 is 8 and your calibrated true airspeed becomes 108 at 10,000ft your CTAS would be 116kt. Remember its CTAS and not C groundspeed and you also refer to it as just Calibrated airspeed not CTAS like I have. I used that as an example only to better show what was happening with the figures. One last thing this formula increases slightly with altitude but we are GA so we won’t spend too much time in altitudes above 10,000

Now the Pitot tube would be a useless wind tunnel if it wasn’t for the second section of the Pitot Static system. Now just to keep the Piper people happy lets move over to your trusty Piper and check out the Static Vents. That’s right I said Vent’s plural meaning more than one and why do you have more than one? Well for lots of reasons but perhaps the most important is covered later on in more detail. In most Piper aircraft you have two Static Vents one on either side and that’s so if your crabbing your way down to the runway (like most Piper pilots do even on nil wind days) you don’t end up with Static errors caused by being outside the relative airflow. If you look behind the wing on the left hand side you will find a large metallic disk roughly the shape of a chocolate wagon wheel with a small pin hole in it. That is the Static Vent and the Static Vent’s job is to measure the airs pressure in its stable no dynamic state and it is the sum of the static pressure + dynamic pressure that our 3 most important instruments need in order to let us know how high we are and how fast we are going.

As we climb our piper the altimeter does its magic and tells us how high we are, but does it tell us the truth? And does it REALLY tell us how high we are? The answer is NO it doesn’t the altimeter is guessing and will never know how high we are because all the altimeter knows is what the outside air pressure is and that is all it’s telling you when you read off the dial. So we have to do something in order for it to work otherwise it’s just wasting space in the dash board. Some of you will know what the International Standard Atmosphere (ISA) is and why it’s important. For the rest I’m about to explain it a little. It’s important because it has given us a datum to work from and it’s important to have a common datum so that all aircraft in all countries are on the same page. So who got to choose what the datum would be the ISA was determined by a United Nation branch going by the name of ICAO International Civil Aviation Organisation and they decided on these figures as the datum so the International Standard atmosphere is +15c at sea level QNH 1013.25 and there might be a prize to the first person that can tell me what QNH means in the forum. Ok so now we have our datum at sea level and we have something to work with to determine our real altitude above the ground anywhere in the world. Now we all should know that the pressure and temperature at sea level is almost never going to be exactly the same as ISA but that doesn’t matter because we now have something to calibrate our altimeter so that it will read the correct height above ground and we won’t hit any mountains. At the bottom of the altimeter there are a few numbers you may have noticed. These numbers are what you use to calibrate the altimeter and there is a knob at the bottom left corner of the altimeter for you to adjust those numbers. Now the numbers may be 1013 or 29.92 but they mean the exact same thing. One measurement is in hPa and the other in inches of mercury which is what the yanks use most of all. Now when you tune up the tower on radio you will hear something like this. “Jandakot terminal information Charlie wind 240/8 temp 25/7 QNH 1020 and what that means to you is that where you are sitting right now they know the air pressure is 1020hPa. They are giving you the calibration number for your altimeter. Only once those numbers are entered into your altimeter will it then read correctly and the altimeter will read zero feet above ground. So you now have accurate information to where the ground is beneath you even if it’s obscured by cloud or fog. But of course it will only work and be accurate as long as its calibrated which is why there are lowest safe altitudes on your navigational charts and AREA QNH on the weather forecasts. That’s why the first thing you should do when entering the boundary of an airport you want to land on is re-calibrate your altimeter to the local QNH.

Static Vent
Static Vent

Now the Pitot and Static systems consist of about 10 components and they combine to give you your airspeed, altitude and the rate at which you are traveling between differences of pressure using the VSI or Vertical Speed Indicator.

The 9 most common components are as follows.

1. Pitot tube
2. Static Pressure Vents
3. Static Drain
4. Static Line
5. Pitot Line
6. Secondary Static source – in case of blockage these are normally inside the cabin and are prone to large errors. Pilots have also been known to smash the glass on the altimeter when a blockage is detected when there is no secondary static source. This will also then read the cabin pressure once the glass is broken in most cases.
7. Airspeed Indicator (ASI)
8. Altimeter
9. Vertical Speed Indicator (VSI)

The Pitot Static system is very prone to error and caution should be taken to avoid situation that might limit or inhibit 2 of your 4 most important instruments.

Maneuver Induced Error

The Pitot Static system works best when pointed into the relative airflow. This is not always possible and when the Pitot or Static vents are not aligned with the relative airflow large errors can occur. The size of the error is relative to the angle of attack and airspeed. These errors can be large but of short duration.

Altimeter Errors

Most commonly caused by incorrect setting of the QNH subscale so if you have landed at an airport and notice your altimeter reading 500feet higher than the Airport elevation you have set your subscale wrong and should probably been listening to the Local QNH a little bit harder. If you were in IMC lets say it was Dark and foggy, you would most likely be getting scraped off the runway and put into little buckets for having made such an error.

Lag Error

Lag Error is dangerous because reading pressure is not instantaneous in most aircraft. Which for you and me means that if we find our selves doing aerobatics and for what ever reason we cant see the ground and have to rely on the altimeter. We may think we have more altitude than we actually do because it’s a little slow and struggling to catch up. Infact say you are in your F/A 18 and diving towards the earth just for kicks but the RAAF have decided they need a little more money to pay for those expensive F35’s they want so much. They try to save some money by installing a Pitot Static system from your old Cessna 152 into your F/A 18. Our poor RAAF pilot is going to look down at his altimeter and its going to say he’s at 10,000 feet and he wont have to start thinking about pulling up for another couple of seconds. But really he will only be 5000ft above the ground and it’s already too late. You will notice that when looking straight down at the ground weather you’re at 10,000ft or 5000ft there is no notable difference yet. Those little houses are still very little if you know what I mean. So Lag Error is something to be weary of.

These are just a few of the possible errors you can expect to come across. I highly recommend you go and study the rest. It may save your life.

I will also see if I can organise something for the person who can best describe the function of the bottom picture.

Thanks for reading.

Gavin Gillett
gavin at ausflightsim dot net

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