Hi everybody, very welcome to Mentour and yet another video podcast. As always, I hope you are doing absolutely fantastic. Today on the podcast guys, we're going to be talking about altimeters. How do pilots judge how high they are above the airport and what is a Flight Level? Stay tuned! Right guys, this video is brought to you in cooperation with Brilliant.org. Briliant.org is a website that will help you perfect your mathematical skills and your physics knowledge, something that you might well need when you're applying for a flight school or an airline job. So check them out and, at the end of this video I'm going to have a small nut for you guys to crack, so stay tuned! Right guys, when we're talking about altimeters, and how we as pilots judge altitude, it's important to have a little bit of basic physics knowledge. This is why I keep pressing with you guys that it's so important that you pay atention during your physics lessons, because an altimeter aboard an aircraft, even in a modern aircraft like this, is essentially just measuring the outside pressure. It is just a pressure gauge. The technology we're using is pretty much the same as the first aviators were using, as the aircraft had back in the 60s and 70s. We're using the same thing. Arround the aircraft, you can see them on the side of the aircraft, close to the nose section, there are static inlets and those take out the static pressure, outside of the aircraft, and we use that to convert it in to altitude. So, the first thing that you need to understand is that, when it comes to altimeters, the reason that we're using pressure to measure our altitude is because the atmosphere has a really really nice feature of having a linear decrease of pressure as you climb. Basically for every thirty feet that you climb, the pressure is going to drop with aproximately 1 hectopascal (hPa). Alright, that's important to remember. So for each 30 feet you go up, it's going to drop 1 hPa. Now, measuring the altitude of the aircraft would been really really easy if the atmosphere was completely uniform over the globe. But that is not the case. The atmosphere essencially looks like the surface of the Earth, where you have high mountains and deep valleys. In the case of the atmosphere, those mountains and valleys are high pressures or low pressures, which I'm sure you've heard of if you've ever watched The Weather Channel. But how does that affect us? Well, since our altimeters are essencially pressure gauges, it also means that it doesn't really know where we are to start with. The only thing that the altimeter knows is if the pressure is decreasing or increasing. Now, what we want to know obviously, is exactly how high we are, so we use something called QNH, which is essncially the local pressure we have at, for example, the airport of departure. Because it is changing all the time, as those valleys and mountains are moving over the globe, you might have high pressure on the airport one day and a low pressure on the airport another day, we don't need to know exactly what the reference pressure is all the time. So, if you've listened to Air Traffic Control (ATC), you've heard them say QNH quite a few times. Basically, every time an aircraft gets cleared to an altitude, they will be getting the local QNH. Every time we're listening to the ATIS (Automatic Terminal Information Service), we will also get the QNH. What we do then is we put that value which might be, say QNH is 1009 hPa. When we get that, we enter that into our altimeter and when we do that, if we have the correct pressure setting, then the altitude that we read here on our altimeter is going to show us our altitude above sea level. So QNH will give us the pressure setting to know how high we are above sea level with reference to the airport we are at this time. Now why that is interesting to know is because the QNH obviously is different between different airports. For example, today the QNH here in Girona is 1015 hPA, and I'm just about to start flying, we're gonna fly to Baden-Baden in Germany, and the QNH there is going to be much lower, it's gonna be 1002 hPa. It is really important that we use the correct pressure setting at the airport we're at, because otherwise our altimeters aren't gonna show us where we are. If I, for example, sit here with QNH 1015, and then I wait for a few hours and the pressure drops, then the altimeter will assume that I've climbed during that time and it will show me, if has dropped for example 3 hPa, then it will show that I'm 90 feet higher, that what I actually am. This is why it is so important that we continuously update the pressure setting when we are either departing from an airport or descending into an airport. That then brings us to Flight Levels. What is a Flight Level? As you might have understood by now, if the pressure differs between different airports, and we have loads of aircrafts taking off and landing at those airports and let's say you're taking off from one airport, and then you're flying towards somewhere else in Europe for example. Well, then you're going to meet other aircrafts that are flying from other airports where the pressure was different. So, unless we continuously update the pressure as we fly along our route, we will not know how much of a difference and altitude we are from other aircrafts, because they might be coming from an airport with a completely different QNH. It's not really practical to be continuously updating the local pressure, and it is not really interesting either because we're flying so high that knowing how high we are above the terrain is not really interenting anymore. What is interesting is being absolutely sure that we have vertical separation we need from other aircrafts. The way that that has been solved in that we have to set, above a specific altitude, the Transition Altitude, we will all set the same pressure. All aircrafts that are flying higher than that altitude will set the same standard pressure: QNH 1013.25 hPa. That is the standard pressure. Whenever you hear an aircraft being cleared to a Flight Level, if we are for example, let's say we're at 3000 feet maintaining, then we've been using the local QNH up until then. Then ATC tells us: "Climb now FL200". Then what I will do is I will set FL200 here, confirm it with my First Officer, check it, we'll start climbing, and I will then set Standard (STD). When I set STD, we are now climbing towards a Flight Level. That means that, all over the world, when you hear Flight Level, all of those aircrafts are flying on the same pressure setting, 1013 hPa, to make sure that everyone they meet have at least the vertical separation distance that is needed in order for safe flight. That is the difference between QNH, which will give you the correct altitude above sea level, and Flight Level, which will just give you a knowledge of how much vertical separation you have from other aircrafts. It will not tell you your correct altitude above ground. We do have other means in the cockpit as well of knowing how high we are, for example we have the radio altimeter, but the radio altimeter only works from 2500 feet and down. So it is not acurate, it's not even showing above that. The times that we use the radio altimeter is basically when we're doing a Cat II or Cat III ILS (Intrument Landing System), and if you're interested in knowing how he we fly a Cat II or a Cat III ILS, I have a collection inside of the Mentour Aviation app that you can get and you can see how I am actually flying one of those approaches including the go-arround. It's a quite good one so check it out. Now, towards the end here, I promised you that I had a question for you. There's a bit of a problem for you to solve. I've been talking a little bit about how we measure altitude and how the pressure is decreasing with increasing altitude. So, I was going into Brilliant.org the other day and I checked through their gravitational physics course, and in there there was a problem where they ask me how high do you need to climb in order to reach the edge of space. The edge of space is defined as when the pressure is 0.1 % of the pressure at the surface of the Earth. That's my question for you guys: How high do you need to climb in order for your pressure to be 0.1 % of the pressure you are feeling right now. I want you to answer here in the comments, or you can send it in via Facebook or my Mentour Aviation app, and if you want to know the answer to this question, then check out the link in the description of the video, and it is going to show you both how to calculate this problem and give you a little bit of background knowledge about it. Check that out guys and as always, I hope you've enjoyed this video and I hope that you have subscribed to the channel and make sure that you highlight the little notification bell, otherwise you won't know when I'm doing livestreams, and when I'm posting new videos, which I'm doing a little bit more regularly now. So, have a fantastic day, wherever you are, I am going to take this baby flying now. Take care and I'll see you next time, byebye! Subtitles by Pedro Almeida
(from Portugal)
