How Aircraft Cabin Pressurization works?

Cabin Pressurization



You must be wondering how you are able to breathe comfortably on a flight from New Delhi to Texas at around 30,000 - 40,000 feet when a well-trained mountaineer finds it difficult to breathe even with an oxygen cylinder at 29,000 feet.


How Aircraft Cabin Pressurization works?


The science behind difficulty to breathe at high pressure

When we move above from the sea level the pressure of air starts decreasing. Though the concentration of air remains to be same at around 70% but as the pressure of air is decreasing hence the molecules of oxygen stretch away from each other making it difficult for human beings to breathe. Just imagine eating jelly from a cup with particles of jelly broken into small pieces and in another case the jelly is in one solid state. Which one would be tastier? The same way our body likes to breathe from the air in which oxygen molecules are closely packed.

Here it is very clear that it becomes more and more difficult to breathe at higher altitudes but according to the flight data received for various flights across the globe, it can be very easily seen that aircraft fly best at the higher altitudes.

Humans love to breathe near ground but aircraft love to fly away from the ground. 

To solve this complex problem which looks very simple aviation industry came out with an idea of cabin pressurization. Assuming that all of you are aware of the basic working of an aircraft engine and various main stages involved in the working of a jet engine. If not I will give a brief introduction of the part of working of an aircraft engine which is of our interest. The ambient air is sucked inside the engine through the fan and after that, it is compressed at high temperature and pressure so that oxygen molecules come closer and facilitate easier burning of the air and fuel mixture.

Moving back to our problem of low-pressure air inside the cabin making it difficult for the passengers -to breathe. The compression process is carried about in a number of stages varying from 7-10 depending on the engine. Now after passing through each compression stage the pressure and temperature gradually. Some amount of air from 6th and 7th stages of compression is taken out from the compressor and is cooled and filtered so that it becomes healthy for the breathing practice of human beings. This is the air you breathe in an aircraft engine while you are flying above the might Mount Everest.


How Aircraft Cabin Pressurization works?


Physics behind the process

The aircraft refrigeration system consists of four basic components.

1. Heat exchanger
2. Cooling turbine
3. Cooling turbine
4. Cooling fan

As you can see in the image that the compressed air from the main compressor and then sent through a heat exchanger where it cools down by giving heat to the ambient air. Now, this cool air is sent to the cooling turbine where it is expanded so that its pressure can be taken down to a value which is most comfortable for humans to breathe easily. The breathable air is then sent to the cabin. The work taken out by expanding and cooling the air from the heat exchanger is used to power a fan which cools the down the heated in the process of cooling the air to be sent to the cabin. The cooling turbine not only lowers the pressure of air but also further reduces the temperature of the air to be sent to the cabin.

What if cabin pressurization fails?

You all must have came across one of the most heated debate of aviation industry these days, the deployment of oxygen masks in a Jet Airways flight due to low cabin pressure. The reason as quoted by various news agencies that the cabin crew forgot to switch on the cabin pressurization switch.


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Suggested article: Dogfights of world war 1 & 2

Infamous Dogfights of World War I & II

Infamous Dogfights of World War I & II


Dogfight of world war 1 ans world war 2

For most of you American Football fanatics out there, Dogfighting is very much like a combination of various offensive and defensive plays that are formulated during gameplay. Just as a receiver or a pass rusher is asked to run a route, it is with that same ideology that is applied to an aircraft in pursuit of enemies or in pursuit of protecting their fellow comrades. 

Dogfighting isn't duck soup

Don't take my word for it but take a look at history itself. We've witnessed several accounts of dogfighting over a span of 100 years! Yes, ten decades of consummate aerial dominance and aerodynamical prowess. The first ever documented aerial warfare had taken place in Mexico in the year 1913, during the Mexican Revolution. Two remarkable pilots who went by the names of Dean Ivan Lamb and Phil Rader had carried out their aerial dispute with a volley of bullets but surprisingly, neither one of them wanted to kill each other so they decided to intentionally miss their fellow targets. I wonder why? Well, one thing's for sure; we have sure come a long way into understanding the concept of dogfighting. Here's a look at some of the most paramount dogfighting battles of the century:

Battle Of Cer (Balkan Wars)

Most aviation pioneers knew the first dogfight had taken place during the Mexican Revolution but the dawn of Dogfighting had approached humanity as a whole in the backdrop of World War I. The first aerial dogfight had taken place during the Balkan Wars over Mišar in Western Serbia. Although this battle hadn't lasted very long, it played a significant role in impacting the world of aviation. Sergeant Miodrag Tomić, a Serbian/Yugoslavian pilot from the 1st Danube Division, was flying his Blériot and was on the verge of completing a reconnaissance mission when he ended up coming face-to-face with an Austrian-Hungarian enemy pilot. Although the initial confrontation showed no signs of animosity within each other, the enemy pilot had decided to pull a pistol and fire at Miodrag. Miodrag then fired back with his own pistol and thus, in the midst of it all, the world's first dogfight had transpired. Miodrag had escaped from this surprise attack but more importantly, within weeks, this iconic confrontation led to the addition of machine guns on top of the Serbian aircraft.

Dogfight of world war 1 ans world war 2

At the cockpit: Sergeant Miodrag Tomić (Left) and Lt. Milutin Mihajlović (Right) aboard the Blériot XI 'STORM'


Spanish Civil War (Rise of the Wingman Tactic)


Dogfight of world war 1 ans world war 2
A Spanish Republican soldier as seen here talks to a journalist. We can see Mr Ernest Hemingway at the back (back facing the camera) who served as a war correspondent during the Spanish Civil War.

During the years 1936 to 1939, Spain had seen a rise in confrontations between the Spanish Republicans and the Spanish Nationalists/Communists. The series of events had led to a full-fledged battle of the aerial aces. The advent of the Spanish Civil War had given birth to a new air combat strategy. Werner Mölders, a Nazi Luftwaffe pilot who had consociated himself with the Condor Legion (Army and Air Force unit of Nazi Germany who sided with the Spanish Nationalists) had come up with the idea of adding a Wingman to the dogfighting formation. Wingmen were imperative in defending and assisting the lead Aircraft during the war. Werner had also suggested that these two aircraft must be kept at a tolerable distance from each other instead of using conventional tight 'V' formations. Werner had also incorporated a system of pilot training during the night! The amalgamation of all these new tactics had been instilled in the Messerschmitt Bf 109 Luftwaffe Aircraft. It is with these very BF 109 Aircraft that helped annihilate the Spanish Republicans' Spitfires and Hurricanes. 


Dogfight of world war 1 ans world war 2
                                                   
Messerschmitt Bf 109 Aircraft


Dogfight of world war 1 ans world war 2

Werner Mölders (Left)


Battle of Britain (Domino effect of Dogfighting during World War II)


Dogfight of world war 1 ans world war 2


Right after the Spanish Civil War, World War II had taken over and dogfighting was prevalent. Nazi Germany's Luftwaffe constantly provoked the United Kingdom which in turn led to the retaliation by the Royal Air Force. This large-scale military campaign had led to the 'Battle of Britain' that consisted of the following belligerents: United Kingdom (Allies), Canada (Allies), Germany (Axis), and Italy (Axis). 
This battle had been a platform for inculcating and developing numerous tactical innovations in order to achieve the zenith of supreme dogfighting prowess. The German Luftwaffe had come up with various aerial formations such as the Rotte (Pack) formation wherein the Rottenführer (Leader) is followed by the Rottenhund (Pack Dogs) at an approximate distance of 200 meters. This allowed effective formation assessment by the pilots and avoided any time delays. It rather focussed on maintaining a close vigilance on a fellow pilot's blind spots. 
The Royal Air Force, on the other hand, had used a Vic Formation tactic in order to keep a tight formation of 4 sections each. Although the Luftwaffe did call it the Idiotenreihen (“rows of idiots”), the formation is used even to this day! The RAF eventually changed the formation to a tight finger-four formation similar to the one used by the Luftwaffe.


Dogfight of world war 1 ans world war 2

Vic Formation used by the Royal Air Force



Dogfight of world war 1 ans world war 2

Battle of the Aces! Spitfire and Heinkel He 111 go head-to-head.
(This picture was captured in 1940 during the Battle of Britain.)



Dogfight of world war 1 ans world war 2

As seen here, Group Captain Sir Douglas Bader sits on his beloved Hurricane. Bader was known for his Aerial Supremacy during the Battle of Britain. The movie 'Reach For The Sky' was a biographical film made in honour of Douglas Bader.


Pearl Harbour (The Battle of Midway and the rise of the 'Thach Weave' manoeuver)

After the attack on Pearl Harbour by the Imperial Japanese Navy Air Service, the United States had directly entered into World War II. This retaliation had given rise to a whole new series of Aerial Dogfights. One such series included the battle between Mitsubishi A6M "Zero" aircraft and the Grumman F4F Wildcats. This dogfight had taken place in 1942 during The Battle of Midway in the Pacific Ocean and its islands. Between the two aircraft, Zeros were considered to be more aerodynamically superior compared to the WildcatsLieutenant Commander John S. "Jimmy" Thach was one of the first pioneers to come up with a tactical plan in order to surpass Japan's Zeros. He ended up coming with a tactic called the Thach Weave with the help of matchsticks on a table. Hence, this tactic was consequently applied in the war. Although most of them deemed it to be less probable, the outcome was beneficial. Most of these Zeros were capable of flying at 295 mph and hence, the tight turns would augment the inefficiencies of the aircraft's manoeuvrability.  This allowed the Wildcats to execute the Thach Weave formation and leave the enemy plane vulnerable to attacks.

Dogfight of world war 1 ans world war 2

Grumman F4F Wildcat in action!

Dogfight of world war 1 ans world war 2

Mitsubishi A6M Zeros in action!

Dogfight of world war 1 ans world war 2

A standard Thach Weave Formation carried out as shown in the figure above.
(Pink denotes Japan's Zeros and Blue denotes the Wildcats)



And thus concludes some of the iconic dogfights that had taken place in the midst of World War I and II. If you'd like to check out more about dogfighting, then I'd suggest you click on the links below:






Until then, stay Turbocharged!

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Bell Boeing V22 Osprey

Bell Boeing V22 Osprey

Bell Boeing V-22 Osprey

© Chief Petty Officer Joe Kane (U.S. Navy)

Operation Eagle Claw - A rescue mission in the 1980s for 52 embassy staff held captive in Iran had failed when a chopper struck a USAF C-130 killing eight servicemen. This mission might be one of those missions, whose failure led to the birth of the most unique aircraft in the aviation industry - Bell Boeing V-22 Osprey.

Design : 


The Bell Boeing V-22 Osprey is the world's first tiltrotor aircraft. It has one three-bladed proprietor, turboprop engine, and transmission nacelle mounted on each wingtip. It might be the first of its kind which has both VTOL (Vertical Takeoff and Landing) and STOL (Short Takeoff and Landing) capabilities. This means that it takes takeoff like a fixed-wing aircraft with a small runway or like a helicopter if no runway is available at all. If it operates like a helicopter, the nacelles are in vertical position and the rotors are in a horizontal position. Once the aircraft takes off, the nacelles tilt forward 90° in 12 seconds for horizontal flight. This technique helps the V-22 Osprey to be a more fuel efficient and higher speed turboprop aircraft. With empty payload, it can fly up to 2100 nautical miles (ferry flight) and with payload, it can fly up to 1100 nautical miles (operational range). The Osprey is powered by 2 Rolls-Royce AE 1107C engines which can power the proprietors both through the wings and driveshaft.



The Osprey has a modern glass cockpit with night-vision-goggle-compatibility and multi-function displays that display the navigation and vital system information. The flight controls consist of a central control stick, thrust control levers, and rudder pedals. The control stick can function like a cyclic control during helicopter mode and also a traditional airplane control stick during fixed-wing aircraft mode! The V-22 Osprey has triple-redundant fly-by-wire flight control systems with FADEC (full-authority digital engine control) systems. There are three flight control computers, three navigation systems, three hydraulic systems, and four generators. The Osprey can safely fly with just one of each of the systems. The Osprey has a Cockpit Management System (CMS) which allows for autopilot functions that fly the aircraft from 50ft without any manual interaction. If we talk about how fast it goes, the Osprey cruises within a speed range of 170 to 240 knots and flies at a speed of 170 knots during an instrument approach to during holding patterns. The Osprey’s fuselage is not pressurized. If the aircraft is flown above 10,000ft, the crew is required to wear oxygen masks

© Senior Airman Julianne Showalter, U.S. Air Force

© Airman 1st Class Russell Scalf
Check Bell Boeing V22 Osprey specifications here!

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Suggested article: What is METAR and how to decode it?

What is METAR and how to decode it?

What is METAR and how to decode it?

What is METAR?

In Aviation, weather plays a very important role in day to day operations. In fact, a pilot needs to check whether several times during the different phases of flight. Before the airplane is ready to leave the gate, he/she always checks the weather at the arriving and departing aerodrome in the pre-flight briefing. If they feel that the weather at and around the airport is not suitable for flying, they might decide not to operate the flight or even divert to their alternate aerodrome.
In aviation, the weather information of an aerodrome is broadcasted in a report called METAR (Meteorological Aerodrome Report). They are published in a fixed format which is common all over the world.

METAR is a string of abbreviations which denote various weather conditions around the aerodrome. You can learn all the METAR Abbreviations by referring to this document. With the help of this document, let us take an example of METAR of Chennai International Airport and decode it!

Eg: VOMM 131530Z 16006KT 4000 HZ SCT020 FEW025TCU BKN100 30/23 Q1009 NOSIG


Abbreviation
Indicates
Meaning
VOMM
ICAO Code
Chennai Intl Airport
131530Z
Date and Time
13 - Day of the month. 1530Z - Time of observation.
16006KT
Wind Direction
Wind direction is from 160 deg at 06 knots (denoted in Meters per second in Metric)
4000
Visibility
Prevailing Visibility at the airport is 4 km or 4000m
HZ
Weather Condition
Haze
SCT020
Cloud condition
Scattered clouds present at 2000ft
FEW025TCU
Cloud Condition
Few clouds present at 2500ft in towering cumulus form
BKN100
Cloud Condition
Broken clouds present at 10,000ft above aerodrome level
30/23
Temperature and Dew Point
The temperature at the airport is 30 deg. Dewpoint is 23 deg
Q1009
Sea Level Pressure / Barometer setting QNH
1009 hectopascals ( denoted in inch of Mercury - inHg in FAA)
NOSIG
Remark
No significant change in weather for the next 2 hours

SPECI Reports: 

A SPECI is a special report of meteorological weather conditions which is issued when one or more conditions match the specified criteria significant to aviation operations. It is also used to identify reports which are recorded ten minutes after an improvement to the SPECI conditions.

SPECI Example: SPECI VMMC 242341Z 10010KT 3500N VCSH FEW010 SCT018 BKN070 28/26 Q1004 TEMPO 5000 SHRA

SPECI Criteria : 



Element
Criteria
Wind Direction
Direction changed by 30 deg when the average speed is 20 KT or more.
Wind Speed
Speed changed by 10 KT when the average speed is 30 KT or more
Wind Gusts
Gust speed exceeds by 10 KT than the last reported speed
Visibility
When prevailing visibility is below the minimum operable limit of the aerodrome
Weather Phenomenon
When any of the weather phenomena such as thunderstorms, hail storms, sandstorms, dust storms, etc are reported in high intensity.
Cloud
When cloud cover (broken or overcast) is below the minimum operable limit of the aerodrome.
Temperature
When temperature increases or decreases by 5 deg since the last reported temperature
Pressure
When pressure increases or decreases by 2 hPa since the last reported pressure
Remarks
If the pilots repeatedly  report wind shear or other such weather phenomena

What is Black box?

What is Black box?

What is Black box?


Accidents such as the Charkhi Dadri mid-air collision, Air India Express flight 812, Air France 447, Korean Air flight 007 were some of the air crashes which were an eye opener for the Aviation industry. Investigators were able to find the cause of these accidents because of the data they found within the black boxes. A Black box is a recording device installed on most modern aircraft as well as mandated by authorities worldwide. They are expensive and cost between $10000 to $15000 each. They record several unique performance parameters throughout the flight which is then stored on solid state drives as well as other high capacity memory devices.

What’s inside the Black Box? 

While a black box has several precision instruments in it alongside some very clever design components, the two major instruments that are of the highest priority to investigators are:

1. Flight Data Recorder (FDR), and
2. Cockpit Voice Recorder (CVR)

1. Flight Data Recorder : 


A Flight Data Recorder is one of the first devices that is searched for and consequently secured after an accident. It records various significant performance parameters such as airspeed, altitude, heading, pitch, etc. A modern FDR records a minimum of 88 parameters which is also the minimum requirement under the  United States Federal Regulations. When data is retrieved from an FDR, the investigators can render the data that reconstructs the flight prior to the crash.  An FDR records up to 25 hours of continuous data and has an impact tolerance off 3400Gs. It can resist up to 1100 Deg Celsius and a water pressure up to 20,000ft.

A sample of FDR Data: Click Here

2. Cockpit Voice Recorder : 

Cockpit Voice Recorder

A Cockpit Voice Recorder (CVR) is used to record the audio inside the cockpit. The audio is usually captured through the microphones of the pilot’s headsets along with the microphones present in the flight deck. A regular CVR is able to record 4 channels of audio data for 2 hours.

Modern-day block boxes store data in Solid State memory devices and use digital recording techniques which make them resistant to moisture, vibration, and shock. They also include a standby power source for redundancy should the aircraft lose its own electrical power supply
The black box is accompanied by an underwater location beacon (ULB) and an emergency location transmitter (ELT) that provide a consistent signal to special receivers which eventually help pinpoint their location after an accident. The beacon can ping up to 30 days with the available power source available inside.
Underwater Location Beacon

Why is the Black box not visually painted in a black color?


The color of the Black box isn't actually black as the name would suggest. The Black box is actually painted in Orange color. The reason for a majority of the vivid orange color on the outside is to aid its detection from a distance. The color is better visible against the dark color of the burnt residue in an accident which makes it easier to spot.

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Suggested article: What is MRO, Line maintenance & Base maintenance?

MRO, Line Maintenance and Base Maintenance

MRO, Line Maintenance and Base Maintenance

MRO, Line Maintenance and Base Maintenance

MRO, Line Maintenance and Base Maintenance

Maintenance is one of the pillars that supports the entire aviation industry. Similar to how all machines require time to time maintenance, even aircraft require regular maintenance to maintain its airworthiness. Airworthiness is the quality check of various aircraft components and structure which determine its suitability for a safe flight. A certificate of airworthiness is conferred from a local governing aviation authority and is maintained by performing the required regular maintenance checks.

MRO, Line Maintenance and Base Maintenance

Maintenance, Repair, and Overhaul (MRO)

The Maintenance, Repair, Overhaul refers to organizations involved with the maintenance, repair, and overhaul of aircraft and associated systems and components and continues throughout the life of an aircraft from its initial entry into service through to its ultimate disposal.
The MRO organizations carry out  activities include the following activities:

  • Maintenance that relates to the process of preserving a complete aircraft in an airworthy condition.
  • Repair or replacement of damaged items that are inoperable
  • The overhaul that relates to reconditioning a system that has degradation in performance or strength.

Line Maintenance

Line Maintenance comprises of any maintenance that is carried out before the flight to ensure that
the aircraft is fit for the flight. The aircraft is visually inspected and its aircraft
logbook is checked for various entries relating to system problems, failures or other maintenance. It includes troubleshooting, defect rectification and component replacement of an aircraft and requires 2 man hours to complete the checks.

MRO, Line Maintenance and Base Maintenance

Base Maintenance 

Base Maintenance is carried out in a hanger and covers a series of checks and MRO activities. Every airline has different maintenance schedules and procedures that relate to their specific aircraft. Base maintenance is classified into 4 categories each denoted by a letter.

A Check: Carried out after every 100 flight hours.
B Check: Carried out after 2 months or 500-600 flight hours. It involves a more thorough check than A Check.
C Check: It involves a very comprehensive and thorough check. Carried out after every 2 years. Some operators carry out a "3C Check" which is a subcategory of C Check and includes light structural maintenance, corrosion check, replacement of seats, etc.
D Check: It is the most comprehensive check-in Base Maintenance. Carried out after every 6 years and can take up to 2 months to complete. An aircraft on average undergo 2 to 3 D Checks before it retires.

Unscheduled Maintenance and Aircraft Recovery 

If there is a need to repair an aircraft is stranded at an airport away from the base airport, the operator may declare the aircraft as “Aircraft on Ground”. Repair to the damaged aircraft may require a specialist team to travel on site and carry out the repair.

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Suggested article: What are Visual Flight Rules (VFR)?

Visual Flight Rules

Visual Flight Rules

Visual flight rulea


If you are a true aviation enthusiast then at one or the other point in your life you must have come across the term visual flight rules (VFR). If you have ever got a chance to fly in an aircraft you must have popped out of the window to see that what does the pilot sees? If not I will tell you that it is just awesome to gaze out that window and the huge huge clear blue sky all around. It's not blue all the time though when you get those shaking cabin rides.

For the sake of this discussion, we would assume that the sky outside the aircraft is blue. Being a passenger you would always think that it's so clear outside, the pilot must be in full control of this craft! But is it correct? Flying around 150-200 pure souls is not that easy. Though the sky is damn clear still there are a set of rules which the pilot have to follow while flying the aircraft. These set of rules and regulations are called Visual Flight Rules. VFR may formally be defined as the set of rules that the pilot have to follow while flying the aircraft when the weather conditions under certain minimum control limits set up by the governing body of that airfield.

The minimum control limits of weather conditions are called as Visual Meteorological Conditions. VMC may formally be defined as a set of weather conditions which are clear enough so that the pilot is able to distinguish between the clouds and any obstruction that may lie on the flight path. This VMC vary in various factors such as what type of aircraft are we talking about,? the airfield over which we are flying? whether we are flying in night or day? etc...

Visual meteorological conditions
Visual meteorological conditions

Let us take an example of VMC conditions as laid down by the United Kingdom government.


Now as we have got an insight into what are the visual meteorological conditions lets take our discussion back to visual flight rules. If the VMC are satisfied then the pilot has to follow VFR while flying the aircraft. If the conditions of VMC are satisfied then the pilot need not fly the aircraft as per ATC directions he/she can fly the aircraft according to his decision-making abilities but still, a transponder is required on the aircraft so that ATC can get the exact location of the aircraft at any instance whenever required.

What are Visual Flight Rules?

As in the case of VMC the visual flight rules are also dependent on various aspects such as the type of aircraft being flown, time of flight etc. All these factors are studied by the federal agency of the country over which the respective airfield lies and VFR are decided accordingly.

For example let us consider that an aircraft is flying over XYZ city where the governing body is ABC , the VMC as prescribed by ABC are that in the case of day flying between 800 hours to 1600 hours above 700 ft from ground or sea with a clear visibility of 5 km then these conditions may be taken as the minima for VMC and the pilot has the right to fly as a VFR with the consent of ATC under which he/she has to maintain a distance of at least 600 m from clouds and to take decisions as per the spot and decide policy in which the pilot spots any obstruction and adjust the flight path accordingly.

What are VMC conditions are not fulfilled?

In case of a condition when the VMC is not being fulfilled then IFC can be carried out. IFC is defined as Instrumental flight control for which another weather and meteorological conditions by the name of Instrumental Meteorological conditions are defined.


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Suggested article: Can an aircraft overspeed?