Why most commercial aircraft do not fly supersonic?

Why doesn't general commercial aircraft like boeing 747, 737, airbus 320, 380 etc, travel faster than speed of sound i.e. supersonic. They usually travel at 0.87 mach (1 mach = speed of sound). Concorde was the only supersonic commercial aircraft.
Let's explore the physics of it.

credit - http://theboard.byu.edu

From your knowledge of aerodynamics and aerofoil, you know that the upper surface of the wing is curved and lower surface is not. Hence over upper surface the air flows faster as compared to lower (see image above).
Now as aircraft accelerates and nears the supersonic speed, over the upper surface of the wing the airflow (or for that matter any curved surface including canopy, nose etc) can potentially be at supersonic speed. Now at this point a shock wave generates a the point where the supersonic airflow slows down to subsonic. As you can see from the image below, at M=0.77, at the middle, the airflow before it is supersonic and beyond that is subsonic. This results in a shock wave. The airspeed where this shock wave starts appearing is called critical mach number. 

credit - wikipedia

This shock wave requires energy and that is supplied by the aircraft itself in form of additional thrust, which means the additional thrust is required to keep the aircraft moving ahead and to compensate the loss of thrust due to shock wave. This acts as a drag and is referred to as wave drag. The engine thrust may not be enough to overcome this drag and move even faster. 
Other aspect is that the supersonic speed can cause structural damage.
These two reasons explains why general commercial aircraft does not fly at supersonic speed. 
Concorde was able to overcome these 2 design issues and fly supersonic.
Hopefully future air travel will be faster.

Pictures from Bayfont park of SFO Airport

Took some pics from the bayfront park. Aircrafts landing at 28L/R 

Communication in Class B and C

Communication is very critical while flying but your responsibility increases when you are in the vicinity of Class B or C airspace in US. Let me put some do's and don't about communicating with controller in these airspace. 
Read about the airspaces here.


Class B:

• Typically large airports (in terms of traffic) are classified as Class B (bravo) e.g. San Francisco, Los Angeles, Washington etc.
• Shape of class B appears as upside down wedding cake. Refer the image below.

Credit - Wikipedia

• Requirements to fly in Class B are list here: Requirements (pilot certification, equipment etc.)
• Communication procedure - You have to be cleared to enter into Class B before you enter the airspace. Most likely you would have to speak with an area controller before you can enter the airspace and may not be directly with the ATC tower of that airport. You should communicate atleast 10 nm in advance. Sample communication with Class B controller

Aircraft Calling: Norcal, Cessna 1234MA at 3500ft over livermore airport, would like to transition through Class B for a bay tour. - Using the standard phraseology of who,where & what. Assuming you are flying through a class C.

Norcal: Aircraft calling standby. -- This means that they have just acknowledged your presence but no permission has been granted yet. KEEP OUT OF AIRSPACE,  may have to circle around.

Norcal: Aircraft calling, say your request.

Aircraft Calling: Norcal, Cessna 1234MA at 3500ft over livermore airport, would like to transition through Class B for a bay tour. - Repeat your request.

Norcal: Cessna 1234MA, CLEARED INTO CLASS B, maintain 3000ft, squawk 1510. -- one marked in red is the golden tagline or "magic word" you need to hear from the controller. Without which you are not supposed to enter that airspace. If you do, its a violation and you may end up loosing your license.


Norcal (Alternate): Cessna 1234MA, remain clear of class B airspace -- STAY OUT of airspace.

Important points:

• Even if ATC does not use the exact words (may use it differently or forget about it), its your duty to confirm that as its your license which is in line of fire. Ask ATC.
• If ATC gives a vector (e.g. turn right heading 130) which may result in a flight path into class B, its your responsibility to be aware of it and ask ATC if you are cleared into Class B. Better ask then be sorry.

Class C:

• Typically medium size airports (in terms of traffic) are classified as Class C (Charlie) e.g. San Jose etc.
• Shape of class C is cylindrical with an outer shell. 
• Requirements to fly in Class C are list here: Requirements (pilot certification, equipment - transponder, comm etc).
• Communication procedure - You have to establish 2 way radio communication to enter into Class C before you enter the airspace. Most likely you would have to speak with an area controller before you can enter the airspace and may not be directly with the ATC tower of that airport. You should communicate atleast 10 nm in advance. Sample communication with Class C controller

Aircraft Calling: Norcal, Cessna 1234MA at 3500ft over livermore airport, would like to transition through Class C for a bay tour. - Using the standard phraseology of who,where & what. Assuming you are flying through a class C.

Norcal: Aircraft calling standby. -- This means that they have just acknowledged your presence but no permission has been granted yet. KEEP OUT OF AIRSPACE,  may have to circle around.

Norcal: Aircraft calling, say your request.

Aircraft Calling: Norcal, Cessna 1234MA at 3500ft over livermore airport, would like to transition through Class C for a bay tour. - Repeat your request.

Norcal: Cessna 1234MA, CLEARED INTO CLASS C, maintain 3000ft, squawk 1510. 


Norcal (Alternate communication): Cessna 1234MA, standby - If the controller speaks up your aircraft tail number then also you are cleared into the airspace as that is establishing a 2 way radio communication.

Important points:

• If ATC gives a vector (e.g. turn right heading 130) which may result in a flight path into class C, its your responsibility to be aware of it and ask ATC if you are cleared into Class C. Better ask then be sorry.

Flying to a uncontrolled airport

Once you get your wings, it opens up the world for you. Your travel will take you to different types of airports e.g. controlled, uncontrolled, military (maybe) etc.
In this post I would like to touch upon a plan, communication and any other important aspects around flying to an uncontrolled airport. For this I will take you for a VFR flight from my home airport of KRHV (Reid Hill view airport) to KCVH (Hollister). 


Step 1 - Prepare a flight plan

1. Get weather from adds.aviationweather.gov or any other aviation weather websites. Most important step for VFR flight. Links on my blog.
2. Pull your sectional chart and identify the start and destination and mark with pen. 
3. identify checkpoints preferably  every 10 nm. The checkpoints should be clearly visible from your altitude e.g. large warehouse, road intersection, water body etc. Identifying checkpoints close enough will help you correct your flight path sooner if you deviate. You can also use VOR, DME if you have the equipment.
4. Do time, distance & fuel calculation, based on the altitude you are flying. Altitude will be decided by the magnetic course if you plan to fly above 3000 ft e.g. 0-179 degree, odd + 500 ft MSL (3500,5500); 180-359, even + 500 ft MSL (4500, 6500). Refer FAR 91.159 . In this flight plan its 130 degrees and hence odd + 500 ft.
5. Do weight and balance and ensure you are within the CG limits of the aircraft you are using.
6. Identify in your flight plan, TOC (top of climb) & BOD (beginning of descent).
7. Once you have the flight plan ready, make a final call to get weather briefing and if required file VFR flight plan by calling FSS on  1-800-WXBRIEF. Important points to obtain are, route of flight, weather at destination, return flight weather etc. You should also obtain any flight restrictions like TFR, NOTAMS (runway closure) etc.
8. Check AFD for details of KCVH e.g. frequencies of ASOS/CTAF, runways available, runway length, runway lighting, fuel stations etc.
9. Usually in my flight plan I draw the runways and the my expected flight direction. This gives me the picture of how I will make an entry.
10. Now you are all set for the travel. Start your engines and contact ATC and take permission to take off. Bon Voyage

See this image below for some help.

credits - skyvector.com

Step 2: Enroute (assuming no flight following)

1. Communication is the key word. Giving your position reports regularly is very important in VFR flight. In the above plan, after taking off from RHV, you should tune   in to the next nearest airport e.g. South County and give position reports. 

• Position call 10 nm ahead
• Position call 5 nm ahead
• Position call abeam

1. Keep scanning for traffic. Use standard scanning techniques.
2. Get ASOS/ATIS/AWOS information of airports enroute if available.

Step 3: Descend and Land

1. Get ASOS/ATIS/AWOS information of destination airport if available atleast 10nm ahead. This will help you visualize the weather condition and if appropriate for VFR landing and also the winds will give you a clue of the runway to land.
2. Start descending based on your flight plan to the destination airport.
3. Descend till 1000 ft above traffic pattern when approx 1nm away e.g. if pattern altitude is 1500ft then descend to 2500ft. This is required so that you can identify the windsock and general direction of wind. Will help you to determine the runway to land
4. Once you have identified the runway to land (by checking the windsock or as advised by any other aircraft in the pattern), then you should plan to enter the traffic pattern. Remember you are at 1000ft above TPA. As per FAR you should use the 45 degree entry procedure. See image below.
5. Make 45 degree entry into the downwind leg after descending to TPA. You should give position report for 45 degree entry, downwind, base, final and after clearing the active runway.
6. Voila you have successfully made it to the airport. 
7. Now taxi to the terminal, fuel up, freshen up, and prepare your flight plan for the return.
8. Don't forget to close flight plan if you have opened any with FSS.

credit - wikipedia 

Sample position report calls. Format is "Airport name traffic; Who; Where; What; airport name". This is suggested only and can be tweaked as required. Idea is to communicate and give as much information as possible so that other aircraft in the vicinity can visualize your position. This is called situational awareness.

1. Position report to South County @ 10nm -- " South County traffic; Cessna 1234MA; Is at 3500 ft, 10 nm North of the airport;Will be overflying abeam the airport for hollister; South County" -- North with respect to the airport and your aircraft. You will have to visualize your position. You can cross check with your heading indicator i.e. if the tail of the heading indicator is pointing to 180 then you are north of the airport heading south. Use N=0,E=90, S=180,W=270.
2. Position report to South County @ 5nm -- " South County traffic; Cessna 1234MA; Is at 3500 ft, 5 nm North of the airport;Will be overflying abeam the airport for hollister; South County"
3. Position report to South County @ abeam --  " South County traffic; Cessna 1234MA; Is at 3500 ft, abeam the airport;for hollister; South County"
4. Call 1 - 10nm away -- "Hollister traffic; Cessna 1234MA; Is 3500 ft, 10 nm North of the airport;Inbound for landing, any other traffic please advise; Hollister" -- Idea is to get traffic pattern or any other useful info from other aircraft.
5. Call 2 - 5nm away -- "Hollister traffic; Cessna 1234MA; Is 2500 ft, 5 nm North of the airport;Inbound for landing, any other traffic please advise; Hollister"
6. Call 3 - 1nm away -- "Hollister traffic; Cessna 1234MA; Is 2500 ft, 1 nm North of the airport;Will be circling above the airport to check wind direction. Any other traffic please advise; Hollister".
7. Call 4 - Circling -- "Hollister traffic; Cessna 1234MA; Is 2500 ft, above the airport;Will be circling  to check wind direction. Any other traffic please advise; Hollister".
8. Call 5 - 45* entry-- "Hollister traffic; Cessna 1234MA; Is at 1500 ft; Inbound 45degree for downwind on runway 32; Hollister".
9. Call 6 - downwind entry-- "Hollister traffic; Cessna 1234MA; Is downwind; Inbound for landing on runway 32; Hollister".
10. Call 7 - base entry-- "Hollister traffic; Cessna 1234MA; Is base; Inbound for landing on runway 32; Hollister".
11. Call 8 - final entry-- "Hollister traffic; Cessna 1234MA; Is at final; Inbound for landing on runway 32; Hollister".
12. Call 9 - After landing and clearing the runway -- "Hollister traffic; Cessna 1234MA; clear of the active runway 32; Hollister".
13. Call 10 - hold short and Taking off -- "Hollister traffic; Cessna 1234MA; is holding short of active runway 32; Will be a straight out departure to north;Hollister".
14. Call 11 - upwind -- "Hollister traffic; Cessna 1234MA; is upwind from runway 32; Will be a straight out departure to north; Hollister".

Comments appreciated from viewers so that I can enhance this post and correct for any mistakes.

Why aircraft's fly at high altitude

Reasons:

1. Terrain clearance - As the land is not flat and has obstructions (man made or natural), to keep it clear of them aircraft need to fly high
2. Less Air resistance - Since air is thin at higher altitude, the resistance is less and it can travel efficiently and faster.
3. Low fuel requirement - As air is thin, the fuel air mixture needs to be leaned. This means that aircraft will get more performance with the same amount of fuel as it would have got at lower altitude.
4. Jet Stream - At high altitude above 30000 ft, the jet stream which moves at excess of 50kts (in winter it goes to 100kts or more) helps aircraft fly faster which are moving the same direction as jet stream i.e. it adds to the ground speed. Though travelling in opposite direction will reduce the ground speed.
5. Higher TAS and ground speed - Since less air at higher altitude the TAS is higher. This means that overall ground speed will increase.

High altitude operations

This topic is usually asked in a checkride by examiners. Let us try to understand what questions are asked and how to respond to those.
Before that I would like all of you to read my previous post about relationship between pressure, temperature, density and altitude. 
Effect-of-temperature-and-pressure


Also see the following mathematical relationship:

where

• L is lift force,
• ρ is air density
• v is true airspeed,
• A is planform area, and
•  is the lift coefficient at the desired angle of attack, Mach number, and Reynolds number

In summary, At high altitude, you have lower pressure, lower temperature and hence lower density (i.e. amount of air molecules per cubic meter). Now the lower density is the main reason which affects the takeoff/landing. Let's see how.

Question1: How is takeoff affected by high altitude?

1. Less engine performance -- Engine requires air to operate (i.e. burn fuel with air). But at this altitude density is less and hence less air for engine. This results in lower performance of the engine. That is one reason why you may (e.g. in cessna 172 its recommended to lean mixture above 3000ft) need to lean the mixture to correct the air/fuel mixture for best performance.
2. Higher TAS -- Higher true air speed. TAS is relative to the air your are currently travelling. Since engine performance is less, you will need to travel faster to get same amount of air required by engine for best performance. Note your indicated airspeed will remain the same i.e. you will still takeoff at the same indicated a/s.
3. Longer takeoff roll -- Since engine performance is low, aircraft needs to travel longer and faster to get the required amount of air to burn fuel optimally. Imagine you are travelling in a car and you open the window and keep your hands out. At low speed you will not feel the amount of air pushing your hands but as speed increases you will feel more air pushing your hands. Its the same principle. As you travel faster in aircraft, more air will start hitting your engine and it will be able to generate higher thrust. As part of flight planning you should calculate the takeoff roll so that its within the runway length. 
4. Reduced maximum takeoff weight -- Higher weight may result in even longer roll as aircraft needs to travel faster to get the amount of lift to balance that weight and runway may not be long enough to achieve that required speed.
5. Reduced Lift -- As you can see from the mathematical relationship above, density is directly proportional to lift. Low density translate to less lift.

Question2: How is landing affected by high altitude?

1. Higher TAS -- As above, since amount of air is less to reach the same indicated airspeed required for landing the TAS is high. 
2. Longer landing roll -- As TAS is higher the aircraft is travelling faster. To slow down it will need longer runway to stop. Also, since air resistance is low that as well contributes to longer landing roll.
3. Less resistance to land -- As air is thin, it provides less resistance for the aircraft to stop and increases landing distance.

These are some of the reasons, why performance is always calculated against density altitude. Note the above reasons also apply to airport with high temperature, as that also results in lower density. Hence if you are at sea level but very hot for the day and lower density, it will behave like high altitude airport.

Effect of temperature and pressure on density

What is density: Density is  mass per unit volume expressed usually in kilogram per meter cube (kg/m³ ). Imagine you have a box of size 1meter x 1meter x 1meter. If you put air inside it, it can hold uptill the entire space is filled up. That is the maximum capacity it can hold. Our atmosphere is also like that. At sea level at 15°C it is 1.225 kg/m³

Effect of pressure and temperature as height increases:

1. As you climb up the atmospheric pressure above you reduces i.e. if at ground level you have 1010 mb or 29.92in of pressure then at 1000 ft above it will be less i.e. the amount of air above you and hence the pressure exerted will be less. Other way to understand is if you apply pressure then molecules are closely packed and hence more dense. Reverse when you remove pressure.
2. Secondly the temperature reduces as you climb higher e.g. at ground it may be 15degree Celsius but at 1000ft height it will be less say 10 degree Celsius. 
3. Density reduces as height increases (at sea level 100%, at 20000 ft 50%). Scientifically it has been put into this formula.

where ρ is the air density, p is absolute pressure, R_specific is the specific gas constant for dry air, and T is absolute temperature

As you can see from the above formula, density reduces as pressure reduces, which means that as per point 1 above, since height increases, pressure reduces and hence density reduces. So far so good. We have established the relationship. 

Now to point number 2, as temperature reduces as height increases density increases (inversely proportional). But that's contradicting or cancelling pt number 1. However that is true but density reduction due to pressure is more than density increase due to temperature decrease, resulting in net decrease in density. 

Effect of water vapor to density: Introduction of water vapor in atmosphere i.e. making the air humid. Humidity reduces density. Why? Cause water has less molecular mass than air and hence when water is added to atmosphere it displaces air in the given cubic meter volume but results in lowering of density.


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Understanding drag

Figure 1: Drag graph

Parasite Drag: Sum of Form drag, Interference drag and skin friction drag.

Induced Drag: by product of airspeed.  caused due to downwash....in a cambered wing, on the upward side air is flowing faster than the lower side. Now this fast moving air above and slow moving air below, meet at the trailing edge and cause turbulence called vortices. These vortices is downwash and it causes opposite reaction in form of drag called induced drag. Refer figure 2 below (RW = relative wind).

At low airspeed to maintain lift, pilot needs to increase AOA (angle of attack), as a result on the upside the air flows even faster as its more curved and lower side it travels the same speed. This causes even more turbulence at the trailing edge. Hence at low speed induced drag is more.
However at high speed, AOA is less and hence turbulence at trailing edge is less causing less downwash and less induced drag. Refer figure 1 above. 

Figure 2: Credits - Civil Air Patrol