May 8, 2018

ETOPS (Extended Twin Engine Operations)

What is ETOPS (Extended Twin Engine Operations)?   You should have heard the word ETOPS, which is extensively used the field of aviation. ETOPS is the acronym for Extended Twin Engine Operations. This is a special operation intended for twin-engine aircraft to utilize them in extended operations. Normally extended routes lie over oceanic regions. Using twin-engine aircraft for these routes pose a threat to the safety of the passengers and the aircraft. If one engine fails due to any reason the aircraft has to fly to an airport with the remaining engine. Lives of passengers and crew utterly depend on the remaining engine until the aircraft comes to a stop in the runway. Hence the integrity of critical aircraft components of twin-engine aircraft should be certified.   An ETOPS certified aircraft should undergo specific inspections, checks and maintenance activities to uplift its airworthiness. Prior to each departure, ETOPS aircraft undergo inspections outlined in a specific checklist to ensure its safety. A concept called Split maintenance is incorporated to inspect a maintenance activity that has been carried out in an ETOPS certified aircraft. A duplicate inspection is carried out to identify hidden flaws in a maintenance task in the split maintenance philosophy. All the systems and components which are mandated by the Minimum Equipment List (MEL) for Extended Range (ER) will be considered as ETOPS significant items.   Oil consumption programs, Engine condition monitoring, and Reliability programs are conducted on ETOPS aircraft to identify and prevent imminent failures.   An ETOPS Verification Flight has to be carried out to identify, rectify, verify and certify the ETOPS significant items before next ETOPS flight. A verification flight will be required for following occurrences,   Reported a defect in an ETOPS significant item and the defect can’t be reproduced or it can’t be tested 100% on the ground. Occurring of intermittent failures and no means to verify the fault has been rectified through maintenance. Maintenance in multiple ETOPS significant items. Engine fuel or oil system component change. When the aircraft was released from a letter “A” check and its multiples.   A verification flight should fly at least for 60 minutes and it can be a NON-ETOPS revenue flight, first 60 minutes of an ETOPS flight under the 60 minutes rule or a non-revenue verification flight.   60 minutes rule – Within first 60 minutes of the ETOPS flight, all the systems related to ETOPS should be verified as serviceable before entering the ETOPS sector.   An ETOPS Proving Flight has to be carried out for following occurrences,   In-Flight Shut Down of an engine (IFSD). After undergoing a “C” check. An engine change, removal, and reinstallation for gaining access. Drop in fixed crew oxygen system pressure.   A proving flight has to conduct for the particular ETOPS diversion of the aircraft and it can be a NO-ETOPS revenue flight or a non-revenue flight.  
March 26, 2018

Deep in to the Bleed Valve Operation

Pneumatic Closure Control   Do you know how the bleed air is controlled by a modern day aircraft engine? If not don’t worry, let’s have a deep look into the system and get things cleared up.   Bleed air is commonly used for cabin pressurization, wing anti-icing, engine anti-icing, engine starting, water tank pressurization and hydraulic reservoir pressurization. For these purposes bleed air is taken from the cross bleed duct. Bleed air outlets from both engines and APU bleed outlet are connected to the cross bleed duct plus a ground air service cart can be connected to the ground. The in-flight cross bleed duct can receive bleed air from all sources except the ground cart. During the flight, bleed air from the engines is used hence APU is kept out of the drama until needed.   Going back to the topic, at what time bleed air from an engine is cut off? There are few instances where engine bleed valve is kept closed. Operation of the bleed valve is controlled by the BMC(Bleed Monitoring Computer) and a twin-engine aircraft consists of two BMCs, one for each engine. If one BMC of an engine is inoperative, the other one takes over and oversees the related operations of both engines.   Bleed valve position is decided by a control logic and the control logic comprises only with OR gates. If one condition agrees, bleed valve will be driven in to close position.   A pneumatic starter is used to start the engines and the starter receives bleed air for engine starting from the APU. But an already started engine’s bleed or air from a ground service cart also usable for this task. When the bleed air is available for engine starting, starter valve is opened and let air pass into the starter. The system is designed in a manner that bleed valve can’t be kept open when the starter valve is opened. If these two valves are kept open at the same time, the starter might not be able to rotate the engine up to the required rpm. So here comes the first logic, (1)if the starter valve is open engine bleed valve should be in closed position. BMC looks into the position of starter valve and adjusts the position of the bleed air valve in accordance with that.   As said earlier APU bleed too can be used to drive the starter, hence if the (2)APU bleed is ON, BMC will command the bleed valve to close. If the upstream (3) pressure or (4) temperature from the bleed valve is exceeded, BMC will command a valve closure. If there is a leak (5) in the bleed air system the process of tapping bleed from the engine should be terminated to avoid huge energy loss and to maintain the rated thrust from the engines.   Out of 8 ways which close the bleed valve we have discussed up to 5. Reverse flow is a phenomenon that should be avoided to protect the engine from surging and due to that bleed air valve is designed as a one-way valve and air can only flow out from the engine. Hence bleed valve is driven to a closed position during a (6)reverse flow situation.   Other two ways of closing the bleed valve are simply by pushing the ENG 1 BLEED push button or by pressing ENG 1 FIRE push button.   Recommended Image:
March 13, 2018

Pressurization of Hydraulic Reservoirs

How Are The Hydraulic Reservoirs Pressurized?   Pressurization of Hydraulic Reservoirs   As we all know each and every modern day commercial aircraft are contained with hydraulic systems. By depending on the design of the aircraft it may have one hydraulic system, two hydraulic systems or even more. Hydraulic fluid is the lifeblood of modern day airliners and used for most of its light-duty and heavy-duty tasks. All the flight control surfaces, slats, flaps, thrust reverses, landing gears and doors and many more actuators are actuated by hydraulics.   Most of the aircraft flying today are equipped with 3 hydraulic systems(Green, Blue, Yellow) and newer aircraft such as A350 and A380 are designed with only 2 hydraulic systems(Green and Yellow). The most important fact to keep in mind when studying about hydraulic systems is all hydraulic systems are separate and independent, no mixture of fluid between each system due to any circumstance and hydraulic fluids are same in color, Skydrol LD-4  has a purple color. This image shows a Skydrol LD-4 hydraulic can where its quantity is 1 quart.   Note: Due to the variation of system designs, from here onward all the information are presented for A320 aircraft. Most of the other aircraft is exactly similar with minor variations. Hydraulic fluid is stored in a reservoir and the aircraft has dedicated reservoirs for each of its hydraulic systems. Below image shows a Green hydraulic reservoir of an A320 aircraft. Green Hydraulic Reservoir Located in the Wheel Well. With the assigned tasks for the system, reservoirs may contain different volumes of hydraulic fluid. In A320 aircraft, Green hydraulic reservoir contains 14.5 L of fluid. Yellow hydraulic reservoir contains 12.5 L of fluid. Blue hydraulic reservoir contains 6.5 L of fluid. In the good old day’s hydraulic reservoirs were not sealed and pressurized like modern day reservoirs, they were vented to the atmosphere. But with the introduction of sophisticated technology, aircraft flew higher and higher. With the increase of altitude, those hydraulic systems faced different kinds of problems. Due to the very low temperature at high altitudes, hydraulic fluid tends to make foam and reduced the fluidity creating blockages. Another problem that aroused with the high altitude was pump cavitation and due to very low head up pressure, hydraulic fluid tends to form cavitation. Ultimately this fluid cavitation can end up with pump failure. As a solution for all those drawbacks pressurized reservoirs were introduced.     Above image shows a sketch of a pressurized hydraulic reservoir and by using pressurized air significant head up pressure(about 50 psi) is created above the hydraulic fluid upper surface. This will terminate pump cavitation and fluid foaming.   So How The Hydraulic Reservoirs Are Pressurized? Bleed air is tapped from the engines and use for various tasks such as cabin pressurization, wing anti-icing, engine anti-icing, water tank pressurization, hydraulic reservoir pressurization and for engine starting. LP(low pressure) and HP(high pressure) bleed is used depending on the requirement and LP bleed is tapped from low-pressure compressor and HP bleed is tapped from high-pressure compressor(In CFM-56-5B engine: it has 9 compressor stages and LP bleed is taken from 5th stage while HP bleed is taken from 9th stage.) That tapped air is passed through PRV(Pressure Regulating Valve) and OPV(Over Pressure Valve) to adjust the pressure while pre-cooler reduces the temperature. Pressure and temperature of the bleed air are sensed by a thermostat solenoid located ahead of pre-cooler and correct pressure and temperature by adjusting the PRV and FAV(Fan Air Valve) respectively. As shown in the above diagram a dedicated duct carrying HP bleed is used to pressurize the hydraulic reservoirs and if this pressure drops due to any reason bleed air from the cross bleed duct can be utilized to pressurize the reservoirs. Air conditioning packs and other bleed air consuming units are connected to the cross bleed duct as shown in the diagram below. Cross bleed duct can receive bleed from engines, APU and from a ground air supply cart. There are 3 positions for the cross bleed valve. In the below diagram it is in the auto position(maybe in SHUT!) and it can be switched to shut off(SHUT) position or open position(OPEN). Controls Related to Air Conditioning System in the Overhead Panel. Even if the engines are not running we can pressurize the reservoirs by using APU or a ground air supply. If we do not have provisions to start the APU or to connect a ground cart we can pressurize the reservoirs by using Nitrogen with the help of a Nitrogen cart.   During an AIR DUAL BLEED FAULT, we will lose both Green and Yellow hydraulic systems due to LOW AIR PRESSURE on the reservoirs. In this situation no point of opening the cross bleed valve because there is no bleed pressure from both engines. Reservoirs are pressurized back to nominal pressure by starting the APU and selecting the APU bleed. When selecting the APU bleed, engine one bleed valves will be closed automatically and engine two bleed valves will be kept open due to the closed position of the cross bleed valve.
March 5, 2018

Fuel Feeding Sequence in Airbus A320

Fuel Feeding Sequence in A320 As we all know there are three fuel tanks in an Airbus A320 and two of them are wing tanks while the other one is located the center of the fuselage belly. Wing tanks mainly divided into two partitions called the inner tank and the outer tank. Apart from those two tanks, there is a tank located towards the tip of the wing and it’s called the surge tank. It collects excess fuel and fuel spillage while venting the fuel tank to the atmosphere.       A318, A319 and A320 aircraft are fitted with booster pumps to supply fuel out from the fuel tanks with a positive pressure. Center tank, inner tank, and the outer tank have two of each booster pumps. When it comes to A321 it has two jet pumps in the center tank instead of ordinary booster pumps. A jet pump is lower in weight and consumes less power when compared to a booster pump. All the newer Airbus aircraft including A320neo will implement this jet pump in the center fuel tank.Jet pump operation is based on the Venturi principle and fuel in the center tanks is sucked into the related wing tank with the aid of fuel flow in a wing tank. Jet Pump   Operating Principle of a Jet Pump   Total fuel capacity of the aircraft is 23,858 L in volume and 18,728 Kgs in weight. So how this fuel is fed to the engines? What is the sequence of feeding?   Fuel in the center tank is fed to the engines initially and when the center tanks if emptied, the fuel from the wing inner tank is taken. Fuel from the wing inner tank is used until fuel level in the inner tank drops down to 750 Kgs. When this limit is reached, two transfer valves open and let the fuel in outer tanks to be filed into the inner tanks. So wing outer tank fuel is used at last. There is a special reason to implement this sequence. The wing is subjected to upward bending force throughout the journey due to the huge lift force created. Heavy upward forces try to bend the wing upwards and weight of the fuel provides the counter-attack for this bending moment. As arm increase when moving towards the tip, even a less amount of fuel will create more counter force to keep the wings straight.   What happens if all the booster pumps failed or inoperative due to a total electrical failure?   Fuel can be fed to the engines using the gravitational force and this method is called gravity feeding. In order to be gravity fed, related tanks should have suction valves. They are held in closed position during normal operations and comes in to act if booster pumps failed. Only wing tanks are fitted with suction valves, hence during a pump failure or electrical failure center tanks, fuel is unusable.
February 19, 2018

Automatic dependent surveillance – broadcast (ADS–B)

Automatic dependent surveillance-broadcast (ADS–B)   Do you know how those flight tracking websites are functioning? If not. Don’t worry, we are going to do a full analysis of the technology behind those flight tracking websites. First, we should have a look at the aircraft and associated equipment. As we all know each and every modern commercial aircraft is equipped with an equipment called Transponder and it assists two systems in the aircraft. They are ATC system and the TCAS system. TCAS stands for Traffic alert and Collision Avoidance System and it helps to detect and alert the pilots about traffic vicinity to the aircraft. TCAS system consists of two antennas, one at the top of the fuselage and the other at the bottom of the fuselage. They transmit at 1090 MHz and receive at 1030 MHz and their range is normally about 30 NM. These signals are transmitted from the TCAS antenna by creating a virtual sphere where it’s radius is 30 NM and alert pilots if there are intruders to this sphere. As mentioned previously these signals do not travel a long distance to reach the earth. When it comes to ATC system they have normally four ATC antennas. Two of them are located at the lower nose fuselage and other two at the upper nose fuselage. Even these antennas transmit and receive at 1090 MHz and 1030 MHz respectively. These signals used for ATC purposes, hence they should reach the earth. Pilots and air traffic controllers use these signals to exchange information between the aircraft and the control center. These signals contain a lot of information about the aircraft including airspeed, altitude, destination, number of passengers and so on. These 1090 MHz signals can be received by two kinds of receivers in the earth, Secondary Surveillance Radar(SSR) and ADS-B Receivers Secondary Surveillance Radar is used by air traffic controllers for air traffic management purposes and the information receiving from the signal can be decoded to present in a monitor. For our purpose, we want to equip with an ADS-B receiver and it performs the same function as SSR except the fact that this receiver can’t transmit signals. Signals coming from the aircraft ATC antennas are received by this receiver and subjected to a decoding process. By decoding the signals it is possible to get real-time data about the aircraft. This decoded information is then uploaded to the server of a particular website. Those flight tracking websites have a system of vast ADS-B receivers located all around the world. Hence they are receiving information all around the globe depending on their coverage. By combining the data from receivers and flight plan data from the airliners they show the real-time traffic of all the aircraft around the globe. This is how a flight tracking website is functioning and it is a c For more understanding: Pilots and ground personnel can communicate in mainly 3 ways, one method is described above, by using ATC antennas. In this ATC communication either pilot or the controller do not use voice and this is a fully automatic method running on the computers. VHF 1, VHF 2, HF 1 and HF 2 antennas are normally used for voice communication (VHF 3 is dedicated to data communication). These communications can be done through the Audio Control Panel(ACP) and this is the second method that the pilots and controllers can communicate with each other. The third method is CPDLC(Controller Pilot Data Link Communication) and for this, we can use either VHF, HF or SATCOM.
February 12, 2018

Growth of Aviation Sector in India in Next 5 Years

Growth of Aviation Sector in India in Next 5 Years Worldwide air travel has grown at a historically brisk pace. Many factors paved the way for immense growth in the aviation sector. Low airfares, the growth of tourism and travel, higher living standards with a growing middle class in large emerging markets, improved safety margins are the driving strengths in the demand for air travel. India is considered as the fastest growing aviation market in the world with 100 million domestic markets and becomes the third largest domestic market in the world. In the year of 2018 domestic market in the country could grow up to 130 million from 100 million by marking a 25% growth while international market could grow up to 12%. The emergence of the middle class made a significant effect for the increment of air travel usage. The middle class in the country has grown to 135 million in 2016 from 80 million which was the figure in 2000.India has maintained a consistent upward curve for the GDP and it’s estimated to escalate the GDP up to a much higher value in the upcoming 5 years. At the end of 2021, GDP will rise to 8.122% from the current value, 7.608%. These figures show a quite promising picture for the aviation sector within the country. Low-cost carriers are the main engines of growth in the sector and they have made remarkable achievements within past years. Contribution from the LCCs for the domestic market share will rise nearly to 75% and they will operate the majority of narrowbodies. Within upcoming years LCCs will acquire few widebodies for the international market to operate long-haul flights. Indigo, Jet Airways, SpiceJet, and GoAir will be the largest LCC contributors who operate international flights. But their contribution for the international market is expected to keep low to a value of 10-15% from the total capacity. Indigo will be predominant within the country for the domestic market by surpassing 50% margin and contributing for exactly half of the domestic share. This pace of growth by Indigo creates a compulsion for other rival airlines in the country to scale up their operations. Infrastructure constraints for the developing field were the biggest problem within the country and this problem will remain as a hurdle for upcoming years as well. This should be addressed by the authorities in a well-organized manner to utilize current facilities while constructing new infrastructures like runways and airport terminals within busy airports. Authorities should come with a strategic plan at least for the 40 largest airports in the country. Introducing sophisticated infrastructures will attract more foreign direct investments which will boost up the economy and hence the aviation industry. With all of these developments projects, it’s utmost important to pay attention towards the means of acquiring resource personnel like Pilots, Aircraft Engineers, Flight Attendants, Air Traffic Controllers and other ground staff. The average age of a pilot in India is 45.8 years and they have limited time to serve. Hence pilot training should be done in an effective way to train native pilots. If not India will have to take the service of expat pilots creating an unfavorable capital flow out of the country. When analyzing the stats and future plans it is quite reasonable to expect a promising future for aviation in India. The authorities should contrive to address above mentioned key problems to maintain the consistent growth for future as well.
February 5, 2018

What is Drag? All Explained Here!- Part 2

What is Drag? All Explained Here!- Part 2   Hope you have gone through the previous article on Lift Induced drag and Parasite Drag. Today will discuss remaining drag types including Skin friction, Foam, Pressure, Interference and Total drag. Skin Friction Drag Skin friction drag is the drag that caused by the adjacent air layer to the moving body. Normally air is at rest before it gets interfered by the aircraft, when the aircraft moves through the air it induced a movement to the air particles adjacent to the aircraft structure and tries to carry the air particles with the aircraft. Energy is transferred to the air particles from the aircraft by causing a loss and this loss is called Skin friction drag. When the aircraft surface is rough, more skin friction is created and by making the exterior surface smooth we can reduce the skin friction drag.     Foam drag When the object moves forward, displaced air changes it’s direction and velocity. Energy to do such changes are extracted from the object and by changing the shape of the object we can reduce the change of direction and velocity of air particles in order to reduce the losses. By making the aircraft parts more streamliner it’s able to reduce the foam drag.   Pressure Drag Let’s take a moving cube as an example and we know it creates a huge foam drag due to the poor frontal shape. Due to the opposition created by the front face of a cube, air particles change their speeds to a very low value while their static pressure increases. Frontal face encounters a pressure greater than atmospheric pressure and aft face of the cube will encounter a pressure lower than atmospheric pressure. This pressure difference creates a sucking effect in the opposite direction to the movement. This sucking effect is known as the pressure drag. Interference Drag If we consider the drag created by the whole aircraft, it is greater than the sum of drag created by individual components. Reason for this is the formed junctions and generating of eddies in the junctions when the air flow passes through them such as wing/fuselage, wing/horizontal stabilizer, wing/nacelle. In order to reduce the interference drag filleting, fairing and streamlining are good solutions. Total Drag Total drag is the sum of all drags. As shown in the diagram induced drag predominates at slow speeds and decreases with the increment of airspeed and become insignificant at high speeds. Form drag is insignificant at low speeds and increases to be the most significant at high speeds. There is a point where the parasite and induced drag become equal and this point is called the minimum drag speed(max L/D ratio). As high thrust is required in both low and high speeds moderate thrust is adequate in medium airspeeds.   The graph shows Coefficient of drag vs AOA for few aircraft and note that coefficient of drag is high at both the extremes. Drag is high at low angles of attack and high angles of attack. For most of the aerofoils, small positive angles will be the best angle of attack with the minimum drag.
January 28, 2018

What is Drag? All Explained Here! Part I

What is Drag? When an object moves through the air it has to displace a certain amount of air mass and this particular air exerts a force which tries to retard the movement of the object. There are different types of drag and they can be classified according to their origin. Lift Induced Drag Lift-induced drag is the drag generates due to the generation of wingtip vortices. As we all know a pressure difference exists on two sides of the wing that creates the lift. High pressure at the bottom side of the wing tries to move towards the low-pressure top side creating a curl. In large aircrafts, this wingtip vortex is strong enough to jeopardize the safety of a small aircraft. Due to this reason, there is a minimum distance that should be maintained in between the two aircraft. In all infinite wings, there is a spanwise velocity component that creates the vortex. Lift-induced drag is a function of lift and when the lift increases pressure difference also increase creating much more strong vortices.   a= geometric angle of attack ai= induced angle of attack ae= effective angle of attack  As shown in the above diagram a downwash component is created due to the vortices and it reduces the angle of attack(a) of the wing by changing the relative airflow direction.  The angle of attack of this aerofoil section reduced to the effective angle of attack(ae). Lift is always perpendicular to the relative airflow and with the changing of airflow direction lift inclined backward creating a component to opposite direction of aircraft movement and this component is called the drag. Wingtip vortices contain a huge amount of transnational and kinetic energy. Where these energies came from?  They are from the engines! So it is a prime importance to reduce the lift-induced drag, if not engine should operate at high power to overcome the drag by burning more fuel and being fuel inefficient. At low speeds and at high angles of attack lift-induced drag is great and it will be greater near the stalling angle of attack. Introduction of wingtips reduce the formation of wingtip vortices and reduce the lift-induced drag to a greater extent.  Parasite Drag Parasite drag/profile drag can be divided into two main drag types called skin friction drag and foam drag. There are other two types of parasite drag called interference drag and pressure drag. This is a function of aircraft speed and when the aircraft speed increases drag to increases. When the aircraft speed doubled the parasite drag is increased by four times. Parasite drag is not much significant at low speeds because lift-induced drag predominates at low speeds. At high speeds, most of the drag is coming from the parasite drag.This drag is independent of the amount of lift generated and parasite drag can exist even the lift is zero. In an aircraft half of the parasite drag is created by the aircraft wings and by keeping the wings cleaner parasite drag can be kept the minimum. Increasing of air density, aircraft speed, and aircraft size increase the parasite drag while being more streamliner reduces parasite drag. Drag= ½ ɋv2scD  Rho: density of air V: velocity V: surface area of the aerofoil CD: represents angle of attack and shape of the aerofoil   Skin friction, Foam, Pressure, Interference and Total drag will be discussed in the next article on the same topic.
January 22, 2018

Growth of Aircraft Fleet in India in Next 5 Years

As air travel is the most prominent means of travel in present day, many new airlines emerged while other operating airlines expand wings to escalate their capabilities. With these developments in the field, the number of aircraft owned by an airline increases in order to accommodate the expanding operations. These expanding processes were supported by the aircraft manufactures with the introduction of more fuel-efficient and sophisticated airliners. By the end of 2017, 3.2 billion passengers have traveled through the air while this figure is planned to appreciate by 50% making the number of passengers up to 4.8 billion in next 10 years. Currently, the world is facing an acute shortage of aircraft to cater the brisk pace of increasing airline passengers.Asia has become one of the biggest aviation markets in the world. Within countries in Asia, Indian emerging market has become the engine of growth due to emerging airliners including many low-cost carriers.Boeing estimates that India will need 1,850 new aircraft worth $265 billion by the year of 2036. In last few years Indigo has made remarkable achievements to become one of the fast-growing airlines in the world. It was started as a domestic low-cost carrier in 2006 with few aircraft and now developed to an airline with nearly 150 aircraft while expanding its wings to the international market by commencing international flights out of the country. Within upcoming years Indigo has planned to buy 400 new A320neo airliners by making it the largest order in the history of commercial aviation. This introduction of aircraft to Indigo will make the aviation field in India superlative by appreciating Indian overall airline fleet. Spicejet, fourth largest airline in India which operates a fleet of 54 aircraft comprehends of Boeing 737s and Bombardier Q400s plays a significant role in the Indian aviation trade. It operates domestic and international flights to 51 destinations worldwide. Spicejet has a 13% of contribution to the Indian passenger market share and this value keeps ticking on. To cater this demand Spicejet has made a plan to upgrade their fleet by introducing more airplanes. They have placed an order for 100 new B737 aircraft and altogether they will acquire 205 aircraft from Boeing, worth $22 billion. With the introduction of these airplanes, Spicejet will be a world-leading airline which flies both domestic and international routes with hundreds of airplanes. GoAir stands with a firm goal of making their fleet of 100 aircraft by the end of 2023. Currently, they own 32 aircraft and contributes to 8.4% passenger market share as the fifth largest airline in India. GoAir has signed a memorandum of understanding with Airbus for 144 new A320neo aircraft. Jet Airways will acquire 100 more narrowbody aircraft by making its fleet up to 175 and Vistara Airlines has made arrangements to expand their fleet by introducing 50 narrowbodies and 50 widebodies. Most of the airliners in India have planned for fleet renewal programs while some airliners already executing their plans. When considering all of these plans Indian sky is about to get busy and upcoming 5 years will be a rebound point for the field with the fast-growing fleet.