A comprehensive overview of the physics involved in climbing and descending in an aircraft. The reader will also gain an insight into the factors that affect climbs and descents.
Introduction
As a student pilot, one of the first basic maneuvers that are taught are climbing and descending, using the primary aircraft pitch control with the elevators (and of course, trim tabs!). All other maneuvers are a combination of basic maneuvers such as climbing, descending, and turning. Once these basic maneuvers are mastered, the student then moves on to more complex maneuvers.
In this article, we will talk briefly about climbs and descents, and the factors that affect climbs and descents, including the pitch control system, pitch angle, and pilot input for aircraft pitch control. We will then discuss Airmanship considerations for climbs and descents, and finish with a suggested flight training exercise for climbing and descending
Physics involved in a climb
In a climb, a thrust is required to overcome drag as well as a component of the overall mass of the aircraft that acts as drag. The ability of an airplane to climb depends on the amount of excess thrust available.
Usually, the take-off and initial portion of the climb after takeoff is flown at maximum thrust or a setting known as ‘max’. Thereafter at a safe airspeed, configuration, and height, thrust is typically reduced to a setting known as maximum continuous thrust (MCT) or a reduced power climb (cruise climb) to prolong engine life and reduce wear and tear on engine components.
For more information, please click the link below:
pilotinstitute.com/principles-of-flight1
“The four forces making up the principle of flight are lift, weight, drag, and thrust. The forces all interact together to determine an airplane’s trajectory. Lift and weight are opposing forces, as are thrust and drag. All are equally important, and they must be balanced to maintain level flight.“
pilotinstitute.com/principles-of-flight1
Factors affecting climb performance
Climb performance is affected by many factors. Some important factors affecting climb performance are:
1. Power setting
It might sound obvious, but power setting has the most impact on climb performance – for example, a military aircraft may climb at MAX power – which is the maximum possible power setting for climb performance to clear near obstacles due to carrying a heavy payload, and then after clearing terrain and retracting gear and flap may set MCP – Maximum continuous power, or even a lower PLA (power lever angle) for a cruise climb based on Standard Operating Procedure (SOPs).
2. Attitude
The climb attitude of the aircraft (with power level set) will determine what type of climb is achieved – a maximum Angle, Maximum Rate or a cruise climb.
3. Speed
The speed at which an aircraft climbs affects how much drag it experiences, and consequently affects both the excess thrust and excess power. Flying an aircraft at the speed for maximum excess thrust results in the best angle of climb, and flying an aircraft at the speed for maximum excess power results in the best rate of climb
4. Altitude:
Since the air is less dense at high altitude / high temperatures, the engine has less excess thrust available and as a result climb performance is degraded.
5. Configuration – Gear and Flap Setting:
Climbing with gear and flaps extended reduces the climb performance due to the drag generated.
Some aircraft are required to take off with ‘Take-off’ or ‘Departure’ flap settings to reduce the take-off ground roll (and get off the ground quicker), and are climbed initially with take-off flap until the landing gear is retracted, but then standard practice is to reduce flap as airspeed increases so you can increase climb performance.
6. Mass (weight):
Higher mass results in poorer climb performances as compared to aircraft with lighter mass.
To understand the effect of these factors on climb performance in detail, please click the link below:
7. Wind
Climbing into a headwind will improve the Angle of Climb, but won’t affect the rate of climb.
8. Thermals and sink
Thermals are rising parcels of air – climbing into a thermal will increase both climb angle and rate, improving climb performance. Actually, Gliders typically exploit this very thing to gain altitude!
However, if you have the misfortune to fly into a downdraught (sinking air) such as below Virga or a thunderstorm, you may not even have the climb performance to outclimb the sinking air – with obvious disastrous consequences. This is one reason why thunderstorms are to be avoided!
9. Wind shear
Windshear is a dangerous phenomenon where wind abruptly changes speed or direction. Flying into wind shear can be incredibly dangerous and has resulted in the loss of many lives – wind shear can appear to either ‘speed you up’ or ‘suck away’ your airspeed in a climb depending on what type or shear it is, with obvious negative consqeuences.
https://www.cfinotebook.net/notebook/aerodynamics-and-performance/climb-performance2
What are the fundamental physics involved in a descent?
A descent can be either powered or unpowered. In case of a powered descent, the power setting is set as required to maintain constant airspeed for a given descent profile. If the descent is unpowered, it is called a glide.
Unpowered or Idle descents can cause issues for piston-engined aircraft due to excessive cooling, so a powered descent profile is more commonly used.
Lift required in a descent is less than lift required for level flight. During the descent, the mass vector that points vertically downward can be resolved into a horizontal and vertical component. The horizontal component balances drag and the vertical component balances lift.
For more information, please click the link below:
What are the factors that affect descents?
Some of the factors that affect descents are the following:
1. Power – the more power setting used, the shallower (lower rate of descent) or faster the descent is flown – depending on the attitude used. When no power is used, this is called a glide descent.
2. Mass:
As mass increases for a given descent power setting, so does the rate of descent, and endurance or time aloft decreases (note the mass of an aircraft does not influence its glide range if flown at best glide speed, but more on this later on down the track)
3. Configuration and Flap Setting:
As flap setting increases, endurance decreases due to higher drag created by the flap extension. Similarly, extending the landing gear increases drag
4. Wind:
Wind has no effect on endurance and rate of descent however, it does affect groundspeed of the aircraft during descents. A headwind steepens the descent angle, whereas a tailwind shallows it.
Descent calculations
You can calculate your descent timings when using a standard descent profile: each company and aircraft has a standard operating procedure that you will get to know. However, a simple descent calculation that is used in General Aviation is a 500ft per minute descent rate. This means for every 500ft you need to descend, it will take you one minute. As a standard descent speed in light training aircraft is 120 Knots or 2nm per minute, each 500ft of descent will require 2nm to descend. We can then adjust this for wind – if you have a 20 knot tailwind, this is about 10% of your speed, so you will need to descend 20% earlier!
As an example – if you are cruising at 6000ft, descending to join a circuit at 1500ft overhead the airport with an elevation of 500ft. You need to descend: 6000ft – (1500 + 200) = 4000ft. This will take you 4000ft/500fpm = 8 minutes, so you will need 8*2=16nm to descend. Your ‘Top of Drop’ is 16 nautical miles away from the airfield. If you had a 10 knot tailwind you would descent 10% earlier, so your ToD would be 17.6nm from the airfield. Conversely, if you had a 10 knot headwind you could delay your descent to 14.4nm from the airfield.
When I was flying for the military, we flew a standard descent profile of 3 degrees, 250 KIAS, and 2000fpm. To calculate our top of descent, we simply multiplied the altitude (in thousands) by 3 to get our top of descent – for example, to descend 30,000ft, the calculation would be 30*3 = 90nm.
Descent calculations are more important when you start learning about cross-country flying and navigation, but it’s worth keeping it in the back of your mind and practicing on the way back from the training area for joining the circuit.
For more information, please click the link below:
principalair.ca/article-the_descent4
Airmanship relevant to climbing and descending
Airmanship is the consistent use of good judgment and decision making. Before continuing with the flying sequence for climbing and descending, it is important to discuss the relevant airmanship considerations relevant to climbing and descending.
- Situational Awareness: An understanding in time and three-dimensional space about where the aircraft has been, is currently, and where it will be, and how this relates to the terrain, other traffic, and the airspace picture. For example, when climbing below controlled airspace, recognizing the restriction and allowing the appropriate buffer, and recognizing the limits and airspace boundaries of the training area.
- Lookout: Lookout is an important part of flying, and critical for VFR pilots. When climbing and descending, aircraft can have significant blindspots which need to be scanned for before commencing a climb or descent, and periodically during one using gentle S turns or momentary leveling the attitude.
- VFR meteorological minima: This might sound obvious but its important to know what your met minima is for your class of airspace so you can maintain appropriate seperation whilst undertaking the flight exercise (for example in Australian Class G airspace, 1000ft vertically and 1500m horizontally from cloud unless below 3000ft altitude then it is clear of cloud and in sight of the ground).
- Minimum height requirements: 500ft AGL over unpopulated terrain and 1000ft AGL over populated areas
- “IMSAFE” personal Checklist before flying: Illness, Medication, Stress, Alcohol, Fatigue, and Eating. Flying with an illness such as a cold may mean you have a blocked eustachian tube which could lead to a burst eardrum on the descent, and many over-the-counter medications can have serious effects on your judgment and flying skills, especially once at altitude.
Human factors
There are several Human Factors to consider when discussing climbing and descending, including trapped gas, hypoxia, decompression sickness, vision and noise.
Trapped gas
One of the major Human Factor considerations for climbing and descending is the effect of altitude on trapped gas within your middle ear. You should never fly with a cold, as you will be able to climb relatively pain-free, however when it comes time to descend, you can experience debilitating pain. This is why I recommend all pilots fly with “Get me down spray” – a nasal decongestant such as oxymetazoline for emergencies.
“On ascent, the Eustachian Tubes open spontaneously allowing expanding gases to travel down into the posterior pharynx and equilibrate pressures. During descent, however, the Eustachian Tube will often not open spontaneously and the middle ear, sinuses, or other closed cavities will experience a vacuum like phenomenon as the gas contracts. Taken to the most extreme case, tissue damage can occur causing hemotympanum (blood behind the ear drum) or even a ruptured tympani membrane (ear drum)”
goflightmedicine.com/2015/04/08/clearing-ears5
Trapped gas can also occur in your gut – so it’s best to avoid foods that make you gassy, and if you need to in flight, just let it out and fart on those long climbs – holding it will make it worse!. Trapped gas can also occur in your teeth – so it’s imperative to maintain impeccable dental hygiene.
Decompression Sickness
Flying after diving can also be problematic, again due to the effect of altitude and reducing pressure. When you dive, the higher pressure forces nitrogen to dissolve into your blood and tissue, and when you climb to altitude, the reduced pressure can cause it to come out of solution and form bubbles – leading to Decompression Sickness (DCS) such as;
- the bends
- the chokes
- the creeps
- the staggers.
Hypoxia
At altitude, the partial pressure of oxygen reduces. Above 10,000ft altitude, you are legally required to be breathing oxygen. This is because your lungs cannot get enough oxygen from the ambient air to sufficiently oxygenate your body – you become hypoxic (hypoic hypoxia – lack of oxygen) which can lead to euphoria, poor decision making, unconsciousness and ultimately, death.
Vision
This lack of oxygen also can affect your vision, reducing your ability to see or detect other traffic (especially at night).
Furthermore, during lookout to a homogenous view (i.e. clear sky or clouds) can lead to empty sky myopia – a short-sightedness that affects pilots staring into the sky causing their focal length to rest only a few meters outside the cockpit. Your lookout can also be complicated due to the surrounding terrain and shadowing. This can be especially damaged if flying with a dirty windscreen covered in bugs or dust – It is imperative to clean your windscreen before every sortie, and use a disciplined ALAP work cycle with a saccade lookout scan as you move your eyes systematically and focus on various points in your field of view.
Aircraft management
Aircraft management is important during climbing and descending. You will use primary controls that will be used to enter and exit from climbs, as well as secondary controls such as trim, and auxiliary controls such as mixture, carburetor heat and cowl flaps which may used depending on your aircraft. Furthermore, you will need to be familiar with performance and engine instrumentation.
Throttle
It’s important to understand the operation of the throttle correctly – for example in most light training aircraft with a fixed pitch propeller, the throttle will be the only power control.
Not all aircraft can climb on full power continuously, so you must know your aircraft type and if you are required to reduce power after a certain time period to a cruise climb. Similarly, you must also understand the RPM limits in order to prevent a propeller overspeed in descent (high power and a fast descent can lead to an overspeed – you must reduce power in a descent to prevent this).
Engine Cooling
A prolonged glide at low or idle power should be avoided, as this can cause excessive cylinder cooling and potential plug fouling. This is why we typically maintain power in the descent, according to the descent profile, as well as close cowl flaps if fitted.
Mixture
For most light training aircraft, full rich mixture is used when climbing to aid engine cooling and prevent possible detonation. Above 5000ft, some leaning may be necessary to ensure smooth operation and maximum power output. Once at altitude, the aircraft mixture is leaned appropriately. It is common practice to use a full rich mixture in the descent, in preparation for a subsequent level off or approach and landing
Carburetor heat
Carburetor heat is typically selected to on during a descent due to the increased chances of carburetor icing at low power settings at altitude. It is important to ensure carburetor heat is selected to OFF or COLD during a climb due to the detrimental effect on engine performance (causing reduced climb power / performance).
Temperature and pressure gauges
In the climb, it is normal to see an increase in oil and cylinder head temperatures with a decrease in oil (and fuel) pressure. In the descent, it is normal to see a decrease in oil and cylinder-head temperatures and an increase in oil (and fuel) pressure.
You should monitor the “T’s & P’s” for any anomalies during climbs or descents which may indicate an issue, as part of your Performance scan in the ALAP work cycle – if you see something slightly out of the ordinary, double-check the position of the cowl flaps, carb heat, and consider levelling from your climb or descent in order to trouble shoot.
Cowl Flaps
Cowl flaps are used to provide additional cooling air intake for the aircraft engine during ground operations and long high power climbs. Depending on your aircraft, these should be left open for start, taxi, take-off and climb, and then closed during level flight and descents
Flight exercise: Climbing and Descending
The following flight exercise is for Climbing and Descending an aircraft so as to become familiar with these phases of flight,
The aim of this exercise is to pitch the aircraft smoothly between level, descent, and climb attitudes, without any of the further effects of controls becoming evident. Imagining the extended longitudinal axis of the aircraft will enable the perception of smaller attitude movements than would be possible if your focus isn’t as far away.
Using elevator input alone doesn’t always accomplish the goal cleanly and rudder pressure is required to anticipate the secondary effects of power application (yaw). A pilot must then hold the required attitude and remain in balance whilst trimming out control forces, using their work cycle.
Climbing the aircraft
To enter a Climb, simultaneously select full throttle and use the pitch control (elevator) to smoothly and positively raise the nose attitude to the climb attitude (sight picture – usually raise the cowl to the horizon). You can use the keywords ‘Power, Attitude, Trim. You do this, using the ‘Select-Hold-Trim’ method;
- SELECT – Apply full throttle and back pressure on the control column or stick to raise the nose to the correct climb attitude. Anticipate yaw with the rudder to keep the ball centered
- HOLD – hold the climb attitude sight picture as airspeed decreases to the climb speed – as airspeed decreases you will need to apply more back pressure to hold the stick back.
- TRIM* – Trim out the residual pressure using the trim wheel or tab so you can fly hands off.
*Note that you don’t need to wait for the aircraft to completely decelerate to climb speed before you trim. Select-Hold-Trim ‘cycles’ will need to be done for any control inputs to ensure hands off flight. Your aircraft may require you to open the cowl flap to provide additional engine cooling.
Once you have entered the climb, continue with your regular ALAP work cycle (attitude, Lookout, Attitude, Performance)
Exiting a climb
To exit a climb, you simply use the same keywords – ‘Attitude, Power, Trim’. However, it is important to remember you will need to allow the aircraft to accelerate back to cruise speed before reducing the throttle to cruise power.
- SELECT – Anticipate the level off by approximately 10% of the VSI (usually 50 to 100ft) and begin to apply forward pressure on the control column to smoothly lower the nose to the cruise attitude.
- HOLD – hold the straight and level flight attitude, as the aircraft accelerates to cruise speed you will need to apply forward pressure to maintain constant attitude. Maintain balance with rudder
- TRIM – Trim out the residual pressure using the trim wheel or tab so you can fly hands off (you can trim before reducing the throttle but will have to re-trim again after).
*Note that you don’t need to wait for the aircraft to fully accelerate to cruise speed and reduce to cruise power before you trim. Select-Hold-Trim ‘cycles’ will need to be done for any control inputs to ensure hands off flight. Once levelled off, if you did open the cowl flaps, you may need to close them.
Descending the aircraft
To enter a descent, use the pitch control (elevator) to smoothly and positively lower the nose attitude to the descent attitude (sight picture), and wait for the aircraft to accelerate to descent speed before reducing to descent power. You can use the keywords ‘Attitude, Power, Trim’. This means using the ‘Select, Hold, Trim’ method
- SELECT – Applying forward pressure on the control column or stick to select the descent attitude
- HOLD – hold the descent attitude sight picture as airspeed increases – as airspeed increases you will need to apply more pressure to hold the stick forward.
- TRIM* – Trim out the residual forward pressure using the trim wheel or tab so you can fly hands off.
As airspeed approaches the standard descent profile speed for your aircraft, reduce power to the descent power setting (and select carb heat HOT if fitted and required, and check cowl flaps shut) required to maintain standard descent speed. For a given descent profile (ie 3 degrees and 250 KIAS for the C-130 Hercules) this will result in a known rate of descent. Remember to maintain balance
*Note that you don’t need to wait for the aircraft to accelerate to descent speed and reduce throttle before you trim. Select-Hold-Trim ‘cycles’ will need to be done for any control inputs to ensure hands off flight. Many light aircraft will use a ‘cruise descent’ at the same speed as normal cruise.
Leveling from a descent
To level off from a descent, simultaneously raise the nose to the cruise attitude and increase power to cruise speed. You can use the keywords ‘Attitude, Power, Trim. You do this, use the ‘Select-Hold-Trim’ method;
- SELECT – Apply back pressure on the control column or stick to raise the nose to the cruise attitude and increase the throttle to the cruise power setting (select carb heat COLD if fitted).
- HOLD – hold the straight and level flight attitude sight picture as airspeed reduces to cruise speed.
- TRIM* – Trim out the residual pressure using the trim wheel or tab so you can fly hands off.
*Note that you don’t need to wait for the aircraft to decelerate to cruise speed before you trim. Select-Hold-Trim ‘cycles’ will need to be done for any control inputs to ensure hands off flight, and for many light aircraft you may use a ‘cruise descent’ at the same speed as normal cruise. Once levelled off, remember to turn CARB heat back to Cold.
Eventually, you should be able to adjust power and attitude between straight and level, climb and descent smoothly and instinctively, and trim each attitude so it stays where you leave it.
Practicing climbing and descending
The basic objective is to control an aircraft’s attitude in pitch (and roll and yaw) as well as prevent undesired secondary effects of power, pitch and airspeed. These exercises build your skillset to form proper core attitude flying skills with reference to the horizon.
To practice climbing and descending, initially begin by pitching the aircraft from level flight to a standard climb attitude, pausing momentarily and then lowering the attitude back to the straight and level – repeat that simple exercise until it is done smoothly and any movement in roll or yaw is corrected with coordinated control pressure.
Once this is achieved, the next step is to hold the attitude until it stabilizes without trimming. This can require a lot of control force!
When you can hold the attitude constant by coordinating the controls, the next step is to trim so the attitude is held constant without any control inputs. If the attitude remains stable after releasing the controls long enough to allow any out-of-trim attitude movement to show (5 seconds or so), then aircraft is trimmed correctly.
Once proficient in entering, maintaining, and leveling off from climbs and descents, a coordination exercise should be flown whereby you repeatedly climb and descend an aircraft through a 500ft block of altitude – for example in the training area between 2500ft and 3000ft altitude.
Once this is nailed – it is time to move on to the next lesson.
Conclusion
The overall idea with these exercises is to enable the pilot to know how the aircraft responds in reality, and for the pilot to make it do what is commanded purely by observing the picture out the front and his/her senses – not the instruments. The whole idea is to ensure you know how to control an aircraft by controlling power and attitude before introducing additional information that isn’t really necessary at this early stage.
By ensuring the foundations of skill and consciousness are sound from the beginning, things will become easier and clearer over time, and you can build upon these core VFR attitude flying skills to further complex flying skills. However, if they’re flawed from the start, flying is a constant struggle against the laws of physics, aerodynamics, and our own natural abilities.
Further reading:
A brief overview can be found here:
southernwings.co.nz/aerodynamic-forces-in-climbs-and-descents/6
Reference List:
- Principles of Flight – The 4 Flight Forces Simply Explained, Pilot Institute. Published: May 31, 2022. Accessed online at https://pilotinstitute.com/principles-of-flight/#:~:text=The%20four%20forces%20making%20up,balanced%20to%20maintain%20level%20flight on June 7, 2023.
- Climb Performance, CFI Notebook.net. Accessed online at https://www.cfinotebook.net/notebook/aerodynamics-and-performance/climb-performance on June 7, 2023.
- How do planes descend for landing?, Aviation.StackExchange. Accessed online at https://aviation.stackexchange.com/questions/30820/how-do-planes-descend-for-landing on June 7, 2023.
- The Descent, Principal Air. Accessed online at http://www.principalair.ca/article-the_descent.htm#:~:text=As%20with%20climb%2C%20the%20weight,all%20affect%20an%20aircraft’s%20descent on June 7, 2023.
- Clearing Your Ears, Go Flight Med. Accessed online at https://goflightmedicine.com/2015/04/08/clearing-ears/ on June 7, 2023.
- Aerodynamic Forces In Climbs And Descents, Southern Wings. Published: Feb 27, 2019. Accessed online at https://www.southernwings.co.nz/aerodynamic-forces-in-climbs-and-descents/ on June 7, 2023.
- Climbing and descending, Aviation.govt.NZ. Accessed online at https://www.aviation.govt.nz/licensing-and-certification/pilots/flight-training/flight-instructor-guide/climbing-and-descending/ on June 7, 2023.
