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Descent Profile

by K.Haroon: Last Update on 29th October 2005
Maintaining The Descent Profile of an F27

Flying a good descent profile is very important in an airline operation. Descending early results in flying too long at low altitudes, wasting time and fuel, while descending late results in unnecessary turns and high rates of descent which are uncomfortable for the passengers. This article is about flying the F27 while maintaing the descent profile.

First thing you need to do is to calculate your top of descent (TOD). It is fairly simple. For example if you are cruising at 12,000 feet and your destination is 1000 feet above MSL then the height you want to loose is 12,000 - 1000 = 11,000 feet. Now take the first two digits of 11,000 i.e. 11 and multiply it by 3. In this example it will be 33, which means that cruising at 12,000 feet you will intercept your profile at 33 DME. (In case you were cruising at 9000 feet, you will take only the first digit i.e. 9 for multiplying by 3).

You can add about 5 miles more so that when you reach your desired height you are slightly below the profile. This will give you a short segment of level flight to intercept the profile, which will help to bleed off your speed and make you compatible with the arrival phase and comfortable in the airport traffic area. So in our example the TOD would be 38 DME i.e. 33 + 5. Another thing which you can cater for is winds if they are significant. Delay the descent for headwinds and vice versa. You can also cater for track miles to go if you are on vectors, but keep it simple for now.

Once you have calculated the TOD e.g. 38 DME in our case. Start your descent when the DME shows 38. Initially pitch down and attain the desired speed (start with 180 knots and readjust later on) and bring the torque to about 100 psi. With this configuration, an average rate of descent of about 800-900 fpm should keep you on the profile. Now you are all set, and all what you need to do is some mental maths to check whether you are on the profile or not so that you adjust accordingly.

From now onwards, for every thousand feet you loose you will do two things. First a simple calculation which is followed by an adjustment to maintain the profile. You can do this in two stages to remain ahead of the aircraft. Start your calculation when the anticlockwise moving altimeter needle (i.e. the 100 feet pointer) is at the 6'O Clock position (or even before if you need more time to calculate) and do the adjustment to maintain your profile when the altimeter needle climbs back to the 12'O Clock position.

Having said about the time for calculation and adjustment, you need to know what the calculation is and how to adjust. The calculation is simple multiplication, one that was involved in the calculation of the TOD. You just need to know the table of 3. As far as the adjustment is concerned, its just pitching up or down to regain your profile. However you will be using your power and flaps for controlling your speed which varies with the changes in pitch. Now lets get back to our example to see how this all is going to be done.

Cruising at 12,000 feet with TOD at 38 DME you start your descent and the altimeter needle drops and starts its anticlockwise movement. Lets say you are descending out of 9000 feet and want to check whether the aircraft is on the profile or not, start the calculation when the needle is at the 6'O clock position (or before). For calculation you need a value. That value will be what the altimeter shows when its needle climbs back to 12'O clock position, as that will be the altitude where you'll figure out how far away from the profile you are. The calculation done before hand is just an anticipation for that. Thus descending out of 9000 feet, the altimeter will show 8000 feet when its needle is up again at 12'O clock position, so take this 8000 feet to start your calculation. Subtract the destination's elevation from this value. In this case it will be 8000-1000 = 7000. Multiply the first digit of this value by 3. It will be 7 x 3 = 21. This 21 is your final answer.

Now a few words about the significance of what you calculated i.e. 21. If at 8000 feet your DME shows 21 or close to it then you are on profile and no adjustment is required. If the DME shows more than 21 then you are below your profile and you need to go up. If the DME shows less than 21 then you are above your profile and you need to go down. See the following figure for visualizing this.

Descent Profile

Another way of calculating and checking if you are on the profile or not is by using the distance available to you. Lets say you are descending and want to check whether you are maintaing the correct descent profile or not. Simply multiply the distance by 3. Lets say the distance of the runway, (elevation 1000 feet), where you are going is 30 miles. 30 x 3 = 90. Add two zeroes to make it 9000 feet. To cater for speed reduction you need a level segment, so you should be below 9000 feet at 30 miles. How much below will vary from aircraft to aircraft depending on its deceleration characteristics. Lets say you need to be 2000 feet below to cater for speed reduction. Considering the runway elevation of 1000 feet the claculation becomes 2000-1000 = 1000 feet which after subtracting from 9000 will give you 8000 feet. So 8000 feet at 30 miles will keep you on the correct profile. If elevation is 0 then 2000-0 = 2000 which after subtracting from 9000 gives 7000 feet. So in this case of 0 elevation, 7000 feet will keep you on the correct profile.

If a point in the descent path has a height restriction e.g. a waypoint ABC has to be crossed at 4000 feet then first determine the distance of the point from your position. Lets say it is at 10 miles. The calculation will be 10 x 3 = 30 i.e. 3000 and then adding the restriction of 4000 makes it 7000. So 10 miles from that point you should be 7000 feet (considering no speed reduction).

So far you have calculated the TOD and managed to figure out whether you are maintaing the profile or have deviated from it. Next thing is to come back to the profile, incase you have deviated. Its just like intercepting the Glide Slope in an ILS approach. There are only two possibilities i.e. either you are below the profile or above it. Lets discuss both the situations separately.

If you are below the profile it means you were descending at a higher rate than desired. All you need to do is to pitch up a little bit for reducing your rate of descent. This will automatically bring you back to your profile just like you intercept the glide slope from below. How much to pitch up and how much rate to reduce depends upon how far below you are from the profile. You might also have to add some power since pitching up will cause your speed to drop.

If you are above the profile it means that you were descending at a lower rate than desired. Now you need to pitch down for increasing your rate of descent. This will bring you back to your profile. How much to pitch down and how much rate to increase depends upon how far above you are from the profile. If you have to pitch down significantly then you will have to control your increasing speed. This will be even more important when you are close to the airfield. Speed reduction can simply be done by reducing the thrust and increasing the drag. Thrust can be reduced by the power levers and drag can be increased by the flaps (since F27 does not have speed brakes) and the gears. However in F27 there is a sequence of using the power, flaps and the gears for speed reduction. The sequence is as follows:

  1. Power Levers

  2. Flaps 16

  3. Gears Down

  4. Falps 26

  5. Flaps 40

Just remember the sequence and apply accordingly. Since your torque was around 100 psi when you started the descent, reduce it further if you are above the profile. You can bring it down to as low as 40 psi. Although you are allowed to go below 40 psi but you'll have to make sure that you dont fly with torque below 40 psi for a longer duration. If you think that flying below 40 psi for a shorter duration will not help then pitch down more and select Flaps 16. If you are still high then extend your gears. Though logically you should be selecting Flaps 26 after Flaps 16 but due to an inherent safety feature you will have to take the gears first i.e. before selecting Flaps 26. If you dont and your power lever position is below 10,500 RPM, your landing gear warning horn will be activated and you will be unable to shut it off unless you move the power levers forward or lower your gears. So after Flaps 16 take your gears and then Flaps 26 if you are still high. by K.Haroon: Last will be selecting Flaps 40. The bottom line is that you keep on pitching down for regaining the profile and control the speed with power, flaps and gears in a sequence mentioned above.

Thus for every thousand feet you loose while descending, keep calculating to check whether you are on the profile or not. Incase you are not, apply the relative correction and recheck at the next calculation time, whether the correction applied has made a difference or not. Keep on applying the corrections untill you are on the profile.


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Disclaimer: Information in the article "Maintaining The Descent Profile of an F27" is a personal feedback, opinion or suggestion by the author that does not sanction any pilot to violate his Company's Standard Operating Procedures, Aircraft Manuals or Manufacturer's Recommendations.


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