# How to calculate the altitude my aircraft normally loses in two minutes of gliding descent?

As this may be a rookie question, I just can't wrap my head around if it is an approximate or established number. Doesn't it always depend on different conditions, such as wind(affecting AGL immensely) or temperature?

This is needed to determine the high key position for the 360 Forced Landing Pattern.

UPD:

I fly a Piper PA-38(best glide 70KIAS)

Once you open Flight Training Manual 4th edition on the page 129 you will find the following section - 360 Forced Landing Pattern.

I will write exactly as it goes:

"The turn is started with the aircraft heading in the direction of the intended landing at an appropriate altitude over the desired touchdown point. This starting position is called the high key. The appropriate altitude depends on the glide perfomance of the aircraft. To calculate this altitude, take the amount of altitude your aircraft usually loses in two minutes of gliding descent, add 200 feet, and you have the height above ground that works well for high key. For example, if your glide rate is 600' per minute, the high key altitude should be 1,400' AGL (2x600 + 200).

What I'm asking is why does the book state it in a way as if I knew my usual ROD every time I'm in the air?

• How do I know precisely what the high key position for my aircraft and configuration is?
• They are, in fact, making calculations after determining the high key and, therefore, I'm presuming that IT IS REAL and can be measured.

Apologies for this perplexing question. I should've probably made it clear from the beginning that its RoDx2 that I'm talking about.

I'm a novice and only two weeks into my flight training(I've had only one flight), yet I'm reading everything in advance to perform better on my flights and make the best of it. Appreciate all of you guys, you're being so helpful.

• What information do you have? (you can probably figure RODx2...) Need glide ratio, speed, rate of descent, or some combination of numbers to determine the answer. Also lift lost due to AOB. Wind won't affect your rate of descent, but will affect your pattern. Commented Aug 27, 2022 at 14:29
• The book states it as if I must know it somehow from the outset, even though the only number I have is the best glide(70 KIAS). It is definitely RODx2, but how do we determine it in the first place? I'm not really sure what the appropriate formula is. I'm about two weeks into my flight training and it seems a bit confusing. Commented Aug 27, 2022 at 14:59
• What aircraft are you asking about? Is there no chart for glide distance in the manual? Commented Aug 27, 2022 at 18:16
• That is a question about the aircraft's sink rate at any given airspeed. Sink rate is completely independent of horizontal wind component, and is only slightly affected by air temperature. If you don't understand why sink rate is completely independent of horizontal wind component (excluding the cross over effect between horizontal wind component and vertical wind component due to slope lift, mountain wave lift, etc), then you need to study up, and maybe ask another question. -- Commented Aug 28, 2022 at 19:33
• I don't mean to be difficult, but this is in a lot of respects a "read the manual to me" kind of question. I suggest that you go through the manual and look at all of the performance charts in it. That may shed some light. Before you ever do an approach from high key for real you will have done one with your instructor so that you can see what it is that the book is talking about. (I spent the better part of 4 years teaching the 'engine out' emergency landing profiles as a flight instructor, but not in this model of aircraft). Sometimes, the demonstration clears up misunderstandings. Commented Aug 30, 2022 at 15:31

This will be much more intuitive if you move on to instrument training, where referring to the climb/descent rate gradient will seem like a daily thing.

POHs are different. A 172N for instance, gives only a speed and a graph. Others will give speeds calibrated by weight. Some will give glide ratios. But often in parametric physics equations, you have a few known values that you have to then convert into additional "intermediate" known values, that you only then can convert into the answer you need.

So, 172 as an example... Best glide is 65, and I can tell by the graph that I'll cover about 1.5 NM per 1,000 feet of altitude. Let's get to work!

1. 1.5 NM = 9,000 feet, which means my glide ratio is 9:1, or expressed as slope is 1/9 (rise over run). We don't need to know the degrees of slope, but if we're interested it works out using the tangent formula to be 6.3 degrees.

2. If I am moving at 65Kts (remember this is indicated, and both calibrated and GS will be different) then lets treat that 65 as horizontal motion. Yes, technically it is the hypotenuse of a 1:9 triangle but the math required isn't justified by the fraction of a knot it will correct. So at 65 Kts, I am covering about 6,500 FPM horizontally before compensating for wind. Beyond a few thousand feet, miles are more useful than feet, so 6,500 feet is 1.083 NM.

3. I know from my glide ratio that if I am covering 6,500 FPM, a 1:9 ratio means I am descending at 720 FPM. I can solve this just dividing 6,500 by 9, or I could have used the Pythagorean formula in step 2. But at these glide ratios, division is accurate enough.

4. How long I will be aloft will therefore be AGL / 720, in minutes. Or that answer times 60 if you need seconds.

5. How far I glide will be the time aloft (answer 4) x 6,500 feet, or if more useful in NM, it's time aloft x 1.08NM.

Now I have all the info I need to actually create a table, remmebering these estimates are in NO-WIND conditions.

AGL Time mm:ss Glide Dist NM Glide Dist Ft
500' 0:42 0.75 NM 4,500'
1,000' 1:23 1.5 NM 9,000'
2,000' 2:46 3 NM 18,000'
3,000' 4:10 4.5 NM 27,000'

Remember!!!:

• This is KIAS which at slow speeds will be slower then your KCAS, see your aircraft's calibration table in Section 5. At 65Kts a 172 only has about 1-2Kts of error at most, but a 182RG could be 4 knots, and that's material to a glide plan.

• Wind can make your groundspeed VERY different than KCAS, and in any ground-reference maneuver, you MUST take wind (and thus GS) into account. And a gliding power-off landing is definitely a ground-reference maneuver, LOL.

• This does not take into account the drag and load factors of a turn. These are HUGE considerations in glide descent planning. The POH assumption is always "land straight ahead, and you hit what you hit."

• This does not take into account recognition time. You can eat a lot of speed or a lot of altitude in just the few seconds it takes you to interpret the problem and commit to action.

• This does not take into account your amount of rehearsal. Recency of practice of power-off returns to field has been determined to be the single largest determinant of success, even for experienced pilots.

• Finally, remember that any extra speed, altitude, or wind toward the field that is occurring at the key position increases your odds of inadvertently over-running the runway and going off the far end. We get so focused on protecting the glide that we sometimes forget we still have to land.

Hope this all helps!

### Adding some PA38 Specific information to better suit the OP

Here's a "typical" glide performance table from the PA38 POH. Piper publishes this detailed table in section 5 (Performance) of your POH, whereas Cessna provides only a cursory graph in section 3 (Emergencies).

Some of the comments on this thread imply it's somehow a legal violation for you to have access to glide performance information, and I find those comments to be unacceptable and have stated that clearly. The information is right in the POH, there's no secrets being revealed here.

You have to be careful about using this information only to form your own "intuition" about how your glide performance changes over a range of conditions, and not as a specific glide ratio you calculate at the moment of engine failure, as there won't be time.

What this chart shows is that at standard temperatures your glide ratio will be on the order of 7.5 to one, which is somewhat steep--which is what we'd expect to see with a glide speed of 70 Kts.

At 70 knots you are moving forward at roughly 7,000 FPM horizontally (no wind), and a 7.5 to 1 glide ratio means that you'll be descending in the neighborhood of 933 FPM. That means that from 2,000 feet you'll be looking at just over 2 minutes aloft. This is useful information to help think about what remedial actions I might have time to take, such as a restart attempt, but can't and must not be used for real "will I make the clearing" calculations.

This chart also tells me on a very hot day, say 10 deg. C above standard, I might see glide ratios more like 10.5 to 1, which is about 666 FPM vertically. From 2,000 feet AGL I now have 3 minutes aloft and cover about 3.5 NM before wind.

This also means that on very cold days, your descent will be steeper at 70 KIAS, and your time aloft will be shorter and distance traveled will be as well.

I absolutely encourage you to spend some time with your instructor both at the white board and in the airplane to determine under what conditions you might adjust your glide speed based on weight and atmospheric conditions. But that's not an exercise for strangers on the web.

### Remember, there are many factors

The factors include weight, density altitude, load factors if turns are required, recency of practice, atmospheric conditions, And WIND, WIND, WIND, WIND. The purpose of this exercise is to give you a mental picture, an intuition that serves as the starting point for your own thinking about how various atmospheric conditions affect glide performance.

As you move to other aircraft, the dynamics will change a lot and it's always worth spending some time with your instructor really digging into the performance tables and understanding "so what does this all mean."

Always, always strive to understand the factors that uniquely affect the performance of every single aircraft you fly, in every atmospheric condition that is common to your region.

• check UPD section Commented Aug 29, 2022 at 0:12
• Thank you for your great work! I will definitely spend some time with my instructor delving into all the subltle details I must understand about flying my aircraft. Cheers Commented Aug 30, 2022 at 0:50

Rate of descent is easy. What you need for emergency landing high key is ground speed.

Because the aircraft "moves with the airmass" rate of descent for a given airspeed and (flaps) configuration does not change with wind conditions. Ground speed does.

Your set up for an emergency landing can be practiced with gliding normal landings. (I combined my emergency and normal landing techniques due to limited flying at one (uncontrolled) field).

Landing without power enables one to practice glide angle control techniques by adjusting drag instead of thrust. A Cessna 172 has 4 flap settings (0, 10, 20, 30) and can also be slipped.

So, when it really counts, come in a little high and set up to make midfield clean based on wind direction, then slip to shorten glide path a bit if needed.

In an emergency, it is way better to land long with 10 knots remaining than to crash short of the runway.

• I covered mostly "the math" but Robert really provides excellent, must-read insights into the practice. Hitting the trees at the far end with 15Kts left to bleed off is infinitely better than the near-end trees at 65. Anywhere on the airport grass is still better than in a neighborhood, etc. And every year we lose so many pilots because we fear the ground more than we fear loss of control. There is no speed you can get down to that makes crashing inverted in a stall a better choice than crashing upright producing lift, PERIOD. If I crash, I'm going to crash producing lift. Commented Aug 27, 2022 at 19:31
• Has nothing to do with the answer, he is asking whether or not, he can know how much altitude he will lose in two minutes, and there is no accurate answer, bottom line.
– user64784
Commented Aug 27, 2022 at 20:10
• @UshbyTechnicalTeam altitude loss covered in paragraph 2 and further more in your answer Commented Aug 27, 2022 at 22:12
• check UPD paragraph under my question. Thanks for the answer! Commented Aug 29, 2022 at 0:14

Why calculate? in the pilot's manual for your plane it should list the descent rate in feet per minute for the plane when trimmed for best glide speed.

(feet per minute) x (2 minutes) = feet.

• Can you show one manual that says that ?, I don't think so Sir !
– user64784
Commented Aug 27, 2022 at 20:11