Emergency Glide Ratio

[Pugachev Cobra]

Hello folks,

I've reached a crossroads on an important matter and want to know your opinion on this.

When I was training an engine failure procedure on a C152 that started on the base leg of the traffic circuit, the aircraft was already configured with 20º flaps.

The flight instructor then discussed with me if I should or should not retract the flaps to reach the runway. Quickly I tried to explain to him that in theory and my intuition told me that you should retract it - thinking that the best glide speed is obtained with flaps up - but I've heard many times people say that once you're configured for landing, you don't change the configuration, even in an engine failure.

So he said "Ok, let's see if we can reach the runway." I maintained the best glide airspeed, but quickly it became obvious that we wouldn't reach the runway: we were losing altitude quickly.

Then I told him we couldn't reach it, and he just retracted the flaps.

Without power or anything, just maintaining the best glide airspeed, the aircraft reached the runway safely.

He then explained to me that the additional drag from the flaps makes it impossible to reach it, and that's why you should retract the flaps immediately after an in-flight engine failure under these conditions.

So I was happy to know that what you learn in theory is applied in real life.

Some days later, I was talking to a far more experienced flight instructor, with many more hours than the previous one, and I explained to him that situation.

He told me the exact opposite, that what happened was pure luck, that atmospheric conditions helped the aircraft and in other circumstances I wouldn't reach the runway.

He then told me a recent case on that same runway that a similar aircraft had a real engine failure on final approach. The pilot judged he could not make it to the runway, so he dived in steeply to gain airspeed, going below the runway height (this runway is located on a higher ground than the sorroundings), and pulled up while lowering almost full flaps (I don't think it was full flaps). So - the instructor said - the aircraft 'inflated' when the flaps were lowered, quickly gaining altitude, and went above the runway, reaching it and landing safely.

As you can see, the two stories contradict each other. In one occasion, you should retract flaps, in the other, you should deploy flaps after diving.

I remember back in my aerodynamics class when I learned that maintaining the best glide speed was the only way to reach the farthest place possible. If you try to dive to gain more speed, and then pull, you're going to decrease your range. It was said that's what some people do when they get desperate.

Now I don't know what to make of this, the theory says one thing, I saw it working in practice, but then a far more experienced pilot, and former instructor, tells me the opposite.

In the aircraft's manual, it references to the best glide speed with flaps up. It doesn't mentions best glide with flaps down, so you assume that the best glide is always with flaps up.

Please, anyone who can shed some light on this would be greatly appreciated, even referencing books or some other source.

Thanks in advance!

[plugster]

Best glide speed refers to best lift to drag ratio. Although there is a best lift to drag speed with any configuration, it will be decreasing the more you increase parasite drag on the aircraft (adding flaps or gear). So when experiencing an engine failure when the aircraft is clean and adding flaps the difference between your actual speed and best L/D = glide speed gets bigger ( in most cases unless you fly the aircraft close to stall speed),i.e. you waste lots of energy and thus gliding distance. Furtheron the L/D ratio itself will greatly decrease as soon as you change from clean configuration. So even when maintaining best glide speed for flaps (and gear) extended your descend gradient will always be steeper than clean. A windmilling prop will make a big tradeoff on gliding distance, so feathering the prop ( if possible) is crucial.

[FCS Explorer]

a question to that: are the flaps running on battery power once the engine is out?

[Intruder]

As with many situations in flying, "it depends"... Overall, though, I agree with your first instructor.

First, it depends on airplane performance. In a light single-engine airplane, there is little difference in stall speed with flaps up or down. Also we almost always fly the landing pattern above flaps-up stall speed, even when the flaps are down. When you aare able to safely raise the flaps without having to dive to gain airspeed, raising them is a good idea when you are at higher altitude (above ~500', depending on conditions). After you establish your flaps-up glide, assess the situation and lower them as drag is needed to make the runway.

Your second instructor gave you a couple examples of techniques that may be used by experienced pilots in extremis. The "dive below airport altitude" situation depends on having enough airspeed, the flight characteristics of the airplane when flaps are extended, and assessing potential ground effect advantages. Similar stories about "jumping" fences abound. However, if you happen to be flying an airplane that does not "balloon" when flaps are initially lowered, the technique may not work, and you need to be VERY aware od airspeed.

The bottom line is that best STABLE glide is with flaps up, though you may use some short-lived tranisent effects to your advantage.

[john_tullamarine]

so feathering the prop

or, for a CSU single, going to full coarse pitch - makes for a very noticeable difference.

[adverse-bump]

Firstly, try not to think of height and speed as two separate things. Think of them as one thing = ENERGY the a/c only has so much energy it cant be gained once the engine has stopped.

With regards what to do with the flaps, its does come down to judgement. If it was me, I would retract them straight away (unless you were taking off) - you can always pop them back down. I never used to like using flaps when i was teaching, firstly because the flaps on the ac I was on were crap! but mostly bcoz there are better ways to manage energy. sideslipping is a great way of loosing energy, as you can be very precise about how much you lose. Another good way to loose height is not fly at the best glide speed (because its the best glide speed) have alook at the drag curve, youll see the faster you go, the more drag you produce.

If your in any further worries about whether you should re-track the flaps, think of it this way. If a certain BA pilot hadnt retracted the flaps on a 777 after the engines both quiet on him, lots of people (on and off the a/c) would have died

[Pugachev Cobra]

Thank you all for the replies, very insightful.

John, what is a CSU Single?

And adverse-bump, are you referring to last year's Heathrow accident?

If so, can you refer to me to where I could read about the retraction of the flaps? Thanks!

[Mysha Da Kat]

The best gliding performance for any given airplane is determined scientifically by engineers and test pilots during the certification process. The speed and configuration required to achieve it are then published in the relevant operating manual. Any other procedure that somehow results in better performance is probably a fluke. Notice that it's only the survivors who can brag about their gliding procedure being better than the manufacturer's. There are probably a lot more who are not in a position to tell us they wished the used the published procedure.

Depending on where you are in the traffic pattern when you lose your engine, retracting the flaps may or may not be a very good idea. For example, if you had 10 degrees of flaps out in your Cessna on downwind, by all means retract them. But if you had full flaps on base leg, retracting them would probably cause you to fall out of the sky.

Making the runway or emergency landing area may require you to fly a less than rectangular pattern. Don't hesitate to cut that base-to-final corner out of the pattern if you are short on available gliding distance. A gently banked turn from the end of downwind directly to short final may be the best route.

I've only had one total engine failure in a single-engine airplane in 37 years of flying. That was in a fully loaded Cessna 206 about 3 minutes after takeoff. I managed to glide back to the airport and land downwind on a different runway than the one from which I took off.

Keep practicing your simulated engine failures and get familiar with how your airplane performs power off. Know what a "good" power-off pattern looks like from any position relative to your intended touchdown spot. Your life may depend on it one day.

[john_tullamarine]

John, what is a CSU Single?

Sorry for the apparently local jargon .. single engine aircraft with a constant speed prop.

[Joshilini]

And adverse-bump, are you referring to last year's Heathrow accident?

If so, can you refer to me to where I could read about the retraction of the flaps

Not last year but the year before - January 2007.

"At 240 ft the aircraft commander selected flap 25 in an attempt to reduce the drag."
- AIIB Interim Report: http://www.aaib.gov.uk/cms_resources.cfm?file=/G-YMMM%20Interim%20Report.pdf

[hawk37]

Adverse-bump said

"If a certain BA pilot hadnt retracted the flaps on a 777 after the engines both quiet on him, lots of people (on and off the a/c) would have died"

Did the pilot in fact do this?

[adverse-bump]

Did the pilot in fact do this?

erm, yes!

taken from the aaib web site

At 240 ft the aircraft commander selected flap 25 in an attempt to reduce the drag.

read the report for yourself if you like!

http://www.aaib.gov.uk/cms_resources.cfm?file=/G-YMMM%20Interim%20Report.pdf

if he didnt, he did a very good job tricking the fdr into thinking he did!

[adverse-bump]

And adverse-bump, are you referring to last year's Heathrow accident?

If so, can you refer to me to where I could read about the retraction of the flaps? Thanks!

same link for you old boy

http://www.aaib.gov.uk/cms_resources.cfm?file=/G-YMMM%20Interim%20Report.pdf

its at the bottom of page 4.

[toolowtoofast]

i'll use easy numbers to make it easy (for me :ok: ).

best glide speed clean = 90kts. that's 1.5NM/minute. ROD is 500fpm. from 1000' agl, that's 2 minutes before you're on the ground. you have travelled 3NM.

best glide speed with flap = 60kts. 1.0NM/minute. ROD is still 500fpm. still 2 minutes til you're on the ground, but you've only gone 2NM.

[hawk37]

Adverse,
thanks for the link and I am reading it. I hadn't realized that the commander decreased flap to stretch the glide. Certainly would have helped increase his air distance.

[clivewatson]

The second (supposedly more experienced instructor) was talking NONSENSE!!!

For maximum glide distance FLAPS UP.

[misd-agin]

The flaps were retracted to Flaps 25 at Flaps 30 Vref -17 or 18 kts.

Retracted flaps at Vref -17 or 18 kts probably does not increase gliding distance due to the increase in angle of attack.

You can check this in the sim - fly level flight at Vref Flaps 30. Note your fuel flow. Then do the same at Vref -17 or -18 with Flaps 25. Note your fuel flow. The higher fuel flow would indicate which configuration has more drag.

I've been told L/D for configured Boeings is typically Vref +10(an estimate from a test pilot, depends on flap settings and different model aircraft). If correct you're in the back side of the power curve and decreasing flaps decreases your gliding distance.

Hopefully the final AAIB report will cover this subject.

[vwreggie]

as an aside. I never understood why general aviation pilots continue to use the threshold as an aimpoint in the circuit. If the landing data is predicated on a 30 or 50 feet threshold crossing height then a circuit and finals to something like the 500 foot marker should allow for a touchdown on the runway or the clearway in the event of an engine failure. been a while for me in a lightie though

[ProfChrisReed]

The "dive, pull up" scenario is only potentially relevant if there is a strong wind gradient - by diving to an altitude where the headwind is lower, you might make the airfield. If you know the aircraft performance well this might be worth trying. The pushover after the pull up is at <1g, and thus the stall speed is lower, but you need to be sure you will regain sufficient airspeed before inertia returns you to the 1 g environment (otherwise we're in plummet territory).

This is a known maneouvre (often demonstrated, but not taught) in gliding. It's last ditch stuff, and you could easily overfly a landable field and still not make the runway unless you are confident that you know what the result will be.

I'd definitely put the flaps away though.

[dashtrash50]

Here in the states, we always have to have the backing of the FAA. If you crash and can point to something in an official government document, they will be less likely to blame pilot error. So here is the FAA take on it, The glide distance with no flaps will be farther, but the loss of lift, and the increase in stall speed related to retracting the flaps is not worth it. If you loose the engine with no flaps, keep them up until the landing is assured to maximise your glide distance. The Fed's have a little picture on glide distance with and without flaps at the bottom of page 8-3 (http://www.pprune.org/www.faa.gov/library/manuals/aircraft/airplane_handbook/media/faa-h-8083-3a-4of7.pdf). They also say "Never retract the flaps to correct for undershooting since that will suddenly decrease the lift and cause the airplane to sink even more rapidly."

[misd-agin]

Your link didn't work.

The decision to retract flaps depends upon how much altitude you have below you. If you are at 10,000 AGL then retracting the flaps would make sense.

[Pugachev Cobra]

Here's a working link for the same filename from the FAA website:

http://www.faa.gov/library/manuals/aircraft/airplane_handbook/media/faa-h-8083-3a-4of7.pdf

[Love_joy]

The decision to retract flaps depends upon how much altitude you have below you. If you are at 10,000 AGL then retracting the flaps would make sense.

I have to agree with FI no.1 - but there is a caveat here.

During my flight training I was also drilled into not retracting flap on approach, the pitfall being that you retract flap at a speed which requires it to maintain a buffer against Vs. On approach, you may not have sufficient height to recover.

That said, if you have drag flap out and need to 'stretch' the glide - get rid! But be aware you need the speed first.

[Pugachev Cobra]

Relevant to this thread, the BA038 777 Captain was interviewed and mentioned his decision to retract the flaps, so that he could extend his gliding range.

BBC News - Hero BA pilot speaks of Heathrow Boeing 777 crash (http://news.bbc.co.uk/2/hi/uk_news/8505163.stm)

Is it wise to apply this experience into lighter aircraft as well, as was the original post's topic?

[FullWings]

Is it wise to apply this experience into lighter aircraft as well, as was the original post's topic?
If you're in a similar situation (developing undershoot) then it's worth a try, especially if you're not going to make it if you don't do something. As has been pointed out, most people fly light aircraft around the circuit (and on approach!) considerably faster than the clean stalling speed, so "falling out of the sky" is unlikely.

There are so many variables involved that there's no real hard and fast answer to this particular scenario. There are three basic outcomes: a) flap or no flap, you'll make it b) the right flap setting/speed/trajectory will get you in, the wrong ones won't and c) you're not going to make the airfield, whatever you do. The "approach cone" containing b) is likely to be fairly narrow. It's energy management: you start off with a certain amount of mgh+0.5mv^2 and wish to make the most of it on the way down.

There's speed<->altitude trading, ground effect, wind gradient, airmass motion and a load of other factors. Unsurprisingly, there's no one answer that fits all. If you can see where your trajectory will end and it's not good, then change the configuration; if it's still looking bad then...

What happens when the engine quits is down to workload, experience and currency. There are also external influences such as wind, whether the prop is stopped or not, aircraft weight, etc. What is important (and stated in the BA38 report) is that if you're going to crash, do it wings level and keep flying the aircraft until the end. As they say, a good approach into a poor area is better than vice-versa. Recognise that you're about to have an accident and make it a controlled one.