Speed on Final to "Make the Runway" at 3 Degrees

RingLaserGyroSandwich

RingLaserGyroSandwich

Pre-takeoff checklist
Joined
Feb 6, 2019
Messages
409
Location
Annapolis
Display Name
Display name:
RingLaserGyroSandwich
A year or two ago I was discussing straight-in approaches with an instructor. I pointed out that a 3 degree glideslope does not allow for us to glide in if we lose the engine. He generally agreed but also pointed out that prior to short final you can maintain a higher speed which allows you to carry some extra energy. This got me wondering, how much faster do you need to go in order to have the runway "made" while on a 3 degree glideslope?

Disclaimer: Do not use these specific results in an actual airplane... consult your POH, CFI, DPE, ASI, and the Administrator for more information.

Assumptions:
  • Typical trainer aircraft
  • 73 kt best glide speed
  • ~10 to 1 glide performance
  • No flaps
A 3 degree glideslope is equivalent to descending ~318 feet per nautical mile traveled. A 10:1 glide ratio is equivalent to ~5.7 degrees or ~608 feet per nautical mile. Here is a graph of both glideslopes side by side (note that x scale and y scale are different and exaggerate how steep the glideslope looks):

UweLW3I.png


If the plane is following the blue line down, in order to reach the runway in the event of a loss of power, it needs to be going faster than the 73 kt best glide speed. The extra kinetic energy needs to make up for the difference in potential energy between the two glidepaths. Here is the speed needed as a function of nautical miles out from the runway threshold:

UqRS8Nv.png


As you can see, when you are on half-mile final, required speed is already up to ~93 kts, which is almost cruise speed. At 1 mile out, you would need a speed of 110 kts. My conclusion for trainer aircraft is that you can only maintain sufficient speed to keep the runway within glide range on a 3 degree glideslope when within ~1 nautical mile of the runway. This may not be recommended for other reasons, though.

So what did you all do with your lunch hour today?
 
Remember that if you’re faster than best-glide speed, you will have a poorer glide ratio. If you can’t reach the runway at best glide speed, gliding faster will only make it worse.
 
Half Fast said:
Remember that if you’re faster than best-glide speed, you will have a poorer glide ratio. If you can’t reach the runway at best glide speed, gliding faster will only make it worse.
Click to expand...
This is a good point. I envisioned, upon losing the engine, the fast pilot immediately converts that excess kinetic energy into potential energy, then takes the best glideslope down from there at 73 kts. Realistically, this is difficult to do optimally. My math is overly optimistic but is really just meant to give a rough idea.
 
I believe the best you can do is to maintain constant altitude, making forward progress, while the airspeed bleeds down to best glide, then hold best glide speed all the way down.
 
Half Fast said:
Remember that if you’re faster than best-glide speed, you will have a poorer glide ratio. If you can’t reach the runway at best glide speed, gliding faster will only make it worse.
Click to expand...

True, but the best glide speed increases if you're gliding into a headwind.

Interesting analysis. You can see an extreme example when sailplanes come in fast and low above the treetops and retain enough energy to pull up and circle for a landing. OTOH, in a draggy ship like my biplane or an ultralight the speed bleeds off too fast to be useful, so like Brad I always come in high and adjust my glide with a slip.
 
Half Fast said:
Remember that if you’re faster than best-glide speed, you will have a poorer glide ratio. If you can’t reach the runway at best glide speed, gliding faster will only make it worse.
Click to expand...
I think the idea would be to trade airspeed for altitude. But looking at the chart it would seem that you'd need to trade ~93kts@150AGL for ~73kts@300AGL from ~0.5 mile final.
 
Too much math and too many charts for a dumb pilot like me. I’m just going to check the seat of my pants, thumb on the windscreen TLAR, declare Leroy Jenkins and do some of that pilot ****.
 
Half Fast said:
I believe the best you can do is to maintain constant altitude, making forward progress, while the airspeed bleeds down to best glide, then hold best glide speed all the way down.
Click to expand...
The answer lies in excess power (which can be negative), not just L/D, and the optimal technique varies from airplane to airplane. The answer can also change depending on *how* you climb and/or bleed to L/Dmax. As for the original premise, I'd rather be high than fast on final, to a point.

Nauga,
high, hot, and overshot
 
I applaud the effort to answer the question. Another reason the whole "stabilized approach" concept seems a bit silly to me in light GA. I thank my instructors for teaching me to fly tight patterns. The chance of losing an engine on final is fairly slim, but never zero. My engine quit once on final due to my own fuel mismanagement, and fortunately I got it back. @Salty also lost his with a less optimal outcome.

Practice power off approaches and stay high enough to always make the runway, IMHO. Now, that's easier said than done in a brick like my Lance, but it's a worthy goal. You can always slip. Save the 3 degree, 3 mile finals for the jets.
 
I am not sure what you were trying to demonstrate. The TCH is listed on the chart and it isn’t much altitude. The glide table is aircraft without flap extended and flaps destroys the glide data. Once you increase the airspeed from LD/max the glide distance get shorter due to increased parasitic drag. So unless the plane was located near the MAP, there no way you are gliding to the runway from the 3° glide path.
 
When flying straight in to a runway (presumably enroute from somewhere else), what is the rationale for when you decide you need to be able to glide to the airport? One could fly as high as possible and dive-bomb the airport, but that doesn't seem to be all that operationally safe, either. The engine could (at very low probability) quit at any time, and the probability of it happening in the last few minutes is a fraction of the probability of it happening during the rest of the preceding flight. Unless you are running yourself out of fuel, but that's a separate issue.
 
Carb ice on some trainers is a thing. I always thought the term glideslope was kind of a misnomer because if I'm on it and the engine quits, I ain't gliding to where I was planning on landing. I think my Cherokee 180 has a little less than 9:1 glide ratio.
 
chemgeek said:
When flying straight in to a runway (presumably enroute from somewhere else), what is the rationale for when you decide you need to be able to glide to the airport? One could fly as high as possible and dive-bomb the airport, but that doesn't seem to be all that operationally safe, either. The engine could (at very low probability) quit at any time, and the probability of it happening in the last few minutes is a fraction of the probability of it happening during the rest of the preceding flight. Unless you are running yourself out of fuel, but that's a separate issue.
Click to expand...

1000 feet is a good rule of thumb. As that is the altitude we often practice power failure from. It is also about where we practice normal landings from. If you are above 1000 feet and have a power failure just do what you were taught and practiced, you should know where you can land from there and set up as normal of landing as possible even if it isn’t at the airport. From 1000 feet you should have time for a quick restart procedure as well. Like Fuel pump on, change fuel tanks. Or whatever is appropriate for your aircraft.

Brian
CFIIG/ASEL
 
So if you’re on an instrument approach and doing 90 kts in a small GA you can make the runway from only about 1/2 away if you lose the engine there?

Lunch today? Racking up more TAA time towards CPL - IFR to Peoria and back to Lafayette. Grissom Apch had us keeping our speed up for a DC9 behind us going back home. 110 kts in the 172 on the ILS...which would’ve put us on the runway a mile out if the engine quit based on your calcs?

We broke out at 400 and 300 ft AGL each leg. Fun day.
 
chemgeek said:
When flying straight in to a runway (presumably enroute from somewhere else), what is the rationale for when you decide you need to be able to glide to the airport? One could fly as high as possible and dive-bomb the airport, but that doesn't seem to be all that operationally safe, either. The engine could (at very low probability) quit at any time, and the probability of it happening in the last few minutes is a fraction of the probability of it happening during the rest of the preceding flight. Unless you are running yourself out of fuel, but that's a separate issue.
Click to expand...

You make an important point. If the engine is going to unexpectedly quit on final, it could quit at any time. To ensure the runway can be reached, the only safe way to fly is take off and circle above the airport gaining altitude until the plane is high enough to glide to the next closest airport. Then it's safe to head towards that airport, but the safety altitude must be maintained.

Once you're over the second airport, punch NRST into the Garmin, and figure out how much altitude will be required to glide to that airport. Hopefully it will be in the direction you wish to travel. Make the glide distance calculation again, gain some more altitude, and off you go towards another airport and safety if the engine quits.

Continue this method of guaranteed safety until you finally arrive within gliding distance of your destination. It might take longer to get there, but if the engine quits at any time during the trip, you'll be able to glide to a runway.
 
I'm firmly in the camp that says, after a 2 hour flight of flawless operation, I am not concerned about the infinitesimal likelihood of my engine deciding to quit on short final with no notice and for no apparent reason...

...other than perhaps my own mismanagement or stupidity, in whuch case I have bigger problems than glide ratio.
 
Clip4 said:
I am not sure what you were trying to demonstrate. The TCH is listed on the chart and it isn’t much altitude. The glide table is aircraft without flap extended and flaps destroys the glide data. Once you increase the airspeed from LD/max the glide distance get shorter due to increased parasitic drag. So unless the plane was located near the MAP, there no way you are gliding to the runway from the 3° glide path.
Click to expand...
I was trying to demonstrate how much extra speed you need to compensate for insufficient altitude when you lose your engine on final. I had to look up what TCH stood for but I'm glad I did. As I mentioned, I assumed no flaps because, presumably, if you lose your engine on say, a 1 mile final, you are not going to deploy flaps. You will just come in clean.

As acknowledged by myself and others above, my method does not account for the energy losses due to additional parasitic drag (and the like) when you lose your engine and try to climb up from the lower glideslope to the higher one by converting kinetic energy into potential energy. Therefore, my "1 nautical mile out" answer is optimistic and you would need to be a bit closer in before you could potentially glide in on a lost engine simply by maintaining cruise speeds up to the point of the lost engine. I think that's okay since I was never trying to make an argument that carrying extra speed is a good answer here. I was simply trying to quantify the effect (roughly). Hey, I'll do what I want during my lunch hour!
 
Engine failure during cruise is very rare unless there's too much air in the fuel tank. An engine is much more likely to quit after a power change, whether after going to full power on takeoff or throttling back for landing (carb ice, bad idle setting, etc.) I believe Ron @wanttaja posted some statistics on this in a previous discussion of the same subject... though those statistics won't include the cases where the pilot got the plane back onto the runway without damage and pushed it off the runway before the FAA or the news found out.
 
RingLaserGyroSandwich said:
I was trying to demonstrate how much extra speed you need to compensate for insufficient altitude when you lose your engine on final. I had to look up what TCH stood for but I'm glad I did. As I mentioned, I assumed no flaps because, presumably, if you lose your engine on say, a 1 mile final, you are not going to deploy flaps. You will just come in clean.

As acknowledged by myself and others above, my method does not account for the energy losses due to additional parasitic drag (and the like) when you lose your engine and try to climb up from the lower glideslope to the higher one by converting kinetic energy into potential energy. Therefore, my "1 nautical mile out" answer is optimistic and you would need to be a bit closer in before you could potentially glide in on a lost engine simply by maintaining cruise speeds up to the point of the lost engine. I think that's okay since I was never trying to make an argument that carrying extra speed is a good answer here. I was simply trying to quantify the effect (roughly). Hey, I'll do what I want during my lunch hour!
Click to expand...
Most people would have at least some flaps out by 1 mile final.
 
Probably true for other than no-flap planes like Velocities. The analysis becomes significantly more complicated if you try to take flaps into account so I'll just say, you can delay flaps if you think the math above justifies it (personally, I don't).
 
Kritchlow said:
That’s like saying you should not descend from cruise until the runway is made without power.
Click to expand...

My post above is sarcasm. It's silly to perform the OP's mental gymnastics every time one is preparing to land an aircraft.
 
3393RP said:
My post above is sarcasm. It's silly to perform the OP's mental gymnastics every time one is preparing to land an aircraft.
Click to expand...
It's foolish to call it mental gymnastics. It's simply a rough analysis of options. I didn't tell you to do anything differently and then try to justify it.
 
RingLaserGyroSandwich said:
I was trying to demonstrate how much extra speed you need to compensate for insufficient altitude when you lose your engine on final. I had to look up what TCH stood for but I'm glad I did. As I mentioned, I assumed no flaps because, presumably, if you lose your engine on say, a 1 mile final, you are not going to deploy flaps. You will just come in clean.

As acknowledged by myself and others above, my method does not account for the energy losses due to additional parasitic drag (and the like) when you lose your engine and try to climb up from the lower glideslope to the higher one by converting kinetic energy into potential energy. Therefore, my "1 nautical mile out" answer is optimistic and you would need to be a bit closer in before you could potentially glide in on a lost engine simply by maintaining cruise speeds up to the point of the lost engine. I think that's okay since I was never trying to make an argument that carrying extra speed is a good answer here. I was simply trying to quantify the effect (roughly). Hey, I'll do what I want during my lunch hour!
Click to expand...

On an instrument approach the airplane needs to be in the approach configuration before the final approach fix. This means you are extending flaps and gear 5 miles from the runway.
 
The discussion mentioned at the beginning of the OP was regarding a straight-in VFR approach to a towered field actually. I didn't think about it, but you are right that the applicability does not extend nicely to instrument approaches for most airplanes. A VFR pilot could still follow the PAPIs down at 3 degrees though which is what we were doing in the plane at that particular time.
 
Dana said:
Engine failure during cruise is very rare unless there's too much air in the fuel tank. An engine is much more likely to quit after a power change, whether after going to full power on takeoff or throttling back for landing (carb ice, bad idle setting, etc.) I believe Ron @wanttaja posted some statistics on this in a previous discussion of the same subject... though those statistics won't include the cases where the pilot got the plane back onto the runway without damage and pushed it off the runway before the FAA or the news found out.
Click to expand...
About 45% of all engine failures on homebuilt aircraft occur away from the airport. About 36% happen on initial climb. When you consider the relative amount of time planes spend in each condition (~2 minutes in initial climb, ~one hour at cruise), it's obvious that the initial climb is the more-critical situation.

I was brought up on the engines-failing-after-power-reduction claim. In my analysis, I didn't see any mention of this. In my Fly Baby, at least, the throttle stays forward until downwind (and longer, if I'm leaving the pattern).

Got curious about what percentage of the power-failure-at-cruise cases involved fuel exhaustion or starvation. Ran a *quick* check (find the fuel-related accidents, compare them to my position-at-failure list). Only about 15% of the engine failures during cruise flight involved fuel starvation or exhaustion.

Ron Wanttaja
 
Last edited:
chemgeek said:
One could fly as high as possible and dive-bomb the airport, but that doesn't seem to be all that operationally safe, either. T
Click to expand...

You just described my stabilized approach. All white VASI//PAPI theb slip it, cut power, fill the windscreen with pavement,kick it straight then land.

Works every time.
 
One solution would be to make every landing a PEL. Fly to high key every time. Or do the RV thing with the overhead break.
 
The important thing is to make sure that you can absolutely, positively, use the appropriate power to maintain a stabilized approach along a 3 degree slope every single time. Practice, practice, practice. That way when your engine does quit over some not so friendly territory and you have to stuff it into some postage stamp sized field with power lines around it, you are totally prepared and instinctively know how to set up your approach.
 
On my last airplane flight review, I flew an approach starting at well over 1000AGL on base and still comfortably landed on my chosen touchdown spot.

The evaluating CFI, also a helicopter pilot was amused, but not impressed that I could get away with a stabilized approach at such a steep angle. We were in an Archer, so that may explain the easy, greased brick glide characteristics contributing to the approach...
 
chemgeek said:
When flying straight in to a runway (presumably enroute from somewhere else), what is the rationale for when you decide you need to be able to glide to the airport? One could fly as high as possible and dive-bomb the airport, but that doesn't seem to be all that operationally safe, either. The engine could (at very low probability) quit at any time, and the probability of it happening in the last few minutes is a fraction of the probability of it happening during the rest of the preceding flight. Unless you are running yourself out of fuel, but that's a separate issue.
Click to expand...

Ahh, the ol' Southwest approach.
 
Based on this discussion it seems that a straight in approach is the biggest risk as far as landing approaches go. If you are landing from a standard pattern if you lose the engine on the downwind it becomes a short approach. If you lose it on the base turn you cut the corner. If you are on short final then you think a little more about that big breakfast you had. That being said a mechanical object develops a trust in it over time. After a 2-3 hour flight of watching the engine behavior and performance I feel much better than during the take off roll after an annual or 100 hour.
 
Star Keeper said:
Based on this discussion it seems that a straight in approach is the biggest risk as far as landing approaches go. If you are landing from a standard pattern if you lose the engine on the downwind it becomes a short approach.
Click to expand...

Huh? If you just flew 3 hours from Podunk to Bohonk, how is flying a straight-in approach (wind permitting) less safe than entering a standard pattern to "get within gliding range" of the airport? When you are within gliding range of the airport straight-in, you can just...land. How does entering the standard pattern in this case enhance safety? If anything, it decreases safety by unnecessarily extending flight and engine operation time.
 
Half Fast said:
I believe the best you can do is to maintain constant altitude, making forward progress, while the airspeed bleeds down to best glide, then hold best glide speed all the way down.
Click to expand...

So, if you were in a jet, doing 500 knots and lost power, you would just hold altitude? Or would you pull the nose up and zoom climb until reaching best glide speed?

Personally, I would zoom climb and get all that speed energy into the bank as altitude.
 
Capt. Geoffrey Thorpe said:
The important thing is to make sure that you can absolutely, positively, use the appropriate power to maintain a stabilized approach along a 3 degree slope every single time. Practice, practice, practice. That way when your engine does quit over some not so friendly territory and you have to stuff it into some postage stamp sized field with power lines around it, you are totally prepared and instinctively know how to set up your approach.
Click to expand...

HUH?

So you train yourself to set X power and hold a stable 3 degree glide slope.

How does that help you when the engine has quit, and you are flying more like a 6 degree glide slope?????

Maybe one should practice more power OFF approaches and get used to the steeper angle.
 
You must log in or register to reply here.
Back
Top