View Full Version : 757 Glide ratio
what is the 757s Glide ratio ? (No engines of course)
expedite_climb
2nd Oct 2002, 12:51
Generally aim to have twice the height you normally would. A normal approach you would have 3 * the height as the dist required, + 10 to slow down. If i was gliding I'd want at least 6 * the height then. So 6 miles for every 1000' - not sure if thats 6:1 or 1:6 tho. !
Intruder
2nd Oct 2002, 14:52
Um... "6 miles for every 1000'" is 36:1, since a nautical mile is about 6000'.
If you want "twice the height," you really want your distance to be HALF what is usually is -- 1.5 times the height, or 1.5 miles (9,000') per 1000'. That would give you a 9:1 ratio, which is in the ball park of the glide ratio (while the landing gear is up, anyhow).
Any 757 pilots know the answer to this question ?
Intruder & expedite_climb what is the excact glide ratio on the birds you fly ? 9 seems kind of low for a 747-400 I would have thought it was closer to 15-20.
Maybe it is because I used to fly single engine (F-16s), but I have never set foot in an aircraft without knowing its capabilities without engines (If it had any. My current bird (AT-38) can't fly without :rolleyes: )
Pilot Pete
2nd Oct 2002, 22:41
Bit of a mixture of the two solutions offered so far............
Normally about 18:1 (ie 3nm per 1000')
Glide (2eo) about 9:1 (ie 1.5nm per 1000')
Tried it in the sim for the first time ever a couple of weeks back and it is tricky! I landed about 1/4nm short............ here's what happened;
15000' above the airfield with both engines out. Planned a 'high key' and 'low key' point at end of downwind and on final approach (OM), planning to be twice as high as normal at VREF30 + 80kts minimum. Put the threshold elevation in the altitude window which then gives you the green arc on the EADI showing your predicted end of descent. The trick is then to keep the green arc over the middle of the runway, if you can achieve this with half speedbrake then you can use the speedbrake either way to constantly adjust rate of descent. My mistake was to keep full speedbrake out at 4nm when I dropped the gear as I looked too high (yeah I know, of course I would look too high!) - green arc had been off the far end for a couple of miles, and delaying putting it away brought the green arc back off the near end of the runway as the drag increased significantly.
Having tried it from above a suitable airfield my hat goes off (purely from the hand flying point of view) to those who have glided successfully in heavy jets to a suitable strip.
PP
We used to do SFO patterns on a regular basis in the Viper (Glide ratio 7) and after doing a couple they became fairly easy to do. (Probably a lot harder to do in one of your big birds) I am surprised the Airlines don't require you to be proficient in SFOs before they let you fly one, but I guess it is because the chances are slim you will ever do one. I know of a couple of dudes who have had to do real FOs in the Viper.
expedite_climb
3rd Oct 2002, 07:28
Intruder / Run,
Okay Guys, I am on the 757, just got my maths a bit backward yesterday. Never was any good after night flights....
Pete has already said what I meant, I concur - not easy ! The other problem we had in the sim was in case of a double engine failure all hydraulics are powered by the RAT, and hence the centre system only is powered. This means youll have alternate gear and flaps extension. The alternate flaps system is very slow compared to normal, so unless you have extra height you risk landing with no (or little) flaps. When we did this of course a tailstrike was the result ! (Very high ROD, big flare and no flaps to reduce landing attitude).
Pete - how does CHE sim this ? BGS does it right but i heard that LTN doesnt ???
Hopefully I'll have made sense this time (if not.... its early.... well for me anyway!)
Could you guys recheck those numbers ? I am positive a glide ratio of 9 must be too low for an airliner. The ones I do know about have glide ratios of 15-23
expedite_climb
4th Oct 2002, 13:56
Yeh, thats definately right. You normally need 3 miles for each thousand feet, and having done it in the sim it is half that engines out. Thats 9:1.
Edited cos I got my half and twice confuzzed again !
saudipc-9
4th Oct 2002, 17:09
Run,
Just curious did they ever flight test what the glide ratio on the Viper was with a siezed engine or was it always assumed that you would have a windmilling motor? Plus you treaching the IFF? I used to be at Randolph teaching at the 550th. Ahhh the Auger Inn:D
Make that the 559th
I don't remember seeing different numbers for windmilling vs frozen engine (It has been a while :) ) But they would of course be different and the glide ratio of 7, which is actually 8.4 after thinking about it (7NM per 5000 feet), is most likely for a windmilling engine. We didn't use different numbers for different configurations, but we were of course aware that the we wouldn't quite get the same numbers if we had a lot of crap hanging. (Weight will not affect the glide ratio, just the speed at which you get the max numbers. But stores that couldn't be jettisoned would of course give a lot of drag - i.e. lower numbers)
If you had altitude to come home and do an overhead you would plan to arrive at High Key over the field at 7000 feet AGL plus 400 feet per 1000 pounds of fuel/store weights over 3000 pounds.
And yes, I do teach IFF at 88th FTS, Sheppard AFB
Auger Inn:D :D
PC-9 Riyadh - How ?
saudipc-9
5th Oct 2002, 15:09
Ex Canadian Air Force did the exchange in Randolph. Then found out how much cash I could make flying PC-9's here for British Aerospace. So said what the hell and that was 3 years ago.
"Sleepy" still flying there or has he been posted back north to the place of permanently stiff nipples?:D
He just left recently.
How do you like Saudi ? Are there a lot of Canadians/British/American people there ? Did you bring a family ? Any open positions :D
747FOCAL
6th Oct 2002, 10:34
Well if you are on approach you better be 3 plus or minus some. The C17 is having trouble meeting civilian cert cause it cant do 3 at max flap on approach.
saudipc-9
6th Oct 2002, 16:07
Saudi is OK if you can adapt to some guy who has been flying for 10 years but only has less than a 1000hrs telling you that you have no idea what you are doing:mad: It is their only way of getting at us because they are so insecure. However, it is only for 8 hours a day and I do have my family here and we just recently had our daughter here but I don't hink we will do that again. The holidays are good, 9-10 weeks a year with a tax free salary of $100'000(US) +. So over all the deal is very good. If you really are interested I could give you someone to get in touch with. There are mainly Brits, Auzzies, Kiwi's and some South Africans here.You can imagine the banter is good and alot of good guys from many different back grounds. We do have a couple of Americans here but only one pilot that I am aware of.
Let me know;)
PaperTiger
6th Oct 2002, 21:04
Unless we are at cross-purposes here (discussing differnt things), 9:1 seems way too low for a 757. Most airliners have L/D numbers in the upper teens, assuming a clean config and windmilling/idling motors.
The Boeing 727 with the landing gear and flaps retracted and the engines at idle has a best glide ratio of about 17:1, but it occurs at a very high airspeed, almost 200 ktas if I remember correctly. This gives it a sink rate of about 1000 feet per minute.
Other numbers I have on a printout (source unknown) are:
Boeing B707-320 19.4
Boeing 767-200 19.0
Douglas DC-8 17.9
Boeing 747-100 17.7
Douglas DC-10 17.7
Airbus A320 17.0
Lockeed Tristar L1011 17.0
Douglas DC-9 (1966) 16.5
Boeing B727-200 16.4
Douglas DC-3 14.7
No reason why the 757 should not be in the same ballpark, is there ? I know those particular wings deflect a lot of lift laterally (hence the gnarly vortices), but surely not sufficient to halve the L/D ratio. :confused:
expedite_climb
7th Oct 2002, 09:30
Paper tiger,
The way Pete and I calculated the drag factor was from experience in the sim, nothing aerodynamic about it. Hence it is a 'real' figure, but allows for making an approach, so I suspect it possible that the best drag factor is slightly higher.
If i get chance to try it again I'll have a look at the ROD at the best glide speed.
Intruder
7th Oct 2002, 11:00
PaperTiger:
It certainly is possible that the idle-thrust figures you have are correct. However, there is a significant difference between idle thrust and windmilling engines, where there id significant drag from the fans.
Just to put it in perspective, does your source have figures for one-engine-out glide ratios?
Dragon Knight
7th Oct 2002, 11:14
I have been digging into my AOM-MD-80, and to do a deadstick landing (in one piece) from 35000 ft. I need 88 NM, and that is 1 to 15, or vice versa.
Actually, having 2 runways side by side, like in LHR, and reaching 3000 ft. on a normal take-off, it is possible, with finess, to deadstick a 180 and still make it back to the other runway, but look out for traffic.