Flying - - CONTENTS - By Peter Gar­ri­son

The down­wind turn re­vis­ited

Last Au­gust, a sightseeing flight crashed in the Swiss Alps, killing all 20 aboard. The air­plane, a Junkers Ju 52, was at­tempt­ing to cross a 10,000-foot sad­dle from south to north. The wind was out of the north, gust­ing to 25 knots. Wit­nesses said the air­plane ap­proached the sad­dle, then banked sharply to the left and sud­denly dived to the ground, “as if fol­low­ing a plumb line.”

The of­fi­cial anal­y­sis of the ac­ci­dent will not emerge for some time. I don’t have an opin­ion as to what ac­tu­ally caused it. But the ac­ci­dent did bring to mind some as­pects of down­wind turns that are worth dis­cussing.

First, as a mat­ter of gen­eral in­ter­est, the air­plane in ques­tion, which was 79 years old, was one of the world’s few sur­viv­ing Ju 52s, cor­ru­gated-skin relics of the era of the Ford Tri­mo­tor. First flown in 1931, four years be­fore the DC-3, Ju 52s served in the Luft­waffe through­out World War II, and be­cause, like the Ford, they could be op­er­ated from im­pro­vised airstrips, some of them con­tin­ued work­ing as util­ity cargo and pas­sen­ger trans­ports in less-de­vel­oped parts of the world into the 1960s. Like the Ford, the air­plane has three ra­dial en­gines, but the Ju 52 looks marginally less quaint than the Ford: It is a low-wing air­plane with wing-mounted out­board en­gines, and at a dis­tance, a my­opic per­son could mis­take it for a DC-3. Just a few are still fly­ing to­day, sev­eral op­er­ated by the Swiss sightseeing air­line Ju-Air.

A Swiss friend of mine, a for­mer Air­bus 340 cap­tain who knew all three crewmem­bers on the ill-fated flight, spec­u­lated that the air­plane could have en­coun­tered a down­draft or strong tur­bu­lence on the lee side of the pass, be­gun a turn back and been caught by a gust that caused it to ex­ceed its crit­i­cal an­gle of at­tack.

His hy­poth­e­sis got me think­ing again about some nu­ances of the down­wind turn.

Down­wind turns — not the turns them­selves, but the dis­course sur­round­ing them — have been a bête noire of mine for decades. Some pilots think turn­ing down­wind in a steady wind makes an air­plane lose airspeed be­cause the headwind be­comes a tail­wind. They there­fore be­lieve that down­wind turns are per se pit­falls for the un­wary.

The sci­ence of this view is sim­ply wrong, but ar­gu­ments about mo­men­tum and frames of ref­er­ence, no mat­ter how of­ten or how loudly re­peated, are sel­dom per­sua­sive. The sim­plest way to sat­isfy your­self about


the mat­ter is to climb up a few thou­sand feet on a day when there is a good strong up­per wind and fly in cir­cles. You will see that the airspeed in­di­ca­tor gives you no hint of when you are fac­ing up­wind and when you are fac­ing down­wind.

Nev­er­the­less, there is con­sid­er­able lore about ac­ci­dents that were sup­pos­edly due to turn­ing down­wind, and so it has been nec­es­sary for com­men­ta­tors like me to mod­ify the “there’s no dif­fer­ence” ar­gu­ment with caveats about mis­lead­ing vis­ual impressions when close to the ground, wind gra­di­ents with chang­ing al­ti­tude, and gusts.

As a prac­ti­cal mat­ter, air­planes make down­wind turns all the time with­out any dif­fi­culty. Every stu­dent fly­ing round and round in the pat­tern makes one or two down­wind turns per cir­cuit. How­ever, stu­dents do not usu­ally fly cir­cuits in ex­tremely gusty weather, and it is about gusts in par­tic­u­lar that the Ju 52 crash makes us think.

The clas­sic thought ex­per­i­ment about gust-in­duced stalls runs like this: An air­plane is fly­ing down­wind at 10 knots above its stalling speed. A 20-knot gust hits it from be­hind. What hap­pens?

The con­ven­tional an­swer is that it must stall, be­cause its airspeed is now 10 knots below its stalling speed. But that is not quite cor­rect, be­cause the an­gle of at­tack does not change — at least not im­me­di­ately. The air­plane cer­tainly loses lift, how­ever, and be­gins to drop. Now the an­gle of at­tack in­creases be­cause of the change in the flight path, but so long as the pi­lot does not make the mis­take of pulling back on the stick, the nose will come down of its own ac­cord, try­ing to re­store the trimmed an­gle of at­tack. The wings re­main level, and the air­plane picks up speed un­til it is again in equilib­rium with the air mass. So, bar­ring pi­lot er­ror, there is noth­ing in this sce­nario to cause a loss of con­trol.

But sup­pose that, in­stead of com­ing from be­hind, the gust is ver­ti­cal, com­ing from di­rectly below. The sit­u­a­tion is now quite dif­fer­ent. The an­gle of at­tack changes by an amount de­ter­mined by the ra­tio of the speed of the gust to that of the air­plane. For ex­am­ple, a 20-knot ver­ti­cal gust strik­ing an air­plane fly­ing at 100 kias changes its an­gle of at­tack by 11 de­grees — cer­tainly enough to stall the wing be­fore the air­plane has time to ad­just its pitch at­ti­tude. In this case, the ad­just­ment would be quite large and an alert pi­lot would feel the need to help it along by force­fully pitch­ing the nose down.

You might en­counter such a gust while fly­ing through a thun­der­storm — one of many ex­cel­lent rea­sons not to do so — or while fly­ing in moun­tains in windy weather. Else­where, strong ver­ti­cal gusts are less com­mon, but when you are in a banked turn, even a hor­i­zon­tal gust has a ver­ti­cal com­po­nent — ver­ti­cal, that is, with re­spect to the air­plane. The re­sult­ing change in an­gle of at­tack is now af­fected by both the speed of the gust and the bank an­gle, with the worst case oc­cur­ring when a strong gust strikes a steeply banked air­plane headed di­rectly across the wind.

The dan­ger of such a gust is aug­mented by the fact that in rolling into a steep turn an air­plane does not au­to­mat­i­cally in­crease its speed; in­stead, the pi­lot is more likely to give up some stall mar­gin. For ex­am­ple, sup­pose you are climb­ing, wings level, at 1.3 Vs — that is, 30 per­cent above your clean stalling speed. Your an­gle of at­tack is about 6 de­grees away from the stall. Now sup­pose you sud­denly, for some rea­son, de­cide to rapidly re­verse course. You roll into a 45-de­gree bank and pull 1.4 G. Your speed is un­changed, or might even di­min­ish slightly, but your an­gle of at­tack is now less than 3 de­grees away from the stall rather than 6. (This cal­cu­la­tion, for the nu­mer­i­cally in­clined, as­sumes a CL­max of 1.4 and a lift curve slope of 0.09.) It does not take a very pow­er­ful gust from below to ex­ceed that small a mar­gin.

Hy­po­thet­i­cally, it is pos­si­ble that a sud­den de­ci­sion to turn the Ju 52 away from pow­er­ful tur­bu­lence or down­drafts cre­ated just those con­di­tions.

It has al­ways been dif­fi­cult to tell whether down­wind turns are re­ally haz­ardous for any rea­son at all. The con­fused su­per­sti­tion that turn­ing from up­wind to down­wind in­evitably en­tails a loss of airspeed could eas­ily have led peo­ple to blame the wind for stall-spins that were re­ally due to sim­ple pi­lot er­ror, for ex­am­ple, get­ting too slow or cross­ing the con­trols in a steeply banked turn, or to wind shear. In­deed, most stall-spin ac­ci­dents in the traf­fic pat­tern oc­cur not on the climb­ing turn from cross­wind to down­wind, but on the turn from base to fi­nal, into the wind.

So, it’s quite pos­si­ble that the dan­ger­ous down­wind turn — even if you ac­cept that the dan­gers might arise from sub­tler causes than dis­ap­pear­ing airspeed — is sim­ply a fig­ment of imag­i­na­tion. But, even if you had never heard of down­wind turns, you would still re­al­ize, if you thought care­fully about an air­plane in a steeply banked turn, that a gust from out­side the turn, strik­ing the air­plane at a steep an­gle to its di­rec­tion of flight, could stall its wing, and that this is worth keep­ing in mind on those rare oc­ca­sions when you might be obliged to make a rapid turn in a gust­ing wind.


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