Narrative:

While I clearly recall the circumstances; I do not recall the call signs; or exact aircraft types. The 7110.65 pg. 3-9-6; 5;D states; 'do not issue clearances for small aircraft to taxi into position and hold on the same runway behind a departing heavy jet aircraft to apply the necessary intervals'; and lists a reference to AC 90-23. There is no further clarification here as to what the necessary intervals are.when I started at my first facility 24 years ago; this 7110.65 paragraph was in effect and I don't think in my 24 years as an atcs there have been any changes. Not a single word has changed. Back then nobody knew exactly what the interval was; and to date; I have never met anyone that has an answer to that question. Being a studious and dedicated developmental controller; I obtained a copy of AC 90-23 (that I still have) to see what I could find hoping to resolve the uncertainty of applying this paragraph. The application of this rule was especially important since at the time; heavy jets comprised 48% of our daily fixed wing traffic. The way we applied the rule back then; and I still do to this day; is to allow the departing heavy to get far enough down the runway; that the following departing small aircraft was not in danger of being upset; or blown over by the jet blast from the heavy. The size and weight of the following small departures are individually considered before being taxied into position. The smaller/lighter the aircraft; the greater the distance you allow to assure their safety. AC 90-23E; dated 10/1/91; of which I do have a copy paragraph 11; 'jet engine exhaust' clearly defines the intent of the 'necessary intervals' that need to be applied in accordance with 7110.65; 3-9-6; 5;B. Basically it states that you must consider jet blast/thrust stream turbulence effects on lighter aircraft; vehicles; etc. It follows with a diagram (figure 19) derived from exhaust velocities versus distance studies; with a graph illustrating jet engine velocity contours at takeoff power and clearly depicts what distances are appropriate behind a departing heavy jet aircraft the aircraft in the illustration is an L101. This aircraft wake turbulence order consisted of 15 pages; with only three paragraphs and one diagram/illustration concerning jet blast and it's considerations concerning light aircraft and equipment. The remainder of the AC concerns wake encounters/avoidance and wing tip vortices.several years ago while defending my position concerning this issue; I obtained a copy of AC 90-23F; dated 02/20/02; which was now the current document to show my supervisor at the time. To my dismay; figure 19; an illustration that graphically depicted distances and velocities of air speed behind a departing heavy had been eliminated! The rest of the paragraph has been watered down and in part states; 'engine exhaust velocities; generated by larger jet aircraft during ground operations and initial takeoff roll; dictate the desirability of lighter aircraft awaiting takeoff roll to hold well back of the runway edge at the taxiway hold line'. Ok ... So now it's only desirable to not taxi behind a departing heavy jet? What happened to the exhaust velocity versus distance studies and accompanying diagram and the corresponding wind velocity contours? I could joke and say that maybe it's not that important any more; but that's just not true. The L101 in the illustration probably had 45-50;000 pounds. Thrust engines. Since the introduction of the B777 almost 15 years ago; the thrust levels are now in the 90-110;000 pounds. Thrust range; likely adding wind speed to the velocity/distance chart that we no longer have for reference. It's perfectly reasonable to review and update this AC; but removing the information from paragraph 11 was a mistake that was not detected and in my opinion should be corrected and if possible [it should] include an update to the study of thrust levels versus distance. To get to my specific incident; it involved I believe a H/B767 departing; with a falcon 50 (not sure of the exact model) taxiing into position awaiting takeoff clearance. I waited until the heavy was approximately 5;000 ft down the runway before clearing the falcon into position to hold for wake turbulence. I estimate the heavy was 10;000 ft down the runway and airborne as the falcon crossed the runway hold short line. So what is the necessary interval? I'm being told by a supervisor that I must have five miles longitudinal separation before I can taxi into position and hold... Essentially radar separation. However; if you do this; you end up with about ten miles separation or more; which is unnecessary and very inefficient. Besides; the five miles separation is meant to protect from wake vortex encounters which begin at the point of rotation. Not immediately taxiing into position behind a departing heavy has everything to do with jet blast/thrust stream turbulence and nothing to do with wake turbulence. These are two separate things that are being lumped into one issue. So; how does one interpret 7110.65 pg. 3-9-6; 5;B? How long do you wait before putting a small in position behind a departing heavy? This leads me to another question. Under the old rules; a small aircraft was under 12;500 pounds and a large was 12;500 or more. The falcon in question was a small+ aircraft; somewhere under 41;000 pounds .the 7110.65 does not differentiate between these two categories in 3-9-6; 5;B. I think it's long overdue that this very vague paragraph is updated with more specific guidance; at least something more than just the 'necessary intervals' that we are left with to interpret. I feel that a defined runway distance should be mandated to protect small aircraft from jet blast/ thrust stream turbulence. This would ensure safe operations and maintain an efficient operation. I suggest a distance of 5;000 ft be mandated. This way controller judgment would be removed from the equation and the safety of small category aircraft; from a C150; to a much larger and heavier corporate aircraft would be assured. This issue is something that has been overlooked for decades. Mandating a minimum distance would make the issue crystal clear and assure the safe; orderly and expeditious movement of air traffic.

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Original NASA ASRS Text

Title: IAD Controller reported that the guidance on taxiing small aircraft into position behind departing jets reference jet blast has been removed from AC 90-23; and the rules in the ATC Handbook 7110.65 are vague; leading to confusion between jet blast separation and wake vortex separation.

Narrative: While I clearly recall the circumstances; I do not recall the call signs; or exact aircraft types. The 7110.65 pg. 3-9-6; 5;D states; 'Do not issue clearances for small aircraft to taxi into position and hold on the same runway behind a departing Heavy Jet aircraft to apply the necessary intervals'; and lists a reference to AC 90-23. There is no further clarification here as to what the necessary intervals are.When I started at my first facility 24 years ago; this 7110.65 paragraph was in effect and I don't think in my 24 years as an ATCS there have been any changes. Not a single word has changed. Back then nobody knew exactly what the interval was; and to date; I have never met anyone that has an answer to that question. Being a studious and dedicated developmental controller; I obtained a copy of AC 90-23 (that I still have) to see what I could find hoping to resolve the uncertainty of applying this paragraph. The application of this rule was especially important since at the time; Heavy Jets comprised 48% of our daily fixed wing traffic. The way we applied the rule back then; and I still do to this day; is to allow the departing Heavy to get far enough down the runway; that the following departing small aircraft was not in danger of being upset; or blown over by the jet blast from the Heavy. The size and weight of the following small departures are individually considered before being taxied into position. The smaller/lighter the aircraft; the greater the distance you allow to assure their safety. AC 90-23E; dated 10/1/91; of which I do have a copy paragraph 11; 'Jet Engine Exhaust' clearly defines the intent of the 'necessary intervals' that need to be applied in accordance with 7110.65; 3-9-6; 5;B. Basically it states that you must consider Jet Blast/Thrust Stream Turbulence effects on lighter aircraft; vehicles; etc. It follows with a diagram (figure 19) derived from Exhaust Velocities versus Distance studies; with a graph illustrating Jet Engine Velocity Contours at TAKEOFF POWER and clearly depicts what distances are appropriate behind a departing Heavy Jet aircraft The aircraft in the illustration is an L101. This Aircraft Wake Turbulence Order consisted of 15 pages; with only three paragraphs and one diagram/illustration concerning Jet Blast and it's considerations concerning light aircraft and equipment. The remainder of the AC concerns Wake encounters/avoidance and Wing Tip Vortices.Several years ago while defending my position concerning this issue; I obtained a copy of AC 90-23F; dated 02/20/02; which was now the current document to show my Supervisor at the time. To my dismay; figure 19; an illustration that graphically depicted distances and velocities of air speed behind a departing Heavy had been eliminated! The rest of the paragraph has been watered down and in part states; 'Engine exhaust velocities; generated by larger jet aircraft during ground operations and initial takeoff roll; dictate the DESIRABILITY of lighter aircraft awaiting takeoff roll to hold well back of the runway edge at the taxiway hold line'. OK ... so now it's only desirable to not taxi behind a departing Heavy Jet? What happened to the Exhaust Velocity versus Distance studies and accompanying diagram and the corresponding wind velocity contours? I could joke and say that maybe it's not that important any more; but that's just not true. The L101 in the illustration probably had 45-50;000 LBS. thrust engines. Since the introduction of the B777 almost 15 years ago; the thrust levels are now in the 90-110;000 LBS. thrust range; likely adding wind speed to the velocity/distance chart that we no longer have for reference. It's perfectly reasonable to review and update this AC; but removing the information from paragraph 11 was a mistake that was not detected and in my opinion should be corrected and if possible [it should] include an update to the study of thrust levels versus distance. To get to my specific incident; It involved I believe a H/B767 departing; with a Falcon 50 (not sure of the exact model) taxiing into position awaiting takeoff clearance. I waited until the Heavy was approximately 5;000 FT down the runway before clearing the Falcon into position to hold for wake turbulence. I estimate the Heavy was 10;000 FT down the runway and airborne as the Falcon crossed the runway hold short line. So what is the necessary interval? I'm being told by a Supervisor that I must have five miles longitudinal separation before I can taxi into position and hold... essentially Radar Separation. However; if you do this; you end up with about ten miles separation or more; which is unnecessary and very inefficient. Besides; the five miles separation is meant to protect from wake vortex encounters which begin at the point of rotation. Not immediately taxiing into position behind a departing heavy has everything to do with Jet Blast/Thrust Stream Turbulence and nothing to do with wake turbulence. These are two separate things that are being lumped into one issue. So; how does one interpret 7110.65 pg. 3-9-6; 5;B? How long do you wait before putting a small in position behind a Departing Heavy? This leads me to another question. Under the old rules; a small aircraft was under 12;500 LBS and a large was 12;500 or more. The Falcon in question was a small+ aircraft; somewhere under 41;000 LBS .The 7110.65 does not differentiate between these two categories in 3-9-6; 5;B. I think it's long overdue that this very vague paragraph is updated with more specific guidance; at least something more than just the 'necessary intervals' that we are left with to interpret. I feel that a defined runway distance should be mandated to protect small aircraft from Jet Blast/ Thrust Stream Turbulence. This would ensure safe operations and maintain an efficient operation. I suggest a distance of 5;000 FT be mandated. This way controller judgment would be removed from the equation and the safety of small category aircraft; from a C150; to a much larger and heavier corporate aircraft would be assured. This issue is something that has been overlooked for decades. Mandating a minimum distance would make the issue crystal clear and assure the safe; orderly and expeditious movement of air traffic.

Data retrieved from NASA's ASRS site as of April 2012 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.