Issue Number 6 : August 1993
by Rowena Morrison
"During taxi out...we were informed on the Ground frequency by Air Carrier B that one of their passengers deboarding behind us was 'blown down and injured' by our jet blast while departing the ramp. All three engines were running as I anticipated a short taxi. No more than idle thrust was required nor used as we were very light...I made approximately a 45-degree right turn toward taxiway before being released from guide person. We could not see, nor were we informed, of any boarding operation behind us. I believe Air Carrier B was remiss for allowing a deboarding operation behind a jet whose engines were running." (ACN 79190)
"...Widebody was cleared for takeoff, and we [commuter twin] were cleared into position and hold. The Captain called for the controls unlocked and runway checklist. While on [taxiway] approaching the runway, just after the hold line, the jet blast hit the airplane. The Captain applied the brakes, however the jet blast lifted the right wing and the right main gear off the ground. The jet blast blew the aircraft into the blast fence at the approach end of Runway 31L." (ACN 145186)
Almost every commercial jet operating manual has one--a single page with the matter-of-fact title, "Jet Blast Data." On this page is a diagram of the aircraft's jet blast "damage profile," as measured from the tail and with engines at low RPM settings (usually 35 to 40 percent N1). This profile extends in line from the outboard wing-mounted engines to more than 200 feet behind some larger aircraft. Within this area, jet engines can generate hurricane-level exhaust forces approaching 100 knots.
The potentially dire results? Before a crew can say "powerback," jet engine blast can up-root trees, flatten building structures, shatter windows, lift and propel heavy objects, weathercock braked airplanes, blow over lift trucks, shift unbraked baggage carts, and create other havoc on airport ramps, taxiways, and runways.
Although the diagrams don't say so, jet blast can also injure or kill crew and passengers who happen to cross its path.
The fact that few serious jet blast incidents and accidents occur during millions of ground operations annually is a tribute to the training and professionalism of air carrier flight and ground crews, and to the continual care they exercise in ground operations. When we looked at ASRS data, we found that ground jet blast incidents (fifty-one reports) represented only a tiny fraction of the total incidents reported. Yet even this small number of jet blast reports contained some surprises:
What are the lessons to be learned from these ASRS jet blast incidents? We begin by briefly revisiting the sites where almost half the jet blast incidents reported to ASRS occurred--taxiways, runup areas, and areas on or immediately adjoining runways.
Collectively, off-ramp sites accounted for forty-seven percent of the jet blast incidents reported to ASRS. These off-ramp locations were taxiways; areas on or immediately adjoining runways, and run-up areas . (See Table 1, below.)
Table 1--Site of Jet Blast Incident
|Site of Incident||No. of Rpts.||% of Data|
Runway (Hold, Landing, Takeoff)
The usual targets of jet blast in these locations were light aircraft, weighing 5,000 pounds or less, that were unexpectedly crunched and pummeled by the blast forces from jet engines. Frequently, the jet blast damage was the result of misdirected engine run-ups and tests. This pilot's experience was typical, down to the inevitable details of a prop strike and bent wing:
Heavier aircraft were also susceptible to jet blast damage in these locations. In an incident reported by two different flight crews, a small transport aircraft (in the 5,000 to 14,500 pound weight category) played "chicken" with a widebody aircraft holding in position on a runway for a nighttime takeoff--and lost. The sense of helplessness experienced by the pilot of the small transport came through clearly in his report to ASRS:
The Captain of the widebody aircraft involved in this incident had several suggestions for preventing jet blast damage during night operations:
Several broad themes emerged from this group of reports:
Both themes were dramatically evident in this controller's report of a large jet's engine run-up calamity:
Several pilots admonished Ground Control for not monitoring run-ups of larger aircraft more carefully, and for failing to provide light aircraft with warnings of jet blast hazard:
In a similar incident, a small aircraft performing a run-up 150 feet behind, and to the side of, an airliner holding short of the runway, experienced a wing and prop strike when the larger aircraft powered up. The pilot of the small plane had succinct advice for both the jet crew and ATC:
Fifty-three percent of the jet blast damage incidents reported to ASRS occurred on an airport ramp during pushback, powerback, taxi-out, or taxi-in. Several different factors appeared to influence these events. The most significant was the position of jet engines in relation to gates, ground equipment, people, and other aircraft on the ramp when breakaway power was applied. Another prominent factor was the proximity of light aircraft, including commuter-type aircraft, on or near ramps with turbojet operations. A final element was ground handling procedures, including gate radio communications and disposition of baggage carts.
Power + Turns = Hazard. More than a third of the jet blast ramp incidents involved aircraft that had engines powered and were turning 45 degrees or more. In a few cases, not even the use of a tractor or tug prevented jet blast damage if engines were running, and an aircraft was in the process of making a sharp turn. This can be explained in part by the power requirements associated with ramp operations--an aircraft initiating movement from a full stop requires relatively more power to overcome inertia and tire friction than an aircraft already in motion. Additional breakaway thrust is needed if the aircraft must also turn during the initial movement. Unless carefully managed, these power applications can result in jet blast damage.
One reporter described unusually severe damage from jet blast that occurred during a hard turn, on a congested ramp not designed for larger aircraft:
Powered 180-degree-or greater turns seemed especially likely to result in jet blast damage, even when crews used "normal" or "only necessary" thrust. Several reports illustrate:
The corrective suggested by several reporters is to position jet aircraft so that their forward thrust is directed away from gate areas, people, and ramp equipment:
Positioning is especially critical to safe powerback operations, which are the turbojet equivalent of "reverse gear." In a powerback operation, the flight crew deploys engine thrust reversers to direct thrust ahead of the aircraft, thus pushing the aircraft backwards. Concluded one reporter of a powerback incident that resulted in jet blast damage:
Close Proximity of Light Aircraft. The policy of parking of light aircraft "tail-to" turbojet aircraft on ramps, or in areas adjacent to congested ramps, appeared to invite jet blast damage incidents. In a number of instances, commuter planes occupying the same ramp area as turbojets were the targets of jet blast:
In cases where a mixture of different aircraft sizes on the ramp could not be avoided, it appeared important that adequate space be left between turbojets and smaller commuter or corporate aircraft, and that ground crews carefully monitor boarding and deboarding operations, particularly those of the lighter aircraft:
It was also clear from reporters' comments that airports' ramp management policies could influence the occurrence of jet blast damage incidents:
Ground Communication & Handling Procedures. A final factor contributing to some ramp jet blast incidents reported to ASRS was inadequate communication between pilots and ATC regarding pushback and power-up, or between flight and ground crews involved in pushback activities. At some airports, initial pushback communications are conducted on gate radio until the aircraft is released to Ground Control. For at least one reporter involved in a ramp jet blast incident, this procedure was a contributing factor:
More frequently, however, inadequate communications between flight and ground crews, coupled with questionable ground handling procedures, were responsible for jet blast damage incidents. In some cases the miscommunication was a result of an ambiguous or absent signal by the ground crew:
In several other incidents, ramp agents or ground personnel did not warn the flight crew of transient light aircraft in the vicinity of the pushback:
Several pilots suggested that damage to baggage carts by jet blast could be prevented by improved ground crew vigilance and cart handling:
Because of the small size of the ASRS study set (fifty-one reports) and the absence of make/model information in database records examined, it was impossible to conclusively identify the aircraft types that were primary sources of jet blast damage. Several interesting findings emerged, however.
Large (LGT) aircraft weighing between 150,001-300,000 pounds, such as the B-727, B-757, and A320, were the most frequent source of jet blast damage by nearly a two-to-one margin, accounting for 45 percent of the damage incidents reported to ASRS. In comparison, medium-size transports (MLGs) weighing 60,001-150,000 pounds, including aircraft such as the DC-9, BA-146, MD-80, and B-737, were the source of jet blast damage in 25 percent of the incidents. In another 24 percent of incidents, widebody aircraft (WDB) weighing over 300,000 pounds, such as the DC-10, L-1011, B-747, and B-767, were the source of jet blast. (See Table 2.)
Table 2--Source of Jet Blast
|Aircraft Type||No. of Rpts.||% of Data|
LGT (Large Transport)
MLG (Medium Large Transport)
WDB (Widebody Transport)
HVT (Heavy Transport)
MDT (Medium Transport)
LTT (Light Transport)
The finding related to LGT aircraft was not completely unexpected: the aircraft types that comprise ASRS's LGT weight category--especially the B-727--account for significantly more worldwide commercial jet operations than other aircraft types. Thus ASRS data may simply reflect these real-world proportions. We also anticipated that there might be fewer jet blast incidents involving the widebody (WDB) category of aircraft, since many air carrier companies prohibit maneuvering widebody aircraft in confined spaces such as ramps.
When we classified the types of jet blast damage, we found that other aircraft--particularly light aircraft--were the primary targets of jet blast damage in 85 percent of the study incidents. Usually aircraft were damaged because of a direct "hit" from jet exhaust, or because objects such as ladders and baggage carts were blown into them. Many of these incidents were preventable, either through safer handling procedures for large jets departing and arriving at ramps; or through stricter ATC separation criteria between large jets and light aircraft in other areas of the airport, coupled with explicit jet blast warnings to pilots of both large jets and light aircraft.
In the high-pressure, quick-turnaround environment of most scheduled air carrier operations, flight crews of turbojet aircraft may be tempted to ignore jet blast avoidance procedures such as requesting a tug or tractor for pushback, or taxiing to a designated area on the airport to do a quick engine run-up. Fortunately, the ASRS jet blast reports we reviewed contained safety suggestions that should help pilots, ground crews, airport managers, and ATC avoid jet blast incidents without causing excessive ground delays.
For Flight Crews:
- Never perform an engine check or run-up on a taxiway or near a runway without first informing ATC. This will allow ATC time to issue appropriate jet blast warnings to lighter aircraft that may be nearby, but unseen by the jet crew.
- When it is necessary to perform an engine run-up, request ATC assistance in ensuring that jet engine exhaust is directed away from active taxiways, runways, and other areas that may be occupied by lighter aircraft.
- If possible, avoid pushback, powerback, or taxi-in procedures that involve 180-degree-or-greater turns with one or more engines running--consider using a tug instead.
- When departing a gate, request pushback to a position where an initial taxi can be made without directing forward thrust into the gate area--or into lighter aircraft parked nearby on the ramp. If a powerback must be made, request that the guide person direct the aircraft to a position that is parallel to the centerline of the taxiway, before applying forward thrust.
- Before engine start and pushback on a ramp where both turbojets and lighter commuter aircraft are parked, ask ground personnel to inform the flight crew of any boarding or deboarding operations by lighter aircraft that may be in progress close behind jet aircraft. If a potential hazard exists, flight crews should delay their engine start and pushback procedure until the area behind them is clear. Be aware that even small air carrier and corporate jets can produce potentially deadly blast.
For Ground Crews:
- Always visually check the ramp and taxiways behind a turbojet before and during a pushback/powerback for the presence of transient light aircraft that may be caught in the jet blast.
- Give decisive hand signals and/or verbal warnings to the flight crew during a pushback or powerback if you see a hazardous situation developing.
- Avoid driving baggage carts between turbojets on the ramp; it is often impossible to know whether aircraft engines are running, or whether power might be applied suddenly by the jet's crew.
- Position baggage carts in the areas designated on the ramp--make sure the cart brakes have been applied.
For Airport Managers
- Ensure that adequate distance buffers are maintained on the ramp between commuter and turbojet boarding and deboarding operations.
- Avoid parking light aircraft "tail-to" turbojets on airport ramps; light aircraft may be damaged during jets' power-up and initial taxi. Make sure that any light aircraft left on ramps overnight, or during daytime operations, are properly secured.
- Encourage owners of light aircraft in parking areas adjacent to taxiways used by turbojets to make sure that their planes are properly tied down, with gust locks engaged or attached when possible.
- When a turbojet requests a maintenance run-up or engine check, direct the aircraft to an airport area designated for this activity, or turn the aircraft to a heading where jet exhaust will not be aimed across active taxiways, run-up areas, and runways. Require one of the aircraft crew members to be on a headset during the run-up procedure so that communications between the tower and aircraft will not be disrupted. Otherwise, the ear-splitting noise of revving jet engines may drown out ATC instructions on cockpit speakers.
- Always warn light aircraft of jet blast hazard if they are near a turbojet that is initiating or performing an engine run-up. This warning is especially important to light aircraft taxiing near a turbojet run-up at dusk or night.
- Before clearing a turbojet into position on the runway, consider warning the jet crew of the presence of lighter aircraft directly behind them, or in close proximity (on an adjacent taxiway or in a run-up area, for example). The jet crew may be unaware of the lighter aircraft. This warning is particularly important when "immediate takeoff" instructions are issued.
- Before clearing a light aircraft to cross a runway where a large turbojet aircraft has just departed, issue a jet blast warning and consider applying time or distance separation between the light aircraft and the residual jet blast. These safeguards may be especially important at night, when pilots lack the visual cues that help them judge residual jet blast.
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