What is jet blast?

Jet blast is the high-velocity, high-temperature exhaust stream of a jet engine that can damage pavement, equipment and aircraft, and injure people.

Jet blast, jet efflux and propeller wash

Jet blast is the exhaust of a jet engine. EASA also writes “jet efflux” in its engine test guidance. A propeller makes propeller wash, which is also called propeller slipstream. The FAA says that propeller wash does not produce the degree of hazard that jet blast produces. It can still raise ground winds and vortices. The wind from large turboprop aircraft can lift and overturn lighter aircraft, vehicles and ground support equipment.

Why jet blast is dangerous

The FAA describes the exhaust of a jet engine as a high-velocity air stream at an elevated temperature. It can damage airport infrastructure, ground equipment and other aircraft, and it can seriously injure people. The air is also turbulent. It makes vibrations and forces that may damage the facades of buildings. Together, the high temperature and the high velocity can also damage pavement close to the exhaust point.

On aprons, the wind forces of jet blast can project objects, displace equipment, cause a vehicle to lose control and damage infrastructure. They put ground crew, passengers, aircraft and structures at risk.

The FAA gives air velocity thresholds for planning. Above 12 mph, air can displace unsecured debris. Above 30 mph (48 km/h), loose objects such as sand and rocks can become airborne. The thresholds in Table C-1 come from the Beaufort scale. They are sensitivity thresholds at which the risk increases. They are not restrictions or structural design loads.

For a rough force, the FAA gives the pressure of a uniform air stream on a surface that faces the stream. The formula is P = 0.00256 V², with P in lb/ft² and V in mph. At 50 mph, this is about 6.4 lb/ft² (about 300 Pa). Multiply the pressure by the area of the object to get the force. The formula ignores turbulence, which makes the real force vary. The FAA says that wind tunnel analysis or computer modeling may be necessary in complex airside places.

How strong it is

The strength of jet blast depends on the aircraft, the thrust level and the atmosphere. The FAA names three thrust levels:

• Idle thrust, common at apron parking positions and at taxiway holding positions.
• Breakaway thrust, which overcomes inertia and tire friction to start the aircraft moving. It occurs on aprons, on taxiways and at runway ends. Aircraft weight, pavement slope, the number of engines that give power and headwind change it.
• Takeoff thrust, which occurs mainly from the runway end along the runway until rotation, and also in run-up areas. Most departures use a reduced thrust level to limit engine wear.

Velocity and temperature extend a long way behind the aircraft before they fall to safe levels. The temperature falls to a safe level at a shorter distance than the velocity does. A crosswind can move the exhaust envelope to the left or the right.

How far it reaches

There is no single safe distance behind a running engine. The distance depends on the aircraft, the thrust level and the atmosphere. The FAA gives no single figure. It tells planners to find the exhaust contours of each aircraft type.

• Most commercial aircraft manufacturers publish exhaust velocity and temperature contours for the three thrust levels in their airport planning manuals.
• For smaller aircraft, the pilot’s operating handbook is the source.
• The contours are usually plan and elevation views with notes on the assumptions, for example standard atmosphere and the thrust level.
• Aircraft path modeling software also includes exhaust velocity and temperature data.

A NASA study from 1993 gives a sense of scale. It says that almost every commercial jet operating manual has a jet blast data page. The page shows a damage profile, measured from the tail at low engine power, usually 35 to 40 percent N1. The profile reaches more than 200 ft (61 m) behind some larger aircraft. In it, the exhaust forces approach 100 knots.

To turn the contours into a layout, planners compare them with the thresholds in the table. For tail-to-tail parking at a terminal, the FAA recommends a 35 mph (56 km/h) maximum wind velocity. For facing terminals, it recommends 50 mph (80 km/h) for the initial blast of an aircraft that enters or leaves a gate. For the facing gate parking and service roads, it recommends 35 mph. The recommended practices also ask for a pushback area, separate parking for general aviation and commuter aircraft, and tie-down anchors for small aircraft.

Where it causes harm

The NASA Aviation Safety Reporting System studied 51 ground jet blast reports. The study says that they were a tiny fraction of all reports. The authors found these facts:

• 27 of the 51 reports (53 percent) came from a ramp. 10 came from a taxiway, 10 from a runway hold, landing or takeoff, and 4 from a run-up area.
• On ramps, sharp turns caused the incidents. The turns happened during an engines-on pushback, powerback, taxi-out or taxi into a gate. A tug did not prevent them when the turn was sharp.
• 85 percent of the damage was to the wings, propellers, flaps and rudders of other aircraft. Most of it was on light aircraft of 5,000 lb or less.
• Buildings, objects or vehicles took 11 percent of the damage, and injuries to people took 4 percent.
• Many off-ramp incidents came from misdirected engine run-ups and tests.

These are pilot reports from 1993. They show the kinds of events, not how often they happen now.

Jet blast on pavement and soil

On asphalt, jet blast erosion is a darkened area where the bituminous binder burned or carbonized. Localized burned areas can reach about 1/2 inch (13 mm) deep. In a pavement condition survey, the PAVER manual numbers it distress 46. It has no severity levels. The inspector records that it exists and measures its area in square feet or square meters. See blast erosion .

Two other distresses can look similar, but the manual defines them differently:

• Raveling is the dislodging of coarse aggregate particles from the surface. Jet blast erosion is about burned binder.
• Oil spillage is the deterioration or softening of the surface caused by oil, fuel or solvents. A stain alone is not a distress.

Jet blast can also erode unprotected soil next to a runway or a taxiway. Wing-mounted engines of some large aircraft overhang the runway edge. The eroded surface can make dust and debris, and an engine may ingest it. Paved shoulders resist this erosion and debris generation. The FAA also says that unprotected soil next to a taxiway is susceptible to erosion that causes engine ingestion problems. The FAA designs airport pavements to give a surface free of debris that propeller wash or jet blast can blow or pick up.

Lighting and navigation aids

Jet blast shapes where lights and navigation equipment stand.

• The FAA recommends that runway edge lights stand 10 ft (3 m) from the pavement edge on runways that jet aircraft use. This avoids possible damage by jet blast. On runways that jet aircraft do not use, it recommends 2 ft (0.6 m).
• The FAA says to use in-pavement taxiway edge fixtures only in two cases. One is where jet blast may damage elevated lights. The other is where elevated lights would interfere with aircraft operations.
• The inboard PAPI light housing may stand up to 75 ft (23 m) from the runway edge. This applies where jet blast and wing vortices may damage it. It is a deviation from the standard, and the FAA must approve it before installation.
• The FAA recommends 600 ft (183 m) at least between any jet blast source and its communications, navigation, surveillance and weather facilities. This limits exhaust deposits on antennas.

How airports protect against jet blast

The FAA lists these mitigation measures:

• Erosion protection: runway shoulders, runway blast pads and taxiway shoulders.
• Apron configuration, with parking positions that point the exhaust away from people, equipment and structures.
• Blast protection, which includes the blast fence .
• Marking of hazardous jet blast zones on the apron.

A blast pad is an area next to the end of a runway. It reduces the erosive effects of jet blast and propeller wash. The FAA requires a paved blast pad where the critical aircraft is in Airplane Design Group IV or larger. Yellow chevron markings identify a paved blast pad. EASA CS ADR-DSN.B.175(d) says that at least 30 m of the strip before the start of a runway should be prepared against blast erosion. ICAO Annex 14 gives the same recommendation in paragraph 3.4.11. ICAO also recommends shoulders on runway turn pads that are wide enough to prevent erosion by jet blast.

A blast fence is a barrier that diverts or dissipates jet blast or propeller wash. It also helps with fumes and noise. The FAA gives these rules:

• Keep the fence clear of the runway and taxiway safety areas and the object free areas.
• Keep the fence clear of the approach and departure surfaces, the obstacle free zone and the ILS critical areas.
• Design the fence members to resist the wind forces of the aircraft with the most demanding jet blast contours.
• Place the fence so that the centerline of the exhaust stream falls below its top.
• Install a fence only if a sufficient safety buffer is impractical. This is a recommended practice.
• Keep at least 50 ft (15.2 m) between the aircraft tail and the front of the fence. This is also a recommended practice.

Fences can be necessary near runway ends, run-up pads and the airport boundary. Embankments or structural walls are alternatives.

Engine run-ups need a place of their own. The FAA recommends a run-up area away from parking positions, gate areas and buildings. EASA guidance says to do engine tests at a remote area where possible. Its guidance does not permit run-ups at or above idle power in cul-de-sacs. It does not permit them where the jet efflux would affect stands, equipment areas or work areas. It limits tests at regular apron stands to check starts and idle power. EASA also asks for care at a holding bay. Jet blast and propeller wash from holding aircraft must not interfere with aircraft operations, damage vehicles or injure people.

EU rules on aprons

Under ADR.OPS.D.055, the aerodrome operator in the EU must:

• make apron users aware of the hazards of jet blast and propeller slipstream, for example by safety training or safety promotion
• require apron users to secure vehicles and equipment, and designate parking areas where the effect of jet blast is minimal
• consider jet blast when it designs or changes apron layouts
• identify jet blast-sensitive places, and either publish a request for minimum thrust to pilots or take other measures

The request for minimum thrust goes in the aeronautical information publication (AIP). Information signs may mark these places.

What an inspection checks

The EASA inspection guidance asks for a check of the runway extremities for blast damage to approach lights, marker cones and threshold lights. For paved areas, it also lists damage to the pavement surface and damage of light fittings. In a pavement survey, a jet blast erosion patch is distress 46, and the inspector measures its area.