Aircraft Hangar Design Considerations

Maintenance Hangars

76 Metre Maintenance Hangar for UPS Courier Services, Shannon AirportNo-one has ever said: “Damn, I made this hangar too big”. The size of aircraft grows continuously and new marks are often wider and longer and higher. And utilisations change, and every foot wider or longer or higher adds to the flexibility in use. Aircraft manufacturers tend to draw dimensional plans of hangars as if the hangar were a single rectangle dedicated to the one type.

This is fine for a huge airline with large numbers of each type, which can find a constant supply of work on the same type. Reality is that there are often mixed fleets and outsourced work filling up every corner of available space. The more space, the more work is possible.
The height of hangars is most often controlled by the radar and ILS cones. Most hangars are close to runways and taxiways and in more cases than not this dictates the height.

In general we would advise that the hangar clear dimensions are 5m (16′) plus aircraft (width or length) plus 5m (16′). If 2 aircraft are side by side, the plan should be for the wingtips to be 5m (16′) apart. The height is more often controlled by other factors, but 3′ or 1m is the minimum recommended clear height under the door track to allow for different loading conditions of aircraft. However users will often shoe-horn in the most aircraft, overlapping wings and tails and using space right up to, and even between, the columns.

If jacking is to take place, either the whole hangar can be say rudder height plus 4m (13′) clear throughout; or part of the hangar can be so. The part that needs this height is dictated by the rudder dimensions, taking into account the pitching of the aircraft in uneven jacking. For B747/400 this is a minimum square of 9m by 9m up to 23.5m high as a ‘jacking slot’. Careful control is needed to ensure the aircraft is in the correct position before jacking. Using jacking pits in the floor relieves the need for this extra height. But of course the pit-mounted jacks have to be in the correct place for each type.

Hangars can be Nose-in or Tail-in. People expect to make savings by choosing Nose-in, so that only the front of the hangar is high, the remainder sloping or stepping down. The part of the aircraft in front of the wings can be slotted into a Fuselage hangar of much smaller dimensions. But such hangars have a problem if any tail docking is needed: the tail docking has to be out of the way and this usually means outside the hangar. And, as discussed below, under ‘Price factors’ the savings are insignificant. So we would tend to advise the full width at full height, allowing Nose-in or Tail-in. In most maintenance hangars, there is a reluctance to turn away work, and it is normal to find a variety of types in all sorts of positions; having the full width and height throughout is a big bonus in flexibility, and often costs less than stepped versions.

Such Nose-in hangars usually present a problem in that the highest part is over the doors; the doors usually face the runway; and this is the part where the height is critical; and the hangars are often startlingly ugly. Another problem is the turning circle for the tractor. This can be resolved with a door in the far end; but because this is airside, special precautions have to be taken to return the tractor to the apron; this factor alone makes a strong case for Tail-in. A good hangar may allow two aircraft tail in, with a gap between them to allow a casualty to be partly wheeled nose-in between them.

Cranes in hangars are a mixed blessing. The main advantage of them is for placing docking over the wing/fuselage area. Most of the heavy lifts are better done with fork-lift trucks or mobile cranes. The disadvantages are: the hangar has to be higher by about 2m, maybe even as much as 5m. Often this mitigates towards a nose-in hangar, to the detriment of the flexible use of the whole facility. The hangar has to be stronger and stiffer to carry the crane. The nature of the hangar is a little different as tolerances are much tighter with a crane. In wide span hangars in windy places (and most airfields seem to be windy places) the roof structure tends to pant up and down, with gusts, or when the doors are cracked open; this can be very annoying while trying to fasten an expensive bit of alloy with a close tolerance pin to an expensive bit of aeroplane. The cranes are themselves expensive to buy and operate. Many operators like to have safety anti-fall tracks fitted under the roof on rails, for safe movement above the wings or fuselage, and these are ruined by overhead travelling cranes. One has to move a big long heavy crane over the whole hangar even for a small load. It is possible to fit a small span 5 tonne crane over a rudder jacking slot covering say the 9m by 9m slot, which can be handy for access cages etc. Likewise a small 1 tonne radome crane at the nose locations may be an asset. The hangar owner should decide whether or not to have suspended cranes, or whether to buy a mobile crane, which can be used anywhere.

DOORS

Rolling Doors – The most simple form of door is the REIDsteel rolling door. This consists of a number of door slabs running on a number of tracks. They can be manual or electric powered. In general the doors should open across the full width, or there may be dead spaces behind the door jambs. The number of slabs required depends on the width needed to be open at any single time within the hangar width. If the full width is needed at the same time, then these doors have to open on to outriggers. The width of outrigger, ie the width available on the apron side, may dictate the number of slabs. Such outriggers use up valuable apron frontage and should be limited to about 1/8th of the span, leading to an 8-slab, 4-track door. An 80m opening in an 80m width needs 97m of apron. If say 70m is needed for the biggest aircraft in a hangar say 97m wide, then this can be provided with an 8-slab, 4-track door which can give the 70m opening to the left; or to the right; or centrally. This gives a much better use of the available space, and consideration should be given to building the widest hangar possible to use the apron width efficiently. 6-slab or 3-slab 3-track doors or even 4-slab or 2-slab 2-track doors (where only two thirds or half the width needs to be open at any time) also have their places. Fewer tracks and fewer slabs give savings in cost, effort and air leakage.

Rolling doors with tailgates – This is often a feature of older hangars, where the door has not been high enough and a hole has been cut into the gable peak and a roller shutter or other door has been added. In most cases the existing structure has just been cut through to make the hole and is only standing up by divine providence. Tailgates can be used but need careful design so that they stand up with allowable deflections. The doors that are fitted often have howling gaps that let in wind and rain and seagulls. The control of such doors is difficult. Our advice would be never to use such doors if there is an alternative. Better to use full width full height doors, than to give the client the long term operating and maintenance cost of the tailgate. This is particularly important in Hurricane areas, as the position, at ¾ of the height on the gable peak, is the exact position of the maximum wind pressure; no roller shutter can resist these max hurricane loads; and, as well, the gable frame will be subject to large uplifts in the hurricane, and the door is in the position of a nut in a nut-cracker, and it will buckle and jam. But sometimes, perhaps when the height of the eaves of the hangar is restricted because of ILS, they may be needed. They do not save money.

Fabric doors – Another kind of door is the lifting fabric door: this is fabric either side of slats which are raised like a window blind by a motor overhead with cables. The span of such doors is limited to about 28m (90′); when bigger openings are needed, one or more columns have to be placed in the opening, and these obviously have to be lifted mechanically to complete the opening. These doors are more costly than sliding doors, and take more coordinated effort and time to open; but they are invaluable when: the full width has to be open at the same time; and there is no space for outriggers outside the hangar wall; and there is no problem with the height needed for the bunched fabric-and slats. (See also ‘Paint hangars’ below).

Doors which turn corners, concertina doors and bi-fold doors should always be avoided in hangars greater than 30m, 100′.

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WORKSHOPS – All maintenance hangars need workshops. You would expect that the workshop on the hangar floor level would be full width and 30% or so of the main hangar length. If the operation includes trim overhaul then there should always be a trim floor of similar area at the height of the jacked (or sometimes unjacked) cabin door height – 6.5m for the 747/400; 6.8m for the A380. These hangars are best built in to lean-tos to the rear of the hangar. The client is paying a lot for the big clear space in the hangar and this should not be wasted with low clearance height structures. An exception is in hangars only dealing with full size commercial aircraft where workshop space may be built along the sides under wingtip height, and in the 2 rear corners (leaving space for any docking). Such workshops, built to the rear, also make construction easier, because they provide stability to the tall end columns before the rafters and bracing systems are installed.

OFFICES – Offices, like workshops, should not take up the valuable high clear-span hangar space. Often they can be built in a lean-to behind, above the workshops. Typically, one level of office above the trim workshop and hangar floor workshop, filling up to 30% of the hangar length full width is enough. Offices with glazing into the hangar will be valuable. Because the depth may be too wide without external windows, light wells may be needed in the office roof. A special room for ‘clients representatives’ should be built with the best view of the countryside outside of the hangar, with television, video, a bar, hostesses, soft music, muted lighting and very comfortable chairs.

PAINT HANGARS – Unlike maintenance hangars, the volume should be reduced to the minimum needed for the biggest type. This would be done with ceilings fitting the aircraft like a glove, with minimum clearances – perhaps 2m (6′) all round and 1m (3′) above. No jacking takes place. The air needs to be cleaned and enters above (perhaps using the void above the ceiling as a plenum), and should be taken out by underground ducting. This is also an opportunity to use fabric doors, which can be made to fit the aircraft; can seal quite well; can have the motors outside the paint area; and can lift less for smaller types to minimise heat loss.

PRICE FACTORS:

WIDTH: The wider the span of the hangar, the more the cost. Some costs vary directly with the span. Some costs go up with the square of the span. There is no direct algorithm to decide this, but a 80m hangar compared to a 70m may cost not 1.14 times but 1.2 times more per sqm, sq ft. The growth in costs tend to grow with spans up to a realistic maximum of 200m (660′), which may cost twice as much per sqm, sq ft. Never-the-less, the increase in flexibility in use will always make the bigger span attractive. Remember always to specify the clear width between columns, not the width overall or width on column centres.

LENGTH: Here it pays to remember that the most expensive items are the two ends, especially the end with the doors. Extra length in the middle is relatively cheap. However a long thin hangar is of less practical use than a wide shorter one. Remember to ask for the clear length.

HEIGHT: Extra height has costs related to the extra areas of walls and doors. But also the wind speeds increase at greater height, so there is double effect on the frames, rails, bracing systems and foundation loads. The height is usually controlled by other factors, but should always allow about 1m, 3′, above the rudder. If jacking is to take place, there must at least be jacking slots to 4m, 14′, above the rudder and these must be wide enough to accommodate the length of the rudder with clearance. It is much cheaper to make jacking slots. But apart from these, if the situation allows, it is much better to have the clear height throughout so aircraft can fit in in different ways. Curiously it is often cheaper to maintain the full height throughout than other arrangements. If cranes are fitted the hangar may need to be maybe 2m higher and the price may rise in the order of 10%. If the cranes are higher than the jacked tail the clear height may be 6m higher and the price may rise by 25%. Remember to specify clear heights.

DOORS: Rolling doors are economical. If the width of opening is greater than the aircraft widths, doors can be made to open to the left, or right, or both ways, within the hangar width without outriggers, which is good. If the full width is needed then outriggers are needed. No money is saved, at the outset or in service, by having lower doors with central tail-gates: better to make the doors full height, full width; only height restriction should be the reason for using tail-gates. If outriggers are not possible then fabric doors will be needed in ascending cost. It is not usually a good economy to cut down on the total width of the door opening, which should cover the whole of the width.

ARCHITECTURAL FEATURES: these have to be looked on as additions. Usually the aim is to make these with no structural function, and to put them on so that they do the least damage to the function of the structure. With care they can be made to unbolt and be discarded when fashions change. Most architects live in houses where the rafters are underneath the roof cladding, where the gutters are external and where there are the minimum of penetrations through the envelope, but there is a tendency to forget this when designing someone’s hangar for the first time. A well designed hangar, like a bridge, needs no gimmicks to make it look good.

QUOTATIONS: In spite of the guidance above, there is little clue to the actual cost of the hangar, door and workshop. The only way is to run each new enquiry through our REIDsteel optimisation programs, which takes only a day to do, when we have someone available to do it immediately.

For more information on aircraft hangars, take a look at our dedicated site www.aircraft-hangars.com or contact us